107. Segregation o f Karyotypes in the F2 Generation o f the Hybrids between Mauritius and Oceanian Type Black Rats with a Note on their Litter Size*'

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1 No. 9] Proc. Japan Acad., 5'6, Ser. B (1980) Segregation o f Karyotypes in the F2 Generation o f the Hybrids between Mauritius and Oceanian Type Black Rats with a Note on their Litter Size*' By Tosihide H. YosIDA National Institute of Genetics, Misima (Communicated by Sajiro MAKINO, M. J. A., Nov. 12, 1980) The Mauritius type black rats occurring in Mauritius Island are characterized by having extra 8 small acrocentrics in the karyotype of the Oceanian type black rat. Comparative karyotype analyses and hybridization experiments have shown that the small extra acrocentrics found in the Mauritius black rat result from the Robertsonian fission of the metacentric pair nos. 14 and 18 occurring in the Oceanian type black rat (2n, 38). Thus, the chromosome number of the Mauritius black rat was shown as 42 instead of 38 in the Oceanian t~7pe rat (Yosida et al. 1979a, b). The laboratory matings between the Mauritius type (2n, 42) and the Oceanian type black rats (2n, 38) successfully produced F1 hybrids (Yosida 1979, 1980). These hybrids were characterized by 40 chromosomes, among them pair nos. 14 and 18 were heteromorphic each consisting of two small acrocentrics and one small metacentric. In mating between the F1 hybrids, 17 F2 offspring were obtained. The present paper deals with the segregation of the small acrocentric and metacentric pair nos. 14 and 18 in the F2 generation, with a note on the size of litters, referring particularly to the evolutional relation between these geographical variants. Material and methods. The Mauritius type black rats (2n, 42) were collected in the island Mauritius in the Indian Ocean, while the Oceanian type black rats (2n, 38) came from Colombo, Sri Lanka, in They were bred in our laboratory and their offspring provided materials for the present study. The F1 hybrids were obtained by mating between the female Mauritius rats and the male Oceanian type rats. In the mating, 17 F2 offspring were obtained and 16 rats were available for the karyotypical investigations. The chromosome slides were prepared from the cultured cells of *' Contribution no from the National Institute of Genetics, Japan. Supported by a grant-in-aid for the scientific research from the Ministry of Education, Science and Culture (nos , ).

2 558 T. H. Y0sIDA [Vol. 56(B), the tail tissue following the procedure previously described (Yosida 1980). The trypsin G-banding technique was also applied to them. Results. 1) Karyotypes of F1 hybrids from Mauritius x Oceania and their litter size. As already mentioned, the Mauritius type black rats were characterized by 42 chromosomes including two large metacentrics (M1 and M2) and 8 extra small acrocentrics. The latter extra acrocentrics were found to be derived due to the Robertsonian fission of two small metacentric pair nos. 14 and 18 generally found in the other type black rats (Yosida et al. 1979a, b). In the mating of the Mauritius black rats with the Oceanian type black rat, 20 F1 hybrids were obtained. Their average litter size was 5.0±2.5, with no significant difference from the litter size of the parental Mauritius and Oceanian type rats (Table I). All the F1 hybrids were found to Table I. Litter size of Mauritius F1 and and F2 hybrids and the parental Oceanian type rats have 40 chromosomes as already described. The karyotypes were outstanding by having heteromorphic pair nos. 14 and 18, each consisting of two small acrocentrics and one small metacentric (Yosida 1979, 1980) (Fig. 1). The two small acrocentrics were derived from two pair nos. 14 and 18 occurring in the Mauritius type and the one small metacentric occurring in the Oceanian type. It is apparent that the large metacentric M1 and M2 pairs are derived from the both parents. 2) Segregation o f the karyotypes in F2 hybrids and their litter size. In mating between the F1 hybrids, 17 F2 offspring were obtained from 4 litters. The average litter size of the F2 hybrids was shown to be 4.3±1.7, slightly smaller than that of F1 hybrids and their parental Mauritius and Oceanian type rats (Table I). In the F2 offspring, karyotypes of the 16 rats were availably analysed. If the nos. 14 and 18 in both F1 parents consist of one metacentric and two acrocentrics, and if they segregate regularly in the meiotic division, the production of F2 rats with 9 different karyotypes is expected as shown in Table II. The segregation ratio of the karyotypes in these 16 F2 rats was not significantly different from the expected value, although

