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Bt - g-r dag Peet D we OF ot oe Conner Begs hogy Bae fect pe We i Oe te a ~ stats Re Tartlg retetaPar stele state Rig therbetie ss Meets ae pi ta ded eee Se eee ee eee SR SARA w Ne pa ae Se ai , ty tail! ol) ne rer ‘ie a ee oe “% a \ \ By — UA - New (a ven) POSTILLA PEABODY MUSEUM YALE UNIVERSITY NUMBER 130. 18 MAR. 1969 KARY OLOGICAL HETEROGAMETY OF DEEP-SEA FISHES T. R. CHEN POSTILLA Published by the Peabody Museum of Natural History, Yale University Postilla includes results of original research on systematic, evolution- ary, morphological, and ecological biology, including paleontology. Syntheses and other theoretical papers based on research are also welcomed. Postilla is intended primarily for papers by the staff of the Peabody Museum or on research using material in this Museum. Editors: Jeanne E. Remington and Nancy A. Ahlstrom Postilla is published at frequent but irregular intervals. Manuscripts, orders for publications, and all correspondence concerning publications should be directed to: Publications Office Peabody Museum of Natural History New Haven, Conn., 06520, U.S.A. Lists of the publications of the Museum are available from the above office. These include Postilla, Bulletin, Discovery, special publications, and available back numbers of the discontinued journal, Bulletin of the Bingham Oceanographic Collection. All except Discovery are available in exchange for relevant publications of other scientific institutions anywhere in the world. KARYOLOGICAL HETEROGAMETY OF DEEP-SEA FISHES ek sCEiEN Department of Biology Yale University ABSTRACT Among 25 deep-sea fish species karyologically investigated, di- gamety is confirmed in 12. Observations were based on the con- sistent appearance of heteromorphic chromosome pairs, of asym- metrical and atypically behaving “sex” bivalents, and of two different chromosome counts from metaphase II. The occurrence of such digamety in other lower vertebrates is discussed. It occurs more frequently among teleostean fishes and other lower verte- brates than previously expected. POSTILLA 130: 29 p. 18 MARCH 1969. 2 POSTILLA INTRODUCTION Probable cytological digamety has been repoorted in several fishes before 1945 (Geiser, 1924; Foley, 1926; Vaupel, 1929; Ralston, 1933, 1934; Bennington, 1936; and Barigozzi, 1937). But its occurrence was questioned by Friedman and Gordon (1934), Makino (1934a, b), and Wickbom (1941, 1943), who doubted that atypical bivalency indicated heterogamety and denied any evidence of cytological sex differentiation. In the gwyniad Core- gonus lavaretus, Svardson (1945) discovered that most karyo- types of blastomeres from about half of the embryos examined included a supernumerary chromosome. But this chromosome was absent in testicular preparations from adult males. Consequently, he claimed that this fish was female heterogametic and that the supernumerary probably was the female-determining chromosome. In two specialized perciform fishes, Mogrunda obscura and Cottus pollux, Nogusa (1955, 1957) described possible XY male hetero- gamety from observations of heteromorphic and heteropycnotic bivalents. Lieder (1963) observed a non-paired chromosome “fragment” in the percid Acerina cernua and a satellited chromo- some in the percid Perca flaviatilis and the freshwater eel Anguilla anguilla, and suggested that the “fragments” or “‘satelliteds’” might be male-determining Y-chromosomes. He cautioned, however, that the partner of the satellited chromosome was probably not an X-chromosome but rather an autosomal homologue, indicating a “YO” sex type. Chen and Ebeling (1966) reported heteromorphic mitotic X and Y chromosomes and their