SPONTANEOUS CHROMOSOME ALTERATIONS IN CREPIS TECTORUM L. BY M. NAVASHIN Ukiversity of California Publications in Agricultural Sciences Yohniio 6, No. 7, pp. 201-206, 1 figure in text Issued April 14, 1931 University of California Press Berkeley, California Cambridge University Press London, England SPONTANEOUS CHROMOSOME ALTERATIONS IN CREPIS TECTORUM L. BY M. NAVASHIN' During the summer of 1930 many progenies derived from Crepis tectonim plants possessing aberrant chromosome sets were grown for the purpose of studying the transmission of chromosomal abnormali- ties. It was found in the great majority of cases, in full accord with expectation, that chromosomal abnormalities were transmitted as such to a certain part of the offspring. Thus, for instance, simple trisomies threw identical trisomies in proportions which varied according to the kind of the extra chromosome and according to the individual. Indi- viduals possessing a typical chromosome structure (for instance, trans- located chromosomes) transmitted their abnormal chromosomes to a certain percentage of their offspring. In three cases, however, the behavior of the tested individuals was entirely different. They pro- duced, indeed, among their progeny not only the expected chromo- somal abnormalities, i.e., aberrations identical with those which were characteristic of them, but there appeared also in the immediate off'- spring some entirely new chromosomal alterations, and, furthermore, these occurred in unusually high numbers. In the following pages will be given a short discussion of these peculiar eases observed among progenies 30.503, 30.511, and 30.515. Progeny 30.503 was grown from open pollinated .seed collected from a simple trisomic. It consisted of seven plants from which two were selected for cytological investigation because of their somewhat abnormal appearance. One of the two plants died early in develop- ment; the other wa.s subjected to cytological investigation. From the seventeen root-tips investigated thirteen proved to be entirely normal. Four of them uniformly contained an abnormal chromosome complement shown in figure 1, d. From an inspection of this figiire one can .see that the alteration involves two chromosomes, namely, the two A-chromosomes (see, for a normal complement of * Now of the Division of Expoiiment.Tl Evolution, Tiniiiiazev Biological Institute, Moscow, U.S.S.R. 202 University of California Publications in Agricultural Sciences [Vol. 6 Crepis ieciorum., fig. 1, /( ) which at first sight seem to b« entirely lacking. Closer examination and comparison with the normal comple- ment make it possible to give to this phenomenon an adequate inter- pretation. It is easj' to see that two conspicuous structures have replaced the two A-chromosomes in this figure and that the large V-shaped chromosome and the other very short one are simply pro- ducts of rearrangement of the material which normally composes the two A-chromosomes. Obviously, the proximal fragment of one of the A-chromosomes became detached as a diminished autonomous chromo- some; the rest of the same A-chromosome (its distal fragment) has fused with the proximal end of the other, otherwise unaltered A-chromosome, making its smaller arm correspondingly longer and thus producing a large V-shaped structure. The upper parts of the plant in question not having been investi- gated, nothing can be said about their chromosomal constitution. It appears probable, however, that certain shoots possessed some chromo- somal abnormalities since they displayed a very low fertilitj'. The latter circumstance would be expected in case the above described chromosomal alteration was present also in spore mother cells. Progeny 80.511 was derived from open pollinated seed produced by a simple trisomic. From the total of forty plants, five appeared to be more or less abnormal and the remaining thirty-five were dis- carded. Cytological investigation of the root-tips of these morpho- logically aberrant individuals showed that two of them were simple trisomies of the triple-B type (like the parental plant), two were apparently normal, and one was a chromosomal chimera. Among many root-tips there was only one cytologically abnormal ; it was uniformly altered in all its cells in a most conspicuous way. The alteration in this case involved the two D-chromosomes (fig. 1, a, h. and c) . As may be easily deduced, the proximal fragment of one of the D-chromosomes became an autonomous small satellited chromo- some, the distal portion of the same chromosome being permanently attached to the satellite of the second (otherwise unaltered) D-chromo- some. The resulting alteration of the chromosome complement was thus expressed by the presence of one minute satellited chromosoiiie and another very large one distinguislied by its unusually large cylindrical satellite. This particular plant did not show any abnormalities as to its fertilit.y, etc., so that it seemed rather probable that this alleraticui was localized in a small part of the root system. 