UNIVERSITY OF CALIFORNIA PUBLICATIONS IN AGRICULTURAL SCIENCES Vol. 2, No. 10, pp. 297-314, plate 53 March 5, 1925 CHROMOSOME NUMBER AND INDIVIDUALITY IN THE GENUS CREPIS I. A COMPARATIVE STUDY OF THE CHROMOSOME NUMBER AND DIMENSIONS OF NINETEEN SPECIES BY MARGARET CAMPBELL MANN ( Contribution from the Division of Genetics, University of California ) Because most of the species of the genus Crepis have low chromo- some numbers, it offers obvious advantages for the study of comparative chromosome relations. The chromosome individuality of certain species is very distinct, so much so that it could be used as a diagnostic character in specific determination. These facts lead to an inquiry to discover first, whether upon careful analysis all species would prove to differ in chromo- some individuality, and second, what relations the chromosome groupings of different species bear to one another. This question has been previously touched upon in several papers by Rosenberg (1909, 1918, 1920) and in a recent contribution by Marchal (1920). Rosenberg (1918) called attention to the fact that the genus Crepis possesses a great variety of chromosome numbers. His summary showed species with 3, 4, 5, 8, 9, and 20 pairs. In order to determine how such numerical differences had arisen within the genus, he measured the chromosomes of a three and a four-pair species, capillaris (Reuteriana of Rosenberg) and tectorum, respectively, and found, on the basis of measurements of homotypic anaphase chromosomes, that three of the chromosomes of the two species corresponded accurately in size and that the fourth pair of tectorum averaged slightly shorter than the shortest of capillaris. He noted that the two shortest chromosomes of capillaris often mate later than the other two in p. m. c. and finds associated with this fact a tendency toward lagging and irregular division. From these data he 298 University of California Publications in Agricultural Sciences [Vol. 2 concluded that the four-pair species have arisen from a three-pair species by the fusion of two gametes each of which has received an extra short chromosome. Although he did not publish measurements on the two five-pair species which he studied (rubra and multicaulis) , he believed that both have three of the short chromosomes, and that these types have originated by a repetition of the process which gave rise to the four-pair types: In his 1920 contribution he changes his count in biennis from twenty to twenty-one pairs and concludes that it represents the three chromosomes of capillaris multiplied fourteen times. Marchal, whose work was done without knowledge of Rosenberg's paper, expressed (1920) the belief that four is the ground number of the genus Crepis. He noted that p.m.c. of a slightly aberrant capillaris plant had what appeared to be a large quadrivalent multiple chromosome plus two smaller but equal elements, and that most of the species of Crepis seemed to have four pairs of chromosomes. He therefore con- cluded that capillaris had arisen from the type by end-to-end union between two chromosomes. He believed that the differences in length which had been noted for C. lanceolata platyphylla (Tahara and Ishikawa, 1911) could be accounted for by bipartition of one chromosome of a species with four pairs. He further suggested that six-pair species might arise by doubling of the three, and an eight-pair species by doubling of the four. He counted sixteen pairs for biennis and noted that, while the individual chromosomes in the p.m.c. of this species appeared somewhat smaller than those of certain four-chromosome species, the total mass was much greater. He then concluded that biennis is an eight-ploid species. MATERIAL AND METHODS A large number of species of the genus Crepis have been grown and identified in the greenhouse of the Division of Genetics of the University of California by Professor E. B. Babcock, thus making it possible to be certain of the specific determination of the material which was studied cytologically. Since the chromosome numbers which have been found to characterize the species thus identified differ in several instances from previously published counts, the data are presented in a convenient form in table 1. The root tips were fixed in chrom-acetic-urea and stained in Heidenhain's iron-haematoxylin. In most species the reduced number has also been counted by Belling's iron-aceto-carmine method. 1925] Mann: Chromosome Number and Individuality in the Genus Crepis 299 TABLE 1 Chromosome Counts of 27 Species of Crepis Number Species N 2N Author alpina L 4 5 10 10 Marchal (1920)* Rosenberg (1920)f Mann (1922)J amplexifolia Willk 4 8 Mann aspera L 4 4 8 Marchal (1920) Mann (1922) aurea (L.) Reichb 5 10 Mann biennis L.. . 16 20 21 20 40 Marchal (1920) Rosenberg (1918) Rosenberg (1920) Mann (1922) blattarioides Vill... 4 4 8 8 Marchal (1920) Rosenberg (1920) Mann breviflora Delile 4 8 Mann bidbosa (L.) Tausch 9 18 Mann bursifolia L. 4 8 Mann capillaris (L.) Wallr 3 6 Rosenberg (1909), Mann (1922) dioscoridis L 4 4 8 Marchal (1920) Mann (1922) foetida L 4 4 5 8 10 Marchal (1920) Rosenberg (1918) Mann (1922) grandiflora Tausch. 4 8 Mann incarnata Tausch 4 8 Mann japonica (L.) Benth. 8 16 Tahara (1910), Mann (1922) myriocephala Coss. et D. R 4 8 Mann (1922) * Marchal gives 1914 as the date of his counts, but they were not published until 1920. t Figured but not mentioned in the text. J Cited from Report of the College of Agriculture, University of California, July 1, 1921-June 30 1922. 300 University of California Publications in Agricultural Sciences [Vol. TABLE 1— (Continued) Number Species N 2N Author neglecta L 4 8 Rosenberg (1918), Mann (1922) palestina Boiss. Bornmuller 4 8 Mann parviflora Desf 4 8 Rosenberg (1918), Mann (1922) pulchra L 4 8 Rosenberg (1920), Mann (1922) rubra L 4 5 10 Marchal (1920) Rosenberg (1918), Mann (1922) setosa Hall 4 8 Mann (1922) sibirica L.... 4 5 10 Marchal (1920) Mann (1922) Sieberi Boiss 6 12 Mann (1922) taraxacifolia Thuill 6 4 12 8 Beer (1912) Digby (1914), Mann (1922) tectorum L.... 4 8 Juel (1905), Mann (1922) vesicaria L 4 8 Mann Table 1 shows that, while four is the most common haploid number for the twenty species studied, five is also fairly frequent. The other numbers (3, 6, 8, 9, and 20) are each represented by a single species. It is obvious that chromosome measurement should show whether cross- division, union into multiples, addition by non-disjunction, or combina- tions of these methods are sufficient to account for the differences in number found in the genus. It is also possible that hybridization between species with different chromosome numbers might account for the origin of certain cytological peculiarities. For some species the cytological material is far more abundant than it is for others, so that it is possible to measure only somatic metaphases in which all the chromosomes are fairly straight. The tendency of the long chromosomes of Crepis to twist is a source of considerable error where relatively poor material is available. The finest metaphase figures are to be found in the upper portion of the rapidly growing region of the root in seedlings, and in roots from adult plants. The region containing fine figures is greater in roots from the latter than 1925] Mann: Chromosome Number and Individuality in lite