presumed bivalent show- ing end-to-end chromatic association from all specimens of the deep-sea fish Bathylagus wesethi examined. Its heterogamety was further substantiated by the occurrence of two X’s in tetraploid cells, probably from testicular supporting tissue, and by the occur- rence of two dissimilar counts from secondary spermatocytes, one including, the other excluding this remarkably large chromo- some. Moreover, by examining different somatic tissues (gill epithelium, spleen and kidney) as well as gonads, Chen and Ebeling (1968) reported cytological heterogamety in the mosquito- fish Gambusia affinis, in which the female karyotype is charac- terized by a large metacentric, which, however, is absent in the male. We concluded that the mosquitofish is female-heterogametic of the WZ-ZZ sex type. HETEROGAMETY OF DEEP-SEA FISHES 3 The cytology of deep-sea fishes, which are adapted to a remote, cold, dark, hyperbaric, relatively impoverished environment, has been little studied. Adaptation to such a stress condition may in- clude various atypical cytological expressions (cf. Stebbins, 1966). The present paper reports the probable occurrence of cytolog- ically expressed digamety among 25 selected deep-sea fishes. MATERIALS AND METHODS Specimens were captured off the coast of southern California as far as Guadalupe Island, Mexico. Most tissues were first placed in 0.9 percent sodium citrate for about 20 minutes, then fixed in 1:3 acetic alcohol, and stored under refrigeration. A few live specimens of Lampanyctus ritteri, Triphoturus mexicanus, and Bathylagus ochotensis were injected intra-peritoneally with 0.05 percent colchicine. They were maintained for about two hours at 5°C, then killed and fixed. These colchicinized specimens provided many good metaphase plates. In total, about one thousand prep- arations from 114 specimens were examined or about 40 prepara- tions from 2-10 specimens per species. Tissues for the squash preparations were mainly testicular; the only female tissues ob- served were ovarian metaphase plates from two halfgrown females of Lampanyctus ritteri. Occasionally kidney tissue provided good metaphase plates. Tissues were usually stained with aceto-orcein. Photomicrographs of the preparations were taken under both bright and phase-contrast optics. OBSERVATIONS The 25 deep-sea fish species are grouped according to their ordinal affinities: (1) the generalized salmoniform family Bathy- lagidae, (2) the related family of hatchetfishes, Sternoptychidae, (3) the evolutionarily intermediate myctophiform families of lanternfishes, Neoscopelidae and Myctophidae, and (4) the more specialized pre-percoid beryciform families Melamphaidae and Anoplogasteridae. A). BATHYLAGIDAE. Bathylagids are typically mesopelagic (mid- depth) fishes, which live between 100 and about 1000 meters in the open ocean. This family includes but three or four deep-sea mesopelagic genera (Cohen, 1964). Cosmopolitan Bathylagus contains five Californian species, four of which were 4 POSTILLA investigated (Figs. 1-10). Their diploid numbers range from 36 in Bathylagus wesethi to 64 in B. stilbius. Their male karyotype is unique in that its largest element, the presumed X chromosome, apparently comprises a considerable percentage of the total nuclear chromatin and lacks any homologue approaching it in size or morphology. Also, in all species but wesethi, it includes a series of very small chromosomes. The presumed X is the largest metacentric chromosome in B. wesethi, ochotensis, and milleri but it is submetacentric in stilbius (Figs 1, 4-6). It is almost 1.5 times as long as the next largest chromosome in all but ochotensis. The presumed Y is © acrocentric