1931] Navashin: Spontaneous Chromosome Alterations in Crepis Tectorum L. 203 Progeny 30.515 came from open pollinated seed yielded by a chromosomally aberrant plant which possessed a very small spherical fragment in addition to its otherwise apparently normal chromosome complement. Among the total of twenty-four plants, tive were selected for cytological investigation since they were distinguished by various morphological peculiarities. Only one of them (plant 3) was probably normal in its chromosome complement, but owing to the lack of good roots, this could not be established with full certainty. The remaining four plants were all chromosomally abnormal, and moreover, each of them in a different wav. Fig. 1. Somatic chroinosomes from tlie root-tips of altered individuals of Crepis tectomm. a, h, o, three nietaphases from the single altered root of the plant SO-.m^. One of the two D-chroniosomes was fragmented; its proximal fragment ("on- stitutes a small satellited ehromosonie, tiie distal portion being permanently fused with the satellite of the other, otherwise unaltered D-chromosome. d, plant 30.503-2. One of the two A-ehromosonies fragmented; its proximal fragment functions as a small autonomous chromosome; the distal portion is attached to the proximal end of the other A-chromosome thus fonning a large V-shaped structure. e, f, g, different plants from progeny 30. .31.5 showing various chromosoina'. alterations. li, a normal chromosomal complement from the chinicral root belonging to plant 30.515-2 (cf. fig. 1, g). i, diagram showing the distribution of altered and normal mitoses in the root of plant 30.515-2 (cf. fig. 1, g). Crosses indicate altered cells, disclets, norma! cells. See also the text. Magnification 1700 diameters, except diagram i which is drawn to a much smaller scale. 204 University of California Puhlications in Agricultural Sciences [Vol. (5 Plant 1 was a simple triijlo-A trisomic, all its roots uniformly possessing an extra A-chromosome. In plant 2 one root out of fourteen was partly altered in its chromosomes. There appeared a sector forming about 75 per cent of the whole root in which the proximal fragment of one of the two D-chromosomes became autonomous (as in the case of progeny 30.503), the distal portion of the same chromosome being fused with the proximal end of one of the B-ehromosomes, thus forming again a large V-shaped structure (fig. 1, g and i) . Plant 4 (fig. \, e) was uniformly altered in all its roots. In this case there was present one C-chromosome extra, and, in addition, a large V-shaped structure apparently derived from fusion, as in the instances described above. Finally, plant 5 displayed the same peculiarity as the parental individual, namely, one of its roots contained in all its cells a very small spherical fragment (fig. 1,/). . None of the above chromosomal abnormalities seemed to influence the mitotic proces>s, nor did they seem to affect in the slightest degree the cell sizes or the vitality of the cells, etc. The latter was especially clear in the case of plant 30.515-2 where the sector of the root con- taining the altered chromosomes did not differ in regard to its cell divisions, cell size, shape, etc., from the chromosomally normal part of the root (cf. fig. 1, i) . It should be concluded, therefore, that even such profound rearrangements of the chromosome material as those described above have no physiological effect whatever, at least so far as roots are concerned. One can hardly doubt, moreover, that they also produce no effect upon any other somatic tissue, at least in the earlier stages of its development. The observations reported here not oidy add some new cases of chromosomal alterations to those previously known to occur in Crcpis spontaneously (M. Navashin, 1926) or after the x-ray treatment (M. Navashin, 1931), but they also throw some light upon the conditions under which such chromosomal alterations are most likely to occur. First, it may easily be seen that in the majority of cases they originate in somatic mitoses and sometimes very late in ontogeny because the majority of individuals developing them are chimeras. Secondly, it seems probable that there exists some peculiar condition in certain individuals, which makes their chromosome structure and the chromo- somal distribution labile and subject to frequent alterations in various ways. It may be suggested that just these three individuals which 1931] Navashin: Spontaneous Chromosome Alterations in Crepis Tectorum L. 20.3 o-ave rise