Genus Crepis 301 in the short root of the cotyledon stage, because there is a longer growing area in which the cytoplasm is less dense than it is at the tip, so that the chromosomes spread out more freely and the picture is less obscured by cytoplasmic inclusions. Table 3 is a compilation of measurement data for somatic metaphase figures in nineteen species of Crepis. In each case, except japonica and sieberi, ten somatic polar metaphases were drawn with a camera lucida. The magnification of the drawings is 4000 diameters. A moistened thread was placed along the center of the drawing of each chromosome, and then straightened and measured in millimeters. The figures were then placed in columns, the two largest in the first, and so on down to the two smallest. A sample of these records for a five- pair species, alpina, is given below in table 2. TABLE 2 Actual Measurements of Drawings Differences from Average 1 2 3 4 5 Total Length 1 2 3 4 5 32 mm 25 mm 14 mm 13.5mm 13mm. 31 27 14 13 12.5 195 mm. +5.8 +5.7 -0.5 +0.4 +0.8 22.5 20 15.5 13 11.5 24.5 18 14.5 13 11 163 mm. -1.7 -1.3 + 1.0 -0.1 -0.7 30.5 21 17 14.5 12 5 22 19 15 14.5 13 179 mm. +4.3 -0.3 +2.5 + 14 +0.8 21.5 17 13 12 10.5 23.5 19 13 12 11.5 153 mm. -2.7 -2.3 -1.5 -1.1 -0.7 23 21. 5 16.5 14 12 29 20 15 12 11.5 174 mm. +2.8 +0.2 +2.0 +0.9 -0.2 It is evident that even measurement by the rather crude method described above gives a fairly definite clue to the individuality of the species. It will also be noted that when the larger figure of each set is compared with the average for the chromosome, obtained by dividing the sum of the ten larger of the twenty chromosomes of one type by ten, the deviations for any one metaphase set are generally in the same direction (+ or — ). (See column headed "Differences from the average.") This deviation indicates that the error of measurement was not sufficient to conceal the fact that the chromosome lengths of a species maintain certain size relations at least throughout the later periods of shortening. It also shows that it is fair to use an average 302 University of California Publications in Agricultural Sciences [Vol. 2 so obtained in a comparative study like this. The larger figure of each set was considered the more accurate measurement and hence was used to secure the 'corrected' totals and averages which appear in table 3. TABLE 3 Measurement Data for Nineteen Species of Crepis Species C. capillaris C. neglecta C. setosa C. parviflora C. bursifolia C. aurea C. aspera C. alpina C. taraxacifolia C. tectorum C. blattarioides . . C. japonica a C. foetida C. bulbosa rubra dioscoridis sieberi a pulchra sibirica Hap- loid chromo- some number Cor- rected average total length 3 61.4 4 61.7 4 63.2 4 69.9 4 78.5 5 83.5 4 82.6 5 87.3 4 88.4 4 88.7 4 91.1 8 92.6 5 93.7 9 100.5 5 102.9 4 109.4 6 109.6 4 112.1 5 143.6 Corrected average for individual chromosomes 26.2 20.4 14.8 24.5 16.2 11.2 9.8 22.3 17.8 14.0 9.1 25.3 20.5 14.4 9.7 24.3 22.0 19.5 12.7 21.0 18.0 16.2 15.1 13.2 23.9 21.5 19.7 17.5 26.2 21.3 14.5 13 1 12.2 26.1 23.3 21.2 17.8 28.1 23.2 20.2 17.2 29.0 23.8 20.6 17.7 15.7 13.5 12.2 11.5 10.8 10.0 9.7 9.2 25.0 20.8 17.7 15.8 14.4 13.9 12.8 12.1 11 7 11.1 10.6 10.1 9.6 29.4 23.9 18.5 16.2 14.9 35.9 29.3 24.9 19.3 26.8 21.4 17.7 16.0 15.2 12.5 36.7 30.6 25.5 19.3 41.9 32.4 27.6 23.2 18.5 8.6 a Averages from less than ten figures. The reliability of such measurements and the evidence for the constancy of specific individuality have been further corroborated by a study of chromosome measurements of the Fi's of two species-hybrids, setosa X tectorum (fig. 1) and setosa X dioscoridis (fig. 2).1 It will be noted from table 3 that all three species involved have four pairs and that the chromosome sizes are far more different in the two latter than in the two former species. In both Fi's, however, it was possible to determine the source of the chromosomes by means of measurement data, and this was facilitated by the peculiar semidetached tip of the longest chromosome of setosa (fig. 3), by which it may usually be identi- fied. Since only one member of a set is present in each Fi figure, it seemed best to compare the averages for the Fi's with the uncorrected averages for the species involved. The results are tabulated below: 1 For the use of these hybrids and the data on hybridization given below, I am indebted to Dr. J. L. Collins of this laboratory. 1925] Mann: Chromosome Number and Individuality in the Genus Crepi-s 303 TABLE 4 setosa X dioscoridis 39.9 33.6 28.9 23 . 1 22.1 22.3 18.1 17.8 13.7 14.0 10.3 selosa 9.1 dioscoridis 34.2 28.9 24.9 20.6 +5.7 +4.7 +4.0 +2.5 -0.2 +0.3 -0.3 + 1.2 selosa X teclorutn 29.4 24.1 21.2 16.8 21.0 22.3 18.9 17.8 13.3 14.0 8.9 setosa 9.1 lector um 28.1 23.2 20.2 17.2 + 13 +0.9 + 1.0 -0.4 -1.3 + 11 -0.7 -0.2 The important point is that one can identify the chromosomes of dioscoridis and of tectorum by measurement when they are in combina- tion with those of setosa in an Fi hybrid, so that it is evident that the specific differences in length noted are not the product of interaction between a certain cytoplasm and its chromosomes. Since abundant material was available for capillaris (fig. 6), the first measurements, which were made on ten figures about as good as the average for all species, were checked by the use, first, of a mixture of slightly different metaphase stages (beginning to almost complete divi- sion) from a very short region of a single root tip, and, second, of a mixture from undivided figures from two different roots. These measurements show that averages for one chromosome in three different sets of ten from the same species may differ by as much as 3.55 mm., but that the averages give, in each case, very nearly the same differ- ences between the lengths of the different pairs. COMPARISON OF SPECIES Crepis neglecta (fig. 7) has a very characteristic individuality, two of the pairs being very similar and distinctly shorter than any of the chromosomes of capillaris. Its total length is very similar to that of capillaris, so much so that one is inclined to test the cross-division hypothesis for this species. If the two shortest averages are added, their sum is practically the same as the average for the intermediate chromosome of capillaris and the other average lengths are very similar. capillaris. neglecta.... 26.2 24.5 20.4 11.2+9.8=21.0 14.8 16.2 ■1.7 +0.6 +1.4 Attempts to cross the two species have as yet been unsuccessful. 