and is the smallest in wesethi but the next largest chromosome in the complements of stilbius and ochotensis. In milleri, which is the deepest-living species, it defies identification amongst the relatively large series of small dot-like chromosomes. In wesethi the sex bivalent, whose X and Y are associated end- to-end, appears satellited during metaphase I (cf. Chen and Ebeling, 1966, and Fig. 2). In metaphase II, two morphotypes, one with the “X”’ and the other without it, are readily identifiable (cf. Chen and Ebeling, 1966, and Fig. 3). Therefore, cytological male heterogamety is best demonstrated in this species. Although in the other species the karyotypes of secondary spermatocytes are unclear, their mitotic karyotypes and their presumed sex bivalent resemble those of wesethi (Figs. 7-10). B). STERNOPTYCHIDAE. Hatchetfishes are small and common vertical migrators of the mesopelagic zone and like most such migrators have light organs, which are ventrally oriented. Four Californian species were studied. Their karyotypes are charac- terized by the presence of several chromosome pairs that are noticeably larger than the rest. Also, satellited pairs are relatively numerous, metacentric and submetacentric chromosomes dom- inate the complement, and heterochromatic bodies (“chromo- centers’) are distinctly expressed. Some of the larger chromosomes have many distinct heterochromatic bands throughout their length. In three species of Argyropelecus, sex chromosomes are not de- tectable. However, the diploid number of Sternoptyx diaphana is always 35 (Figs. 11, 12). The largest among five acrocentrics in the male complement is apparently unpaired. This may be the X of an XX-XO sex type and is the fifth largest pair in the HETEROGAMETY OF DEEP-SEA FISHES 5 complement. Among leptotene cells, an elongate deeply stained body, which morphologically resembles the unpaired “sex chromo- some” from somatic cells, is presumably a heterochromatic sex element (Fig. 13) but is not observable, however, during later stages of meiosis. Occasionally, one to four other ““chromocenters”’ occur simultaneously, but invariably they disappear earlier than the morphologically persistent “sex element.” In metaphase I the presumed univalent X, which is morphologically indistinguishable from the mitotic X, is clearly observable (Fig. 14). In metaphase II two morphotypes are detectable (Fig. 15): one with 18 ele- ments and the other with 17 elements and presumably lacking the X chromosome. C). NEOSCOPELIDAE and MYCTOPHIDAE. Comprising more than 30 genera (Fraser-Brunner, 1949; Bolin, 1959, 1966), lantern- fishes are the most speciose and among the most abundant of all deep-sea fishes. Species are often distinguished by differences in the patterns of light organs on the flanks and belly. Of about 25 species that occur off California, 11 were investigated cytologically (Figs. 16-29). Most karyotypes are 2n=—48, with acrocentric chromosomes predominating. The chromosomes are subequal in length, excepting the presumed X and one or two pairs. The times of occurrence and disappearance of heterochromatic bodies during prophase I distinguish taxonomic groups within the Myctophidae. In Scopelengys, the only Californian representative of the rela- tively primitive Neoscopelidae, an atypically behaving bivalent, whose univalents are associated end-to-end, always occurs at the periphery of the metaphase plate; it is probably composed of sub- equal sex chromosomes (Fig. 17). In Symbolophorus californiensis of the family Myctophidae, a “‘sex bivalent” occurs at the periphery of the metaphase I plate. It is formed of two submetacentrics asso- ciated end-to-end and lags behind the others