to the progenies reported above possessed some inherent and perhaps heritable instability of chromosome behavior, which could account for the fact that, of their offspring, an unusually high pro- portion of individuals was altered in several different ways. It is necessary to point out in this connection that the frequency of spon- taneous chromosomal alterations normally does not reach even 0.1 per cent, while in the above cases the corresponding frequency was almost one hundred times as high. While chromosomal alterations representing only rearrangements of the chromosome material (dislocations) could hardly produce any immediate visible effect, some of them would inevitably lead to changes in chromosome affinity. Thus, for example, if plant 30.515-2 (fig. 1, g) could transmit its chromosomal peculiarity in homozygous form, indi- viduals could arise which would possess an entirely new mode of chromosome conjugation in meiosis. From what is known now of homozygous translocation in Drosophila melanogaster (Dobzhansky, 1930), such organisms can really exist and, moreover, can be fertile. If, now, such an individual should be crossed with the orig'inal normal form, the hybrid would inevitably suffer from meiotic disturbances involving the translocated chromosomes, in the case referred to above, chromosomes B and D. For the normal B-chromosome will be attracted by one part of the fusion chromosome, the D-chromosome will tend to conjugate with the other part of the fusion chromosome, and the small free fragment of the D-chromosome will be attracted by the cor- responding part of the D-chromosome. In such a hybrid normal reduction division would be impossible and the homozygous form possessing- the dislocated chromosomes would become more or less genetically isolated from its progenitor. Thus it seems probable that certain chromosome alterations of the type described above may play an important role in the initial steps of species formation. It should be noted that alterations originating in undifferentiated sporophytic tissue would have a decided advantage over those arising during gametogenesis. For, in case the altered sporophytic tissue should produce reproductive organs, altered functional gametes might arise at once in great numbers and thus the new chromosome organiza- tion would be transmitted to the offspring more certainly than in the case of occasional formation of single aberrant germ cells during sporogenesis. The above facts may be advantageously applied also to the explana- tion of some characteristic chromosome relations existing among species Thus, for instance, the origin of the large V-shaped chromo- ol .vp,cal of many species, especially of the p,Wc.™-gro„p m C,"l Babcoek and NavasMn, 1930) tnay he attnhuted w,th a fa^^ d le of probability to fusion of the sort descnhed above. The c^rrenee'of very small satellited f'^^-^^^X^rZ Crevi^ parvifiora. loc. cit.) may be explamed as a result of 1™?™™" « rice the D-chromosomcs are especially apt to detach free prox,- malfrlTments. Finally, chromosomes with unnsurily large satelhtes. 7\1Z,U ..o» with its unique satellited chromosome, co-dda.s Lough Lnslocation of a considerable chromafn port.on to the t^ these conside;ations are concerned with translocattons or mere „,« fir»t designated dislocations (M. Navashm, 1926). "fdXn I gai" ^r ,o.s of chromosome material UUes place, one wold deal with the change of genie balance, and, «-*^ / : ■ h more or less far-reaching changes in the ^'^^^^^'^^^^ , , • • uc PPlk these new genetic conditions, it is odmous ^^:::^::Z:^^o. ..lu uUlmately .smt m .a. ana that anj heterozj. succeeding generations owing to segrega- lossof chroma,tinma,terialm ucceedi gg _ ^^^^ ^^^^ tion of chromosomes. And, i£ not mcompatiD result in variations of evolutionary significance. Fox *^^« ^^ilv ;, A^..\.i that the evolution of Crep^s species was primarily hardly any doubt that the evoiut contained in the based upon changes m the quantity of the materia individual chromosomes (cf. BabeocU ^^^^^ ^^^^^^.^^.^a Further investigation is now m progress. Attention wiu u ^ -flTto the study of the transmission of the presumed hereditarv chiefly to the stuaj ui i nroblem of obtaining lability of chromosome behavior and to the problem homozygous translocations. LITERATURE CITED TSarcock E B., and Navashin, M. ,. c i on 1930 The Genus Crepi... Bibliographia Genetiea, 6:1-90. DOBZHANSKY, T. , fourth chromosomes of 1930. Translocations involving the third '""j ^ Drosophila melanoffaster. Genetics, 15.347 39J. NAVASHIN, M. (NAWASCHiN, M.) -...^^en in Bezug auf die ArtbQdung. 1926. Variabilitat des Zellkerns bei Crepes ^^" . Zeitschr. Zellforsch. mikr. Anat., *(;);"\ "Ij" ^^,^^„. ^at.. 65: 1931. On chromosome alterations induced b> xiajs. A (in press).