304 University of California Publications in Agricultural Sciences [Vol. 2 Setosa (fig. 3), like neglecta, differs little from capillaris in total length. It contains, however, only one pair of chromosomes shorter than any in capillaris; otherwise it is rather similar to it. capillaris 26.2 20.4 14.8 setosa 22.3 17.8 14.0 9.1 -3.9 -2.6 -0.8 +9.1 It has already been noted that the longest chromosome of setosa has a semidetached tip by which it may be recognized. This tip is usually at an angle to the main portion of the chromosome. In the figures given above the longest chromosome of setosa appears to have lost a portion of its length, while another pair of chromosomes averaging about ten units has been added. It is also possible that the longest chromosome has cross-divided, and that the peculiar chromosome of setosa really corresponds to the intermediate of capillaris. capillaris 26.2 20.4 14.8 setosa 17.8+9.1=26.9 22.3 14.0 +0.7 +1.9 -0.8 If either of these possibilities represented the whole truth concerning the difference between the two species, we should expect reduction to be fairly normal following hybridization. As a matter of fact, no pairing occurs in the Fi setosa (N = 4) X capillar is (N = 3) (Collins and Mann, 1923), and as a consequence gametes are formed with 3, 4, and 6 chromosomes as shown by five plants (backcrosses to setosa), which have 7, 8, and 10 somatic chromosomes. It seems possible that new types differing in number and combination of chromosomes may be obtained by selfing such plants as the backcrosses with ten chromosomes. Crepis parviflora (fig. 8) has a chromosome individuality much like that of setosa; the longer chromosome, however, averages slightly longer and does not appear to have a semidetached tip. setosa 22.3 17.8 14.0 9.0 parviflora 25.3 20.5 14.4 9.7 +3.0 +2.7 +0.4 +0.7 It is evident that parviflora is more similar to capillaris than setosa, but like setosa it has an additional short pair of chromosomes. capillaris 26.2 20.4 14.8 parviflora 25.3 20.5 14.4 9.7 -0.9 +0.1 -0.4 +9.7 The first hypothesis for setosa appears to be the more probable for parviflora. If it were true, one would have to account for the additional chromosome of 9.7 units by hybridization between two such forms as 1925] Ahum: (.'hromtisnmc X umber and Individuality in the Genus Crepis 305 neglecta and capillar is. The hybridization results for setosaX capillar is given above indicate that new types with new combinations of chromo- somes may arise in this manner. It will be interesting to observe the results of crossing setosa and parviflora. Bur si folia (fig. 9) appears to have an extra element of the size of the intermediate chromosome of the capillaris series: capillaris 26.2 20.4 14.8 22 + 19.5 bursifolia 24.3 =20.7 12.7 -1.9 +0.3 -2.1 It's average total length is 17.1 units longer than that of capillaris. Crepis taraxacifolia (fig. 10), tectorum (fig. 5), and blattarioides (fig. 11) have very similar chromosome groups. taraxacifolia 26.1 23.3 21.2 17.8 blattarioides 29.0 23.8 20.6 17.7 tectorum 28.1 23.2 20.2 17.2 All the chromosomes of these three species tend to average slightly larger than those of capillaris, but the differences do not greatly exceed those of the different averages for capillaris. If we suppose that the intermediate chromosome of capillaris has been duplicated in this group of species, the correspondence is somewhat bettered. Average of taraxacifolia, tectorum, and blattarioides 27.7 22.05 17.6 Average of capillaris 26.2 20.40 14.8 + 1.5 +1.65 +2.8 It is obvious that the relative lengths of the chromosomes in these three species are very similar to those in capillaris. Tectorum and capillaris were repeatedly crossed by Collins (1920), but the Fi developed only as far as the cotyledon stage. This indicates an incompatibility of the chromosomes or cytoplasm hard to account for on the basis of mere addition of similar material, especially when one considers that trisomic forms which come to maturity appear to be not uncommon among plants and animals. It will be very interesting to know whether others of the group of species indicated above will behave like tectorum in crosses with capillaris, and whether they will intercross. Aspera (fig. 12) is like the group discussed above except that the longest chromosome appears to be rather short. capillaris 26.2 20.4 14.8 21 .5 + 19.7 aspera 23.9 =20.6 17.5 -2.3 +0.2 +2.7 306 University of California Publications in Agricultural Sciences [Vol. 2 Crepis bursifolia, taraxacifolia, tectorum, blattarioides, and aspera might all be derived from capillaris by duplication of the intermediate pair of chromosomes. The five-pair species listed below, although generally rather similar in chromosome individuality, show certain distinct differences. Total length aurea 21.0 18.0 16.2 15 1 13 2 161.9 alpina 26.2 21.3 14.5 13.1 12.2 174.6 foetida 25.0 20.8 17.7 15.8 14.4 187.4 rubra 29.4 23.9 18.5 16.2 14.9 205.8 Aurea (fig. 13) is outstanding since it lacks a long chromosome of about twenty-five units. The figures are excellent, so that the averages must be considered as very nearly accurate. Aurea is also very dis- tinctive morphologically. Alpina (fig. 14), foetida (fig. 15), and rubra (fig. 16) are much more alike in chromosome individuality. Alpina seems to have three pairs resembling the shortest chromosome of capillaris, and to be cytologically very like it otherwise. capillaris 26.2 20.4 14.8 14.5 + 13.1+12.2 alpina 26.2 21.3 — ' = 13.2 0 +0.9 -1.6 Foetida might also have three duplicates of the shortest chromosome of capillaris. capillaris 26.2 20.4 14.8 17.7 + 15.8+14.4 foetida 25.0 20.8 ■ ! = 15.9 -1.2 +0.4 +1.1 The figures for rubra compare better with those of capillaris if we average the two intermediates and the two shortest together. capillaris 26.2 20.4 14.8 23.9 + 18.5 16.2 + 14.9 rubra 29.4 -=21.2 - = 15.5 2 2 +3.2 +0.8 +0.7 It was noted above that Rosenberg (1918) suggested that probably the small chromosome of capillaris had been duplicated twice for rubra. It will be seen from the figures that duplication of the intermediate and of the short chromosome appears more probable on the basis of the measurements presented here. 1925] Mann: Chromosome Number and Individuality in the Genus Crepis 307 Crepis japonica (N = 8) (fig. 17) and bulbosa (N = 9) (fig. 18) are rather similar in chromosome individuality, but are totally different from all the rest of the species studied in chromosome number and size. japonica 15.7 13.5 12.2 11.5 10.8 10.0 9.7 9.2 bulbosa 13.9 12.8 12.1 11.7 11.1 10.6 10.1 9.6 8.6 It is, of course, possible that japonica might have been derived from a species like tectorum by cross-division of every chromosome, or vice versa. When we test this hypothesis by adding the averages for the two largest, the next two, etc., of japonica together, the results are rather striking. 15.7 12.2 10.8 9.7 japonica | 13.5 11.5 10.0 9.2 i 29.2 23.7 20.8 18.9 tectorum 28.1 23.2 20.2 17.2 + 1.1 +0.5 +0.6 +1.7 It is at least obvious that tetraploidy could not explain the chromosome individuality of japonica while cross-division might do so. Crepis sieberi (fig. 19) is the only species so far studied which has six pairs of chromosomes. It looks as if it might have four pairs of short chromosomes: capillaris 26.2 20.4 14.8 17.7 + 16 + 15.2 + 12.5 sieberi 26.8 21.4 — =15.3 4 +0.6 +1.0 +0.5 or two intermediate and three short pairs: capillaris... 26.2 20.4 14.8 21.4 + 17.7 16 + 15.2 + 12.5 sieberi 26.8 ! =19.5 ! ! =14.6 2 3 +0.6 -0.9 -0.2 Crepis pulchra (fig. 21) and dioscoridis (fig. 4) are very similar to one another in chromosome length. pulchra 36.7 30.6 25.5 19.3 dioscoridis 35.9 29.3 24.9 19.3 Difference 0.8 1.3 0.6 0 C. sibirica (fig. 23), with five pairs, resembles pulchra and dioscoridis in choromosome measurements, and the average length of the two longest chromosomes, 36.5, indicates that it may have two instead of one of the longest type of chromosome. 