during anaphase I when its four arms are clearly detectable (Figs. 19-21). In the mitotic complement the first and fourth largest chromosomes are submetacentric and probably constitute the sex chromosome pair. In the above two species, a heterochromatic “sex element” (cf. Sternoptyx diaphana) is clearly detectable in leptotene cells (Figs. 16 and 18). In Lampanyctus ritteri the diploid number is 47 in males but 48 in females (Figs. 22, 23). The largest chromo- 6 POSTILLA some in the male complement is submetacentric but two such elements are observable in females. Probably, therefore, this species is male heterogametic of the XX-XO sex type. The X-chromosome usually but not always forms a characteristic Y- shaped trivalent, probably with a particular autosomal bivalent (Fig. 24). In anaphase I (Fig. 25) and metaphase II (Fig. 26) counts clearly are n=24 with the X and n=23 without the X. This asymmetry is also observable in the specialized deep-living species Parvilux ingens, whose diploid number is 49 in males (Fig. 27). During metaphase I the presumed X, which behaves: differently and forms either a V or ring, may appear chromatically associated at both poles (Fig. 28). In metaphase II, counts were n=25 with the X and 24 without the X (Fig. 29). D). MELAMPHAIDAE and ANOPLOGASTERIDAE. These families generally live at greater depths than the previous species. Like many bathypelagic fishes, they lack light organs and probably do not undergo extensive diurnal vertical migrations (Ebeling, 1962). Five melamphaids and monotypic Anoplogaster cornuta were studied (Figs. 30-35). The complement is made up either of subequal chromosomes in most species or of many very small acrocentric chromosomes in others, e.g., Poromitra crassiceps. Generally, acrocentrics dominate the complements, which are unusually variable in number among the genera. The hetero- chromatic bodies (“‘chromocenters”) are clearly observable during early prophase I. A heteromorphic “sex bivalent” is detectable in Melamphaes parvus and Scopeloberyx robustus (Figs. 34-35). In Scopelogadus mizolepis bispinosus a large, “lampbrush-like” bivalent, which differs from all others in having broad sections between its nar- rowed ends, is observable during zygotene and pachytene (Fig. 32). This “sex bivalent” is observable until metaphase I, shows no chiasmata, and may be associated end-to-end (Fig. 33). In the mitotic complement a pair of relatively long chromosomes obviously differ in length and probably constitute the heteromor- phic sex pair (Fig. 30). DISCUSSION Cytologically expressed digamety had previously been reasonably verified in only seven of 260 teleost fishes hitherto investigated HETEROGAMETY OF DEEP-SEA FISHES v | (cf. Chen, 1967); however, the present results indicate that 12 of 25 deep-sea species have heteromorphic chromosome pairs, presum- ably of the XX-XY or XX-XO sex type. This disparity may be due primarily to technological difficulties in properly preparing slides for detailed study and to the generally small chromosome size of shallow-water fishes (Chen, 1967; Chen and Ebeling, 1968). Measuring five to six microns, however, most deep-sea fish chromosomes in mid-metaphase are two to three times as long as those of the shallow-water fishes which I and others have studied. Also the use of aceto-orcein has facilitated the present study because of its deep-staining affinity for chromatin. This allows examination of detailed chromosomal structures, which often are obscure in shallow-water fishes. Giemsa stains shallow- water fish chromatin very well, but is less effective than orcein for staining deep-sea fish chromatin. Although in most deep-sea fishes, observations of sex