3 No. 9] F2 Hybrids between Mauritius and Oceanian Type Black Rats 559 the rats with the karyotype regarding nos. 3, 7 and 9 shown in the table were not obtained in contrast with the expectation. The karyotype of F2 hybrids regarding 1 and 9 was quite similar to those of their grand parents (Oceanian and Mauritius type rats), although Figs Two G-banding karyotypes of the F2 hybrids (Mauritius>< Oceanian type black rats). 1: Karyotype of F1 type (no. 5 in Table I). Pair nos. 14 and 18 are both heteromorphic (2n=40). 2: Karyotype no. 6 which is heteromorphic pair no. 14 and acrocentric pair no. 18 (2n=41). Table II. Segregation of pair nos. 14 and 18 in F2 hybrids by MauritiusxOceanian type black rats occurred

4 560 T. H. YOsIDA [Vol. 56(B), the karyotype regarding no. 9 was not obtained in the segregants. The black rats with the karyotype no. 5 (Fig. 1), which was the same chromosome condition as the F1 hybrids, were most frequent as expected. A rat with the karyotype regarding no. 8 was obtained in the natural population of Mauritius Island (Yosida et al. 1979b), but the other three karyotypes involving nos. 2, 4 and 6, were new in the F2 hybrids so far obtained in the laboratory breeding (Figs. 2-4). Figs Two karyotypes of the F2 hybrids (MauritiusXOceanian type black rats). 3: Karyotype no. 2 having metacentric pair no. 14 and heteromorphic pair no. 18 (2n=39). 4: Karyotype no. 4 having heteromorphic pair no. 14 and metacentric pair no. 18 (2n=39). Remarks. Based on the occurrence of the large metacentric pairs M1 and M2, the conclusion is available that the Mauritius black rats are derived from the Oceanian type black rats (2n, 38) as a result of the Robertsonian fission of the small metacentric pair nos. 14 and 18. In the laboratory mating between the Oceanian and Mauritius type rats, the karyotypes of the F2 offspring showed a regular segregation as expected. The litter size of the F2 hybrids from Mauritius and Oceanian rats was also not significantly different from those of the F1 as well as from their parental lines. However, F2 hybrids between the Oceanian and Asian type rats and between the Oceanian and Ceylonese type rats were very poorly obtained. The average litter size of the F2 in the former combination was about 1.2, while that in the latter was 2.6±1.0 (Yosida et al. 1971; Yosida 1976, 1979, 1980). It is then emphasized that the evolutional relation between three geographical variants can be possibly suspected on the basis of the fertility of their F1 hybrids. The above consideration has

5 No. 91 F2 Hybrids between Mauritius and Oceanian Type Black Rats 561 made possible the following conclusion that the Mauritius type black rats are veryy closely related to the Oceanian type rats. Noticeable is the fact that the sexual anomalies such as XXY and XO rats were frequent in the F2 hybrids between the Oceanian and Ceylonese type rats (Yosida 1977, 1978, 1979), while no such anomalies have never appeared in the F2 hybrids between the Oceanian and Mauritius type rats. This is also in favor of the view that the evolutional relation between Oceanian and Mauritius type rats is closer than the relation between Oceanian and Ceylonese types. The meiotic analysis of chromosomes in the hybrids specimens has now been in progress in order to inquire into the mechanism of segregation of hybrids chromosomes. Details will appear elsewhere in the near future. Summary. The segregation ratio in F2 hybrids from Mauritius type black rats (2n, 42) x Oceanian type rats (2n, 38) and their litter size were studied. The average litter size of the F2 offspring was 4.3±1.7. The segregation ratio of chromosome pair nos. 14 and 18 in the F2 hybrids was nearly identical with the expected one. It was concluded that the Mauritius type black rats are very closely related to the Oceanian type rats on the bases of litter size and segregation ratio in the F2 hybrids. Acknowledgments. The author is very grateful to Emeritus Professor Saj iro Makino, M. J. A., for refinement and revision of the manuscript with keen interest. He is also indebted to Mr. Hisaharu Iwasaki and Miss Etsuko Hamada for technical assistance. References Yosida, T. H.: Proc. Japan Acad., 52, (1976). Cytogenet. Cell Genet., 19, (1977). Japan. J. Genet., 54, (1978). Proc. Japan Acad., 55B, (1979). Cytogenetics of the Black Rat-Karyotype Evolution and Species Differentiation. Univ. Tokyo Press, Tokyo/Univ. Park Press, Baltimore (1980). Yosida, T. H., Kato, H., Tsuchiya, K., and Moriwaki, K.: Chromosoma (Berl.), 34, (1971). Yosida, T. H., Kato, H., Tsuchiya, K., Moriwaki, K., Ochiai, Y., and Monty, J.: Proc. Japan Acad., 55B, (1979a). Chromosoma (Berl.), 75, (1979b).