308 University of California Publications in Agricultural Sciences [Vol. '2 41.9+32.4 sibirica =37.1 27.6 23.2 18.5 2 dioscoridis 35.9 29.3 24.9 19.3 Difference 1.2 1.7* 1.7 0.8 If we suppose that this group of species has been derived from a type like capillaris, we must consider that the longest chromosome represents a multiple. If we subtract the intermediate average for capillaris (20.4) from the average of the longest chromosomes of all three species in this group (36.3), the remainder, 15.9, is only 1.1 units longer than the shortest chromosome of capillaris, indicating that an intermediate and a short chromosome might have united end to end to form an element averaging 36.3 units. Then if we average the two shortest chromosomes of these three species with the chromosome of 20.4 units, which, we have supposed has united with a short element, the average, 19.9, is so like the intermediate of capillaris as to suggest that it may have been duplicated in the group under consideration. When we look at the averages now, the figures compare very well. capillaris 26.2 20.4 14.8 pulchra, dioscoridis, ,>0 c i oq q_i_27 fi and sibirica —=29.1 19.9 15.9 3 +2.9 -0.5 +1.1 These species obviously form a group by themselves, especially since it has been shown that the great size of the chromosomes in dioscoridis is maintained upon hybridization with a species like setosa. DISCUSSION For two reasons it is impossible to make any sweeping general- izations at this time concerning the data presented here. First, we do not yet know how species differing in chromosome number can arise, and second, we know too little about the genetics of Crepis. There are two known methods by which a single pair of chromosomes can be added to a complex, non-disjunction and species-hybridization, but in neither case has it been proved that stable types would ever result; and the formation of new species presupposes stability. It has. been suggested that it is very improbable that stability is to be expected of tetrasomic individuals because the complex as a whole is unbalanced by the addi- tion of chromosomes. This view seems to be borne out by observations on the cytology of tetrasomic plants of Datura (Belling and Blakeslee, 1925] Mann: Chromosome Number and Individuality in the Genus Crcpis 309 1924) and Matthiola (Frost and Mann, 1924). Both of those tetrasomic types are even feebler than the trisomic plants, and hence would have little chance of survival under unfavorable environmental conditions. The possibilities of species-hybridization as a source of differences in chromosome number within a genus are still less known. It might be argued with some plausibility that if a tetrasomic condition is unbalancing and associated with lessened viability, even less in the way of stability and viability should be expected of organisms having a pair of chromosomes from another species added to a complete specific complex. The Drosophila workers have found, however (Morgan, 1922), that a similar genie structure characterizes the chromosomes of several species of that genus, and if this is true of Crepis, one method may be as probable as the other. It has been shown (Collins and Mann, 1923) that new types with more chromosomes than either species possesses are formed when the Fi C. setosaXC. capillaris is backcrossed to setosa. It is only through further work on such types that the question of stability can be answered. The theoretical and practical value of such work is self-evident. While the little work that has so far been done on tetrasomic plants tends to show that they would be expected to be somewhat unstable genetically, tetraploid plants, e. g., Oenothera gigas, breed true. That Crepis biennis may be an octaploid from a five-pair species is indicated by the following experimental evidence : 1. In the Fi C. setosaXC. biennis the twenty pairs of chromosomes from biennis form ten pairs. 2. In the backcross of this Fi to biennis the thirty chromosomes from C. biennis form fifteen pairs. The great size and vigor which distinguish it from the other species studied also indicate that it is polyploid. The evidence from chromo- some measurements indicates strongly that Crepis biennis is the only one of the twenty species discussed in this paper that could owe its origin to polyploidy. It would seem possible that, if the whole complex of one species were added to that of another by segregation following species-hybridization, zygotes formed by the union of two such gametes might be expected to give stable races differing in chromosome number from other species of the genus. There is no evidence that such a procedure has occurred in any of the species of Crepis discussed above. There is at present little evidence that whole chromosomes can be lost and the resulting organisms be expected to give rise to new species. Genet ical and cytological results on Drosophila (Bridges, 1921) indicate 310 University of California Publications in Agricultural Sciences [Vol. 2 that while 53 per cent of the expected flies lacking one of the small fourth chromosomes live, they are imperfect, weak, and often sterile. That a small portion of a chromosome may be lost or inactivated is indicated also by work on this fly (Bridges, 1919). Loss of this strain is attributed to the injurious effect of the deficiency upon viability, fertility, and productivity. While loss of chromosomes appears to be somewhat improbable as a method by which one species can come to differ from another in chromo- some number, the chromosome number of some species may be reduced as a result of permanent end-to-end union of certain chromosomes to form multiples. The differences in number noted for the Acrididae (McClung, 1917) appear to be of this type. One species, Hesperotettix viridis, shows considerable variation in chromosome union in different individuals, indicating that it may be in the process of producing new types of chromosome grouping. It is also decidedly variable morpho- logically. There is some observational evidence that species differ from one another in chromosome number due to cross-division of all chromosomes of a complex. Marchal (1920), for example, reported that in the section Medium of Campanula the size of each chromosome of pollen mother cells is less when the haploid specific number is thirty-four than when it is seventeen. It is difficult to understand how cross-division or union of chromo- somes to form multiples could cause specific differences. In fact, a case from Drosophila reported by Mrs. Morgan (1922) indicates that while end-to-end union of the X-chromosomes may affect genetic results it has no effect upon specific characters. It seems simpler to suppose that such changes in chromosome complexes are the result rather than the cause of genetical differences between individuals, such as have been noted for Hesperotettix viridis and for the different species of the Acrididae. In the genus Drosophila, it has been shown that chromosomes that look alike may carry very different genes. For example, in D. willistoni, Metz and Lancefield (1922) report that the X-chromosome is a V- shaped element similar to the second and third autosomes of D. melano- gaster. Without this genetic evidence one would have said that these two species had the same type of chromosome complex. Such evidence is a timely warning to those who would draw hasty conclusions on the basis of data like those given above for Crepis. The genetical results from Crepis are still too scanty to permit of such tests. 