Studies on sterility in hybrid birds.

Abstract

As the third report of the series of studies on sterility of hybrid birds, which have been continuously carried on under the direction of Prof. K. Oguma of the Hokkaido Imperial University, the results of cytological investigations on the hybrid between the Muscovy duck (Cairina moschata, _??_) and the domestic duck (Anas platyrhyncha var. domestica, _??_) will be presented in this paper. The results of the present investigation are not in accord in many important points with those already recorded by Crew & Koller ('36) on the same subject.The intergeneric cross between the Muscovy duck and the domestic duck as above mentioned produces always completely sterile hybrids consisting of infecund males and abnormal females with rudimentary ovary and no sexual behaviour.The majority of the material employed in this study was obtained in Formosa. The gonads from 13-17 days embryos and also those from adult birds were used for the study of the chromosome. The embryonic material was fixed in Hermann's mixture and the adult testes were preserved in modified Champy's solution after Makino. The sections were stained with Heidenhain's haematoxylin method. The gonial chromosomes were observed in the embryonic gonads and the meiotic processes in the adult testes. Here the author wishes to express his sincere gratitude to Prof. Oguma for valuable suggestions and for laboratory facilities.The results obtained in this study are summarized as below : (1) The number of chromosomes of the Muscovy duck (Cairina moschata) was determined to be 80 for the male diploid complement (in the spermatogonium) (Fig. 1), and 79 for the female (in the oogonium) (Figs. 4 and 48).The male diploid complement consists of 12 (in 6 pairs) (or 16, in 8 pairs) macro-chromosomes which arrange in the peripheral zone of metaphase plate surrounding the remaining micro-chromosomes scattered in the central region of the plate (Fig. 1). Among the macro-chromosomes four are conspicuous V-shape forming two homologous pairs and the other pairs are telomitic rod-shape tapering at their inner ends (see Figs. 15-16). The micro-chromosomes range in shape from short rod to minute spheroid. The haploid number was determined to be 40 in the primary spermatocyte (Figs. 2-3).The female complex nearly resembles that of the male in general morphology. But in close comparison it is found that the fifth largest chromosome (e) has no homologous mate in the female complex as clearly recognizable by means of the serial alignment of chromosomes (Figs. 17-18). In other words, the chromosome e remains always unpaired having no mate of corresponding size in the female complement, while it is present in a paired condition in the male. This fact can be accepted only by considering that the chromosome e is nothing but the sex chromosome. The largest V-shaped chromosomes which were designated as the sex chromosome by Crew & Koller ('36), are always present in a paired condition in both sexes, and thus the conclusion of Crew & Koller ('36) is not correct.(2) The diploid number of chromosomes of the domestic duck (Anas platyrhyncha var. domestica) is 80 in the male and 79 in the female showing complete agreement with the record already published by Oguma ('38) in the other morphological details (Figs. 13-14). The sex chromosome is represented also by the fifth largest chromosome (e) (Figs. 19-22). The discrepant statements concerning the chromosome number and the sex chromosome of the duck as announced by Werner ('27), Sokolowskaja ('35), Alichanian ('36) and Crew & Koller ('36) are all found to