chromo- somes were made on testicular preparations only, abundant evi- dence substantiates the common occurrence of heteromorphic sex pairs: (1) these pairs are consistently observable among the tis- sues of different individuals, (2) males of some species have odd- numbered diploid counts, which were based on examination of at least three individuals from each species, (3) asymmetrical bivalents are always observable in metaphase I, and (4) the expected different haploid counts occur in MII in those species of the presumed XX-XO sex type. During metaphase I the atypi- cally behaving “sex element” of Scopelengys tristis and Parvilux ingens always occurs outside the concentration of other bivalents in the metaphase plate. In Scopelogadus m. bispinosus, the only morphologically distinct chromosome during prophase I and meta- phase I is probably the sex bivalent. In leptotene, a distinctly stained body, which is presumably of heterochromatic sex element and is easily distinguishable from “‘chromocenters,” is observable in Bathylagus milleri (Fig. 10), Sternoptyx diaphana, Scopelengys tristis, Symbolophorus californiensis, and Scopelogadus m. bis- pinosus (Fig. 31). The characteristic X of bathylagids is always distinctly longer than other chromosomes in the complement. This is substantiated in Bathylagus wesethi, whose two different types of metaphase I cells, one with and the other without this longest chromosome occur in equal frequency (Chen and Ebeling, 1966; and Chen, 8 POSTILLA 1967). This strongly suggests that metaphase II cells without this element are not eliminated as zygotic lethals, i.e., that the chromo- somal heteromorphy is an incident of isochromosomal fusion (Chen and Ebeling, 1966). Such a high frequency of deleterious cells would appear disadvantageous to this species and others in the family which have similar karyotypes. But, in fact, Bathylagus stilbius, whose karyotype is also characterized by the presence of a single distinctly hypertrophied chromosome in the complement, is one of the most abundant (i.e., most successful) mesopelagic deep-sea fishes in the eastern North Pacific Ocean. Therefore,. it is most reasonable to assume that the large chromosome is the female-determining chromosome. The Y_ is_ interspecifically variable. It is the largest acrocentric chromosome in Bathylagus ochotensis but the smallest in B. wesethi. Several investigators have suggested that intra-individual karyo- typic polymorphism may commonly occur in shallow-water fishes (Ohno, Stenius, Faisst, and Zenzes, 1965; Ohno and Atkin, 1966; Becak, Becak, and Ohno, 1966).However, the intra- and inter- individual consistency of heteromorphic pairs observed in the present study seems unlikely to be the result of chromosomal interchanges. For example, not one bathylagid cell among about thirty specimens examined lacked the characteristic X chromo- some. In the three hatchetfishes and lanternfishes of the presumed XX-XO sex type, the odd diploid counts in males were always based on more than 30 well spread metaphase plates. Also, the regular presence or absence of the large unpaired chromosome in secondary spermatocytes further substantiates their heterogamety. Consequently, the occurrence of heteromorphic chromosome pairs may be much more widespread among teleosts than pre- viously suspected. Also, the ‘“Superorder” Teleostei is an evolu- tionarily diverse group, which has undergone extreme adaptive radiation. It is hardly conceivable that sex chromosomes remain primitively undifferentiated among its generalized and specialized species alike. Evidence of cytological digamety is accumulating in other lower vertebrates. For example, Nogusa (1957b) reported