1925] Maim: Chromosome Number and Individuality in the Genus Crepis 311 SUMMARY AND CONCLUSIONS 1. With the exception of neglecta and possibly setosa, all the species of Crepis studied show significant increases in total length of the chromo- some complex over that of capillaris, the single species with three pairs of chromosomes. 2. Generally speaking, increased number is associated with increased total length, but there are certain exceptions. 3. In so far as studies on chromosome individuality can determine, five of the species with four pairs of chromosomes might have two pairs like the intermediate chromosome of capillaris. 4. In Crepis neglecta (N = 4) the two shortest chromosomes might have been derived by cross-division of a chromosome of the length of the intermediate chromosome of capillaris. 5. Crepis setosa (N = 4) and parvi flora (N = 4) are very similar in total length and quite unlike all of the other species. 6. Crepis dioscoridis (N = 4) and pulchra (N = 4) have a long pair of chromosomes which is not represented in capillaris or in the other four chromosome species. It is possible that it might be a multiple chromosome. That this difference in length is not due to a difference in physiological condition or to error is shown by the fact that it is maintained when the dioscoridis chromosomes are in setosa cytoplasm in an Fi between these two species. All the chromosomes of these two species can be distinguished in this Fx. 7. Aurea stands out among the species with five pairs because of its lack of an element like the longest chromosome of capillaris. The complexes of rubra, foetida, and alpina might all have been derived by duplication of certain chromosomes of capillaris. Sibirica seems to possess two chromosomes like the large element of dioscoridis and pulchra. 8. The single species with six pairs, sieberi, has chromosomes which are enough like those of capillaris in length to have been derived from it by chromosomal duplication. There appear to be but one pair of the large and the intermediate types, and four pairs like the short chromosomes. 9. Japonica with eight pairs might be derived by cross-division of all chromosomes of a species like tectorum. 10. Bulbosa (N = 9) has short chromosomes like those of japonica. 312 University of California Publications in Agricultural Sciences [Vol. 2 11. Biennis (N = 20) has chromosomes comparable in size to those of capillaris, and there is some experimental evidence which indicates that it is a polyploid from a five-pair species. 12. It is well understood that these data are simply suggestive, but it is hoped that they may be of some use in taxonomic and hybridiza- tion studies on Crepis. The evidence, based on especially favorable cytological material, shows that it is entirely unsafe to assume that even closely related species which have the same chromosome numbers are identical in chromosome individuality; or to assume polyploidy unless the sizes of the chromosomes have been compared. LITERATURE CITED Beer, R. 1912. Studios in spore development. II. On structure and division of the nuclei in the Compositac. Ann. Bot., vol. 26, pp. 705-726. Belling, J. 1922. The cytology of Datura mutants. Carnegie Institute Year Book, vol. 21, pp. 99-100. Belling, J., and Blakeslee, A. F. 1924. The distribution of chromosomes in tetraploid Daturas. Am. Nat., vol. 58, pp.. 60-70. Bridges, C. B. 1919. Vermilion-deficiency. Jour. Genera] Physiology, vol. 1, pp. 645-656. 1921. Genctical and cytological proof of non-disjunction of the fourth chromo- some of Drosophila melanogaster. Proc. Nat. Acad. Sci., vol. 7, pp. 186-192. Collins, J. L., and Mann, M. C. 1923. Interspecific hybrids in Crepis. II. A preliminary report on the results of hybridizing Crepis setosa Hall with C. capillaris (L.) Wallr. and with C. biennis L. Genetics, vol. 8, pp. 212-232. Digby, L. 1914. Critical study of the cytology of Crepis virens. Arch. f. Zellforsch., vol. 12, pp. 97-146. Frost, H. B., and Mann, M. C. 1924. Mutant forms of Matthiola resulting from non-disjunction. Am. Nat., vol. 58, pp. 569-572. JUEL, H. O. 1905. Die Tetradenteilungen bei Taraxacum und anderen Cichorieen. Kungl. Svensk. Vetensk. Akad., Handl., vol. 39, no. 4. McClung, C. E. 1917. The multiple chromosomes of Hesperotettix and Mermiria (Orthoptera). Jour. Morph., vol. 29, pp. 519-590. Marchal, E. 1920. Recherches sur les variations numeriques des chromosomes dans la serie vegetale. Memoires de l'Acadcmie royale de Belgique, ser. 2, vol. 4, pp. 1-108. 1925] Mann: Chromosome Number and Individuality in the Genus Crepis 313 Metz, C. W., and Lancefield, R. 1922. The sex-linked group of mutant characters in Drosophila willistoni. Am. Nat., vol. 56, pp. 211-241. Morgan, L. V. 1922. Non-criss-cross inheritance in Drosophila mclanogaster. Biol. Bull., vol. 42, pp. 267-274. Morgan, T. H. 1922. Croonian lecture on the mechanism of heredity. Proc. Roy. Soc, Sec. B, vol. 94, pp. 162-197. Rosenberg, O. 1909. Zur Kenntniss von den Tetradenteilungen der Compositen. Svensk. Bot. Tidskr., vol. 3, pp. 64-77. 1918. Chromosomenzahlen und Chromosomendimensionen in der Gattung Crepis. Arch. f. Bot., vol. 15, pp. 1-16. 1920. Weitere Untersuchungen liber die Chromosomenverhaltnisse in Crepis. Svensk. Bot, Tidskr., vol. 14, pp. 319-326. Tahara, M. 1910. tlber die Zahl der Chromosomen von Crepis japonica. Bot. Mag., Tokyo, vol. 24. PLATE 53 Somatic metaphases of Crepis species magnified 4000 diameters, using a B. and L. camera lucida mirror at 50, bar at 110, and a 1.8 mm. oil objective with an 18X Zeiss compensating ocular. Reduced in reproduction to 1800 diameters. F\ setosaXtectorum Fi setosaXdioscoridis setosa dioscoridis tectorum 1. 2. 3. 4. 5. 6. 7. 9. 10. 11. 12. capillaris neglecta parviflora bursifolia taraxadfolia blattorioides aspera 13. aurea 14. alpina 15. foetida 16. rubra 17. japonica 18. bulbosa 19. sieberi 20. arrvplexifolia 21. pulchra 22. grandifolia 23. sibirica 24. biennis [314] UNIV. CALIF. PUBL. AGRI. SCI. VOL. 2 [MANN] PLATE 53 r -» 41 \f. J m ^v ;.'f a: i^*" # s 10 ii A/ .£*-£ \^> ""£ >» ** ,>/>£ „#f ,«^ ,.$? 16 -& > 17 8 "9 19 *£\ "*>$ 20 V 21 22 23 24 UNIVERSITY OF CALIFORNIA PUBLICATIONS IN AGRICULTURAL SCIENCES Vol. 2, No. 11, pp. 315-341, 7 figures in text March 6, 1926 CHROMOSOME NUMBER AND INDIVIDUALITY IN THE GENUS CREPIS II. THE CHROMOSOMES AND TAXONOMIC RELATIONSHIPS BY ERNEST BROWN BABCOCK and MARGARET MANN LESLEY CONTENTS PAGE Introduction 315 Material and methods 316 Acknowledgments 316 Taxonomy and cytology of twenty-one species of Crepis 317 Literature and discussion 332 Summary and conclusions 338 Literature cited 339 INTRODUCTION For the past three years we have been accumulating data on the taxonomy and cytology of the genus Crepis. The present paper repre- sents only two phases of our general project, which also includes exten- sive genetic research on species and species hybrids, the whole under- taking being an effort to establish a natural classification of a genus which has been a source of considerable difficulty to taxonomists and which presents a wide array of chromosome numbers. In addition to number we have examined the size of the chromosomes in the species studied, in the hope that this might also prove useful as a criterion in classification. We are confining our discussion to species which we have been able to cultivate in the greenhouse or garden and to identify with certainty, a procedure which has thrown considerable light on the classification. Ideally the taxonomist should know his species as they appear under natural conditions, but obviously this is impossible for any one botanist in the case of such a large and widely distributed genus as Crepis. 316 University of California Publications in Agricultural Sciences [Vol.2 But, even though field studies of most of the species could not be made, it was yet necessary to cultivate them in order to study them eyto- logically, and hence it has been possible to supplement the examination of herbarium material by observations on cultivated plants which were grown under fairly uniform conditions. By this method it has been possible to show that certain characters (for example, nodding position of the young flower heads) which have been used by some authors to separate sections of the genus, are variable within a single species. Crepis was chosen in the first place because certain species have small chromosome numbers and because the chromosomes are compara- tively easy to study in some detail. A previous paper on chromosome size and number in the genus (Mann. 1925) contained a majority of the chromosome data herein considered, together with a suggestion as to how a cytologist would be tempted to group the species studied. In this paper we have added somewhat to the cytological data and have attempted to utilize both the cytological and the taxonomieal modes of attack. Generally speaking, this method has proved of the greatest usefulness; and. while certain irreconcilable situations still appear to exist, we have reason to hope that future developments — as we obtain more species and make further studies — may show how such situations have arisen and lead the way to a (dearer understanding of the genus. MATERIAL AND METHODS The species of Crepis upon winch this study is based are all from the Old World, and have mostly been obtained through the cooperation of European botanists. Since we desire to make our study as complete as possible, we shall greatly appreciate any assistance towards obtain- ing viable s Is in- roots of additional species. The taxonomic studies have included the examination of both dried and living specimens, and much care has been exercised in the determination of all this material. The cytological methods were described in Mann (1925). Acknowledgments The investigations herein reported were conducted in part through an allotment from the Adams Fund. It is with pleasure that we acknowledge the assistance of Dr. J. L. Collins and Mr. C. W. Haney in the growing of cultures and in providing us with certain data on species hybridization. All the drawings were made by Helen E. Rearwin, whose attention to accuracy of detail is gladly acknowledged. Our thanks are also due to the curators of herbaria and directors of 1926] Bdbeoek— Lesley : Chromosomes and Taxonomic "Relationships 317 botanic gardens in numerous institutions. Many taxonomic and other treatises on the Compositae have been consulted, which cannot be cited in this brief paper. TAXONOMY AND CYTOLOGY OF TWENTY-ONE SPECIES OF CREPTS In the present paper we do not wish to discuss the taxonomy of Crepis in detail or to propose any taxonomic revision of the genus, but merely to set forth the general features of the group and its sub- divisions in such a way as to enable the reader to appreciate some of the difficulties involved in attempting to classify the species according to a natural system. Also, it is hoped that the significance of the cyto- logical data herein presented will be clearer after a preliminary con- sideration of the outstanding morphological resemblances and differ- ences to be found within this group of plants. No thoroughgoing investigation of the entire genus has been made.. Some of the species have been studied since the time of Linnaeus or even earlier, and at least forty-four other generic names have been applied by twenty-four authors in attempting to classify various por- tions of the assemblage. The purposes of the present paper can be best served by a discussion of the treatment of the genus given by Hoffmann in Engler and Prantl's Pflanzenfamilien. This treatment, represented in condensed form below, includes all but six of the twenty- one species for which complete data as to chromosome size are avail- able and one other (C. patula) which Ave have not yet been able to secure. The six species referred to — blattarioides Vill., bursifolia L., neglecta L., parviflora Desf., montana d'Urville, and setosa Hall. f. — are all easily placed in Hoffmann's categories with the exception of neglecta, which is referred to Eucrepis in most recent floras (see p. 327). A translation of Hoffmann's description of the genus is given below "for the information of readers who are not familiar with this groups of plants. His analysis of the genus and key to the sections appear in table 1. Crepis L. — Heads small to rather large, yellow- or seldom recb flowered, borne singly or in panicles of variable form ; involucre cylin- drical or bell-shaped, often with loose or appressed outer calyx, the inner fructiferous bracts often becoming stouter and harder through- out or along the middle nerve ; receptacle naked or ciliate ; fruit 10-30 ribbed, with a short callosity on the base, reduced or beaked at the apex, the outer fruits sometimes shaped differently from the inner ones ; pappus in most species composed of soft pliable hairs, seldom somewhat brittle and brownish, in the marginal fruits sometimes lack- ing.— Herbs, very seldom half-shrubby plants. Perhaps 170 species mostly from the northern hemisphere. 318 University of California Publications in Agricultural Sciences [Vol. 2 a a- Fig. 1. Achenes of Crepis alpina — a, marginal; a', inner. X 7 circa. L'JiliiJ Bab cock-Lei It y : Chromosomes and Taxonomic "Relationships 319 Fig. 2. Marginal and inner achenes of: b, b', Crepis rubra; c, <•', C. foetidu. X 7 circa. 320 University of California Publications in Agricultural Sciences [Vol. 2 TABLE 1 Hoffmann 's Key to the Sections of Crepis with the Addition of Six Species Not Listed by Him and References to Original Drawings of Achenes A. Pappus bristles very short, unequal, the longest scarcely as long as the width of the fruit, very readily deciduous; fruit short-beaked. Sec. I. Ceramiocephalum Schultz Bip.* C. patula Poir. B. Pappus bristles longer. (a) Inner or all the fruits long-beaked. Sec. II. Barkhausia Much.* Fruits all beaked (outer sometimes shorter than inner), involucre mostly with outer calyx, seldom imbricate. Fig. 1, a, a' ; Fig. 3, d, e, e' g, g'. C. alpina L., turaxaci folia Thuill., bursifolia L., setosa Hall. f. Sec. III. Anisoderis Cass.* Outer fruits short-, inner long-beaked. Fig. 2, b, b', c. <■'. C. foetida L., rubra L. Sec. IV. Nemauchenes Cass* (in part). Marginal fruits not or scarcely beaked, enclosed within the much hardened involucral bracts; ribs prominent, the innermost enlarged wing-like so the fruits seem to be compressed; inner fruits prismatic long-beaked. Fig. 3, h, h' . C. aspera L. (b) Fruits reduced at the apex, but not beaked or only short-beaked. Sec. V. Nemauchenes Cass.* (in part). Except for the scarcely beaked inner fruits, like TV. Fig. 4, k, I'. C. Dioscoridis L. Sec. VI. Cymboseris Boiss.* Marginal fruits compressed, 3-angled, the edges winged, enclosed by the inner much hardened involucral bracts, without pappus. Fig. 4, m, m' , m". C. palaestina Boiss. (Boriim.). Sec. VII. Phaecasium Cass.* Fruits alike in shape with readily deciduous pappus which is mostly absent in the marginal fruits, inner fructiferous involucral bracts much hardened. Fig. 4, ft, ft', n". C. pulchra L. Sec. VIII. Aetheorrhiza Cass.* Distinct from others by tuberous root-stock, fruits all similar in shape. Fig. 6, «. C. bulbosa (L) Tausch. Sec. IX. Eucrepis DC. Roots not tuberous (fusiform or root-stock as though bitten off); fruits all alike; involucre with outer calyx; inner fructiferous involucral bracts mostly moderately thickened. Fig. 5, o, p, q, r, s, t. C. capillaris (L) Wallr., neglecta L., parviflora Desf., tcctorum L., biennis L., montana d'Urv. * Described as a genus. 1 * * — * > J Babcock-Lesley: Chromosomes and Taxonomic Relationships 321 Sec. X. Youngia Cass.* Distinct from preceding section in the small few-flowered (8-15) heads. Stem few-leaved; involucre in mature fertile heads little changed. Pappus readily deciduous. Fig. 6, v, v'. C. japonica (L) Benth. Sec. XI. Catonia Much.* Involucre imbricate, often black hairy; outer bracts shorter but at least half as long as inner bracts and forming no distinct outer calyx, in mature fertile heads flat and unchanged. Fig. 6, w, x,; fig. 7, y. C. sibirica L., aurea (L) Cass., blattarioides Vill. We shall first discuss Hoffmann's grouping of the twenty-one species now before us, and then suggest a more natural grouping, in order that the cytologic data to be presented may be more intelligently considered. It will be noted that the genus, as treated by Hoffmann, is divided into three subgenera but without designating them as such. The first consists of the monotypie section, Ceramiocephalum ; the second (a) contains three sections all characterized by having fruits with definite beaks; and the third (?>), comprising the remaining seven sections, contains species none of which have manifestly beaked fruits. It was long ago pointed out (Bischoff, 1851) that all degrees of development of the beak are found in group (a), while some of the species included in group (b) have fruits with very short or obscurely developed beaks. But this seems to be generally looked upon as merely part of the evidence of relationship within the whole group and as part of the argument for treating it as a single genus. Section I is set apart from all the other species, probably justifiably. but, as we have not yet been able to work with living material of this interesting species, it is unnecessary to give it further consideration at present. Subgenus (a), on the basis of fruit characters alone, would be better rearranged as follows: Sec. II. Fruits large, the inner ones 10-18 mm. long. C. alpina, foetida rubra (cf. figs. 1 and 2). Sec. III. Fruits small, all alike, the inner ones 5-8 mm. long. C. bursifolia, setosa, taraxacifolia (cf. fig. 3, d, e, g). Sec. IV. Fruits small, of two shapes, marginal ones winged. C. aspera (cf. fig. 3, h, h'). Furthermore, the above rearrangement is not inconsistent with other morphological characters of diagnostic value. This is especially interesting in connection with the cytological evidence, the species * Described as a genus. 322 University of California Publications in Agricultural Sciences [Vol. 2 3.S o *> « 01 d O , ■si pairs of very large chromosomes resembling those of Dioscoridis, puichra, and palaestina. Three other species in this section have been counted recently, but as no measurements have yet been made, they arc not included in table 3 (see p. 331). TABLE 3 Tabulation of Twenty-one Species of Crepia A.cc6rding to a Tentative New Taxonomic Grouping and with Reference i<> Ni mber and Length of Chromosomes. (The Lexuth Values Represent Averages from Tex Differext Cells.) Number of Chromosome Pairs Sec. II Anisoderis nl inn, i ioi tula rubra Sec. Ill Barkhausia bursi folia 8< tOSQ taraxacifolia Nemauchenes S,T IV. Sec, V.* Sec. VI. Sec. VII Sec. VII Sec. IX. Sec. X. aspera Dioscoridis palaestina puichra., Eucrepis capillaris parviflora tectorum montana ' 12 8 18.0 32.4 23.8 1 1 7. 17 7 is :, in ;. i i n 21 2 19.7 24.9 24.6 2.". :> I 1 8 It I 20.2 17 7 12 2 12 1 16.2 27.6 20.6 13. 1 l.-, 8 16 2 12 7 '.'. 1 17 s 1 7 :» 19 3 21 2 19.3 17.2 16.0 11.5 11 7 15. 1 23.2 17 7 12 2 1 1 I 1 I 'i I A . 2 10.8 II 1 L3 2 is :, 12 .". Ill (I 1(1 (', 9.7 Id 1 '.I 2 9.6 S li *Gatyona, Cymboseris, and Phaecasium combined. t Not measured; size range much like that of species in this group. L926] Bdbcock— Lesley : Chromosomes and Taxonomic Relationships 33] Our analysis of relationships among these twenty-one species, as based on comparative morphology, is summarized in table 2. This analysis is presented only in a tentative way, as an aid in the study of eytologicaJ evidence and a step toward the classification of the entire genus. The correspondence of the new taxonomic grouping with chromo- some number and size is shown in table .'{. Since the foregoing was written, the chromosomes have been examined in the following additional species of Crepis. The classifica- tion into sections is according to the tentative new arrangement shown in tables 2 and 3. IV. Nemauchenes C. amplexifolia (Godr.) Willk N= 4 size medium VI. Eucrepis C. hjrata Froel N= 6 size medium C. mollis (Jacq.) Asch N= 6 size medium C. pygmaea L N= 6 size medium C. chondrilloides Jacq N= 4 size large C. Blavii Asch N= 4 size large C. ciliata C. Koch N = 20 size medium VII. Youngia C.fuscipappa (Thw.) Bent ,h N= 8 size medium IX. Omalocline C. Hookeriana Ball N= 4 size medium X. Soyeria C. conyzaefolia (Gouan) Dalla Torre N= 4 size large C. tingilana Salz. ex Ball N= 5 size medium C. paludosa (L) Mnch N= 6 size large With reference to the six species classified under Eucrepis, the first group of three lyrata, mollis, and pygmaea, must be grouped with montana on the basis of morphology, and they have similar chromo- somes. The next two, chondrilloides and Blavii, represent a subdivision of Eucrepis not previously studied and are very distinct from other members of Eucrepis. Lastly ciliata is certainly in Eucrepis, and its chromosomes indicate relationship to biennis, to which species there is considerable resemblance in the rosettes of our immature plants. Evidently Eucrepis is too heterogeneous a group to be retained as a section, and in the taxonomic revision of the genus which is now in preparation it will become a subgenus containing several sections. 332 University of California Publications in Agricultural Sciences [Vol. 2 It is evident that, generally speaking, there is a definite correspond- ence between the taxonomic position of the species studied and their chromosome number and especially with chromosome size, and that the new taxonomic grouping increases this correspondence. It is almost perfect in Section II, and in Section III (cf. table 3), and the species that stand apart in the classification also differ markedly from the rest in either size or number of chromosomes (Sections V. VI, and VII). It will be noted that Section III and Section VI contain species will) similar chromosome numbers and sizes, parviflora and setosa having very similar size differences, as do also twraxacifdlia and tectorum. It would seem worth while to test these groups by means of species-hybridization. Sections VII and VIII as compared with Sections V and X exhibit the most extreme differences in chromosome size. LITERATURE A XI) DISCUSSION The numerous summaries of chromosome numbers which have appeared in recent years clearly indicate that there is some parallelism between chromosome number, size, and shape and relationship in the plant and animal kingdoms. In general, members of the same genus usually have similar chromosome numbers. In the Liliaceae, for instance, each genus has a characteristic number of chromosomes. On the other hand, in wheat, instead of exact numerical correspondence within the genus, the species fall into three groups with respect to chromosome number (Sakamura. 1918), einkorn having 7, emmer 14. and vulgare 21 pairs of chromosomes. These groups also differ from one another in susceptibility to rust, serological relations, and morphology (Sax, 1921). Thus in the genus Triticum the most similar species are most alike in chromosome number. Winge (1!)17, pp. 166- 168) cites an interesting case from the Compositae. Species were described as having 8, 9, 14, 16, 18, 24, 27, 32, 36, and 4."> pairs. When these species were classified by tribes, the numbers formed two series with 8 as the ground number for the Ileliantheae. and 9 for the Anthemideae. Marchal (1920) recently noted that the species of the genus Campanula which belong to the section .Medium have X values of 17, 34, or .31, but finds that the other section of the genus fails to show a similar numerical seriation, including such X values as 8, 10. and 13. He suggests (p. 66) that "The results of the cytological study of species of section II [Rapunculus] tend to show that this grouping is much less natural and less homogeneous than the preceding." 1920] Babcock— Lesley : Chromosomes and Taxonomic "Relationships 333 McClung (1908), on the basis of observations on many genera of Orthoptera, says, Merely as :i result of the study I have made of the germ cells I would have classified these insects into two groups, one having a complex of twenty-three chromosomes and the other of thirty-three. On the other hand, many taxo- nomists, from careful and minute examination of the external anatomy of these same species, had agreed in placing them into family groups which they call the Acrididae and Locustidae. McClung (1917) has made an especially thorough study of the genera Hcsperotettix and Mermiria, and lias had the benefit of the cooperation of experts on the classification of the Orthoptera, with similar results. Metz (1914, 1916) has shown that the Drosophilidae have rather similar chromosomes and that the species form several groups on the basis of their cytological characteristics. Metz and Lancefield (1922) state that the 13 species belonging to class A, of which D. melanog aster is an example, are scattered throughout the genus. The Drosophilidae are of especial interest from the standpoint of cytology and taxonomy, since something is known of the arrangement of genes within the chromosomes of several species, and it is therefore possible to com- pare the chromosomes from a genetical as well as a purely morpho- logical viewpoint. Sturtevant (1921) says, "44 recessive mutant genes in 41 loci of D. melanogaster and 12 recessive mutant genes of D. simulans (in 12 loci) are also recessive in melanog aster-simulams hybrids." Some of these genes are found in each of the 4 chro- mosomes indicating that "The data from D. simulans show what was suggested by the other results and by much cytological data, that the constitution of a chromosome may be essentially the same in two different species. ' ' Both of these species belong to type A cytologically (Metz and Moses, 1923) and are closely related taxonomically. The evidence from I), obscura and D. willistoni, on the other hand, shows that the chromosomes which one would naturally suppose to be identical on the basis of purely cytological criteria are not the same genetically, since Metz and Lancefield (1922) state: "In the two species having V-shaped X chromosomes, then, yellow and scute are 'located' near the middle of the chromosome map, while in melano- gaster with its short rod-like X chromosome, yellow and scute are on one end." Metz and Moses (1923) emphasize the importance of genetical evidence in any attempt to evaluate the significance of similarities or differences of a cytological type. Lists of chromosome numbers also contain what appear to be many flagrant exceptions to the view that the species of a genus will be cyto- 334 University of California Publications in Agricultural Sciences [Vol.2 logically similar. In fact, the summaries of Ishikawa (1916) and Tischler (1916, 1922) contain very few genera with either the same number throughout, or even a single ground number. Even in the Liliaceae certain species have been reported as having chromosome numbers different from that typical of the genus. Time and further work alone will tell how many of these exceptions are real and how many are due to error. At present few genera have been much studied, and even where a large number of counts have been published, the same error may appear in a whole series of observations. For instance, in both Triticum and Rosa numerous species were included in recent summaries as having 8 and 16 pairs of chromosomes. Il has been shown by Sakamura (1918) and Sax (1918, 1921) for Triticum, and by Tiickholm (1922) for Rosa, that 7 and not S is the ground number for both genera. Another very real source of error in any attempt to generalize from summaries lies in the fact that few eytologists are trained taxonomists. Our experience with Crepis indicates thai seeds which are obtained from the most reputable sources may be incorrectly labeled, and, unless the seeds are grown and the plants classified, we cannot always be positive that they even belong to that genus, much less to the species to which the sender has attributed them. While lists of chromosome numbers include such errors as are indicated above and are, therefore, not suitable as a basis for very sweeping generalization, no one can doubt that chromosome number and. in some cases, size and shape, are good specific characters. We venture the prediction that chromosome number and size will sometime lie given with taxonomic descriptions. Crepis contains species with 3. 4. 5, 6, 8, 9, and 20 pairs of chromo- somes; but 3, 6, 8, 9, and 20 are much less frequent numbers than 4 or 5, each of the former characterizing only one of the twenty-one species represented in table 3. A similar condition has been described for a closely related genus, Lactuca (Ishikawa, 1921), most of the species having 5, 8, 9, or 12 as the haploid number, while single species have 7, 16, or 24. It is especially interesting that Ishikawa finds that his grouping of species according to chromosome number and size cor- responds very strikingly with the taxonomic classification of Nakai (1920). In Lactuca, as in Crepis, great differences in chromosome size exist, and because of this and the numerical differences, Ishikawa is inclined to think that Lactuca is really an assemblage of genera. It is particularly interesting that two varieties of L. dentata have 12 pairs, while one has 7 pairs of chromosomes. 1926] Bdbcock— Lesley : Chromosomes