male heterogamey in the protochordate lancelet Branchiostoma belcheri, whose X is the largest and Y the smallest in the com- plement; Yosida (1957) observed it in the treefrog Hyla arborea japonica; and Gorman and Atkins (1966) and Gorman and Holzinger (1967) reported digamety in several species of the HETEROGAMETY OF DEEP-SEA FISHES 9 lizard Anolis, whose multiple sex-chromosome mechanism consists of X,;X,X2X_ in females and X,XeY in males. ACKNOWLEDGMENT I am grateful to: Dr. A. W. Ebeling (University of California at Santa Barbara), who advised me on collection methods, species identifications, and taxonomic relationships, for his most helpful assistance throughout the present work and also for reviewing the manuscript; Drs. F. H. Ruddle (Yale University), who also provided facilities in the later period of preparation of the manu- script, for reviewing the manuscript, C. L. Remington (Yale University) and S. Ohno (City of Hope Medical Center) for their stimulating discussions and valuable criticisms; and Mrs. A. W. Ebeling for her encouragement. My sincere gratitude goes to all personnel and organizations allowing my participation in cruises in the waters off southern California and Mexico from the General Motors R/V Swan and the University of Southern California R/V VELERO. This study was supported mainly through National Science Foundation grants in Systematic Biology, GB 1654 and 4277. The ship time for the collection of specimens was financed by NSF grants G-10691 to the University of Southern California, and GB 2867 and 2698 to the University of California at Santa Barbara. Further support was obtained from USPHS grant GM- 9966-06. LITERATURE CITED Barigozzi, C. 1937. La gametogenesi e la sessualita di Cyprinus carpio var. specularis. Atti Soc. ital. Milano 76: 88-104. Becak, W., M. L. Becak, and S. Ohno. 1966. Intra-individual chromosomal polymorphism in green sunfish (Lepomis cyanellus) as evidence of somatic segregation. Cytogenetics 5: 313-320. Bennington, N. L. 1936. Germ cell origin and spermatogenesis in the Siamese fighting fish, Betta splendens. J. Morph. 60: 103-125. Bolin, R. L. 1959. Iniomi. Myctophidae from the “Michael Sars” North Atlantic deep-sea expedition 1910, p. 1-45. /n Rept. Sci. Results “Michael Sars” N. Atlantic Deep-sea Expedition 1910, 4 (pt. 2, NOw/)e ——— 1966. Interim account of family Myctophidae, p. 190-191. Interim account of family Neoscopelidae, p. 192-193. Jn Fishes of the western north Atlantic. Mem. Sears Foundation Mar. Res. | (5). Chen, T, R. 1967. Comparative karyology of selected deep-sea and shallow- water teleost fishes. Ph.D. Dissertation, Yale University. 10 POSTILLA and A. W. Ebeling. 1966. Probabie male heterogamety in the deep-sea fish Bathylagus wesethi (Teleostei: Bathylagidae). Chromo- soma (Berlin) 18: 88-96. 1968. Karyological evidence of female heterogamety in the mosquitofish Gambusia affinis (Baird and Girard). Copeia, 1968 (Dies 70-75: Cohen, D. M. 1964. Suborder Argentinoidea, p.1-70. Jn Mem. Sears Founda- tion Mar. Res. | (4). Ebeling, A. W. 1962. Melamphaidae I. Systematics and zoogeography of the species in the bathypelagic fish genus Melamphaes Ginther. Dana Rept. 58: 58 p. | Foley, J. O. 1926. The spermatogenesis of Umbra limi with special reference to the behavior of the spermatogonial chromosomes and the first maturation division. Biol. Bull. 50: 117-147. Fraser-Brunner, A. 1949. A classification of the fishes of the family Myctophidae. Proc. Zool. Soc. Lond., 118 (4): 1019-1106. Friedman, B. and M. Gordon. 1934. Chromosome numbers in xiphophorin fishes. Amer. Nat. 68 (718): 446-455. Geiser, S. W. 1924. Sex ratios and spermatogenesis in the top-minnow, Gambusia holbrooki. Biol. Bull. 47: 175-212. Gorman, G. C. and L. Atkins. 1966. Chromosomal heteromorphism in some male lizards of the genus Anolis. Amer. Nat. 100: 579-583. and T. Holzinger. 1967. New karyotypic data on 15 genera in the family /nguanidae, with a discussion of taxonomic and cyto- logical implications. Cytogenetics 6: 286-299. Lieder, U. 1963. Uber vermutiliche Gonosomen bei Perca, Acerina und Anguiila (Vertebrata, Pisces). Biol. Zbl. 82 (3): 296-302. Makino, S. 1934a. The chromosomes of the sticklebacks, Pungitius tymensis (Nikol’sky) and P. pungitius (Linnaeus). Cytologia 5: 155-168. 1934b. Notes on the chromosomes of some fresh-water tele- osts. Jap. J. Genet. 9 (2): 100-103. Nogusa, S. 1955. Chromosome studies in Pisces. IV. The chromosomes of Mogrunda obscura (Gobiidae), with evidence of male heterogamety. Cytologia 22: 11-18. 1957a. Chromosome studies in Pisces. VI. The X-Y chromo- somes found in Cottus pollux Giinther (Cottidae). J. Fac. Sci. Hokkaido Univ., Ser. VI— Zool., 13: 289-292. 1957b. The chromosomes of the Japanese lancelet, Bran- chiostoma belcheri (Gray), with special reference to the sex-chromo- somes. Annot. Zool. Jap. 28: 249-255. Ohno, S., and N. B. Atkin. 1966. Comparative DNA values and chromo- some complements of eight species of fishes. Chromosoma (Berlin 18: 455-466. C. Stenius, E. Faisst, and M. T. Zenzes. 1965. Post-zygotic chromosomal rearrangements in rainbow trout (Salmo irideus Gib- bons). Cytogenetics 4: 117-129. Ralston, E. M. 1933. The chromosomes of Xiphophorus, Platypoecilus and their hybrids during the maturation stages. Science 78: 124-125. 1934. A study of the chromosomes of Xiphophorus, Platypoe- cilus and Xiphophorus X Platypoecilus hybrids during spermatogenesis. J. Morph. 56: 423-433. HETEROGAMETY OF DEEP-SEA FISHES 11 Stebbins, G. L., Jr. 1966. Chromosomal variation and evolution. Science 152 (3728): 1463-1469. Svardson, G. 1945. Chromosome studies on Salmonidae. Rept. Swedish State Inst. Fresh-water Fish. Res. 23: 151 p. Vaupel, J. 1929. The spermatogenesis of Lebistes reticulatus. J. Morph. 47 :555-587. Wickbom, T. 1941. The sex chromosomes of Cyprinodontidae and of teleosts in general with a list of new chromosome numbers of Cyprinodontidae. Arkiv fir Zool. 33B (10): 1-6. 1943. Cytological studies on the family Cyprinodontidae. Hereditas 29: 1-24. Yosida, T. H. 1957. Sex chromosomes of the tree frog, Hyla arborea japonica. J. Fac. Sci. Hokkaido Univ. 13: 352-358. 12 POSTILLA PLATES (All figures reproduced at same scale: 104 = 27 mm) FIG. 1. Karyotype of Bathylagus wesethi (2n=36). a. Idiogram. b. Meta- phase plate corresponding to a. Presumed X and Y chromosomes are underlined in the idiogram and indicated with arrows in the metaphase plates. (All figures follow same citation as mentioned here. All figures: reproduced at same scale: 10u—27 mm) FIG. 2. Premetaphase I (a) and metaphase I (b) of B. wesethi, indicating the satellite-like “X-Y” bivalent (arrow). 13 HETEROGAMETY OF DEEP-SEA FISHES » © @ 83 & ey yb D8 BS 14 POSTILLA = — 3-a gy *,. > “% a) wt i ~~ .. a % @. S.. a - . © _* Be % - F rs os .* > X24 es * 9 a il : a # Y * ox wre 4 A o* a , oe foe ei. " $&-, * FIG. 3. Metaphase Il of Bathylagus wesethi. N=18 with the small “Y” (upper left) and with the large “X” (two cells at right). HETEROGAMETY OF DEEP-SEA FISHES 15 ra | és 47 @©0@ ga 46 wOe sh esr tes neaasi 8eesee mae? § @Pt*® war. ae A-b °* .e* i -,. * “es a > * a # * @.e + os Xs a * head ¥ ¢ + * * ~@ » od X . e FIG. 4. Karyotype of Bathylagus ochotensis (2n=54). a. Idiogram. b. Metaphase plate corresponding to a. 16 POSTILLA Se a € 8. PR eas te ea ae CP GSE ET fg tear cogas B® ee gta ape eae weg geriiges ¥ @e ata FIG. 5. Karyotype of Bathylagus stilbius (2n=64). a. Idiogram. Seven identifiable metacentric (M), 4 submetacentric (SM), and 1 acrocentric (A) pairs are placed separately. The rest are morphologically unidentifiable. b. Metaphase plate corresponding to a. HETEROGAMETY OF DEEP-SEA FISHES U7/ ss — CUECis2e PIED ze GULCERAREBREF ET SEEDED atebhteeatteaewetesae asaeeer FIG. 6. Karyotype of Bathylagus milleri (2n=60). a. Idiogram. Morpho- logically identifiable autosomal pairs are placed separately. (cf. Fig. 5). b. Metaphase plate corresponding to a. 18 POSTILLA 7 8 FIG. 7. Metaphase I of Bathylagus ochotensis. FIG. 8. Metaphase I of B. stilbius. FIG. 9. Metaphase I of B. milleri. Fic. 10. Preleptotene of B. milleri showing the presumed heterochromatic sex element (arrow). HETEROGAMETY OF DEEP-SEA FISHES 19 20 POSTILLA FIG. 12. Metaphase plate of Sternoptyx diaphana. FIG. 13. Preleptotene of Sternoptyx diaphana showing the heterochromatic sex element (arrow). FIG. 14. Metaphase I of Sternoptyx diaphana showing the univalent “X” (arrow). HETEROGAMETY OF DEEP-SEA FISHES 21 15-b = &, FIG. 15. Metaphase II of Sternoptyx diaphana showing 18 chromosomes (with the “X”, a and a’) and 17 (without the “X’, b). FIG. 16. Preleptotene of Scopelengys tristis showing the heterochromatic sex element (arrow). FIG. 17. Metaphase I (sideview) of Scopelengys tristis (2n=48, n=24) showing atypically behaving presumed X-Y bivalent (arrows). 22 POSTILLA FIG. 18. Two leptotene cells of Symbolophorus californiensis (2n= 48; n=24) showing the heterochromatic sex element (arrows). FIG. 19. Metaphase I (sideviews) of Symbolophorus californiensis show- ing atypically behaving presumed sex bivalent (arrows). FIG. 20. Early anaphase I of Symbolophorus californiensis showing the sex bivalent and its distinct heterochromatic bands (arrow). FIG. 21. Anaphase I of Symbolophorus californiensis showing two lagging, presumed sex univalents (arrows). HETEROGAMETY OF DEEP-SEA FISHES 23 pea } NP CUeB Nal beate Odteieeact@ebaasse Q@Raasiagsasadteaa 22-C . @ 9098 ®.°8 08° .° %e - 404 te* ee ee} ® #é @ ot * ®”« Se oa?*, e ; “one * we 8 ,°. >, @ wali J “eeese le “— x—> & FIG. 22. Karyotype of male Lampanyctus ritteri (2n=47). a. Idiogram. b. Metaphase plate corresponding to a. c. Two late metaphase plates. POSTILL (eUebat iy ein aD Waka aa 16 OR OR OY DD UE isan HETEROGAMETY OF DEEP-SEA FISHES 25 24-a FIG. 23. Karyotype of female Lampanyctus ritteri (2n=48). a. Idiogram. b. Metaphase plates corresponding to a. FIG. 24. Primary spermatocytes (MI) of Lampanyctus ritteri showing the “X”-autosome trivalent (a and b). FIG. 25. Anaphase I of male Lampanyctus ritteri showing one (lower) with and the other (upper) without the “X” chromosome. FIG. 26. Secondary spermatocytes (M II) of Lampanyctus ritteri showing: a, n=23 without, and b, n=24 with the “X”. 26 POSTILLA ow VOCED CITI SPROUL GEEEaeeTy Orig chasccruh tees 270 27. Karyotype of Parvilu: s (2n=49). a. Idiogram. b. Meta- ae ase plate corresponding i a. Cc. nother metaphase plate showing the ae, Caer HETEROGAMETY OF DEEP-SEA FISHES 27 FIG. 28. Metaphase | of Parvilux ingens showing atypically behaving “X” univalent (a and b). FIG. 29. Metaphase II of Parvilux ingens, (n=25, a and a’) with and the other (n= 24, b and b’) without the “X”. 28 POSTILLA 30-a MW Nw wos 96.6€ 88 te te 20 6F G6 48 6) a8 Ga 06 Df be ae FIG. 30. Karyotype of Scopelogadus mizolepis bispinosus (2n= 46). a. Idiogram. b. Metaphase plate corresponding to a. FIG. 31. Preleptotene of S. m. bispinosus showing the presumed hetero- chromatic sex element (arrow). FIG. 32. Pachytene of S. m. bispinosus showing a lampbrush-like “X-Y” bivalent (arrow). HETEROGAMETY OF DEEP-SEA FISHES 29 eicle ad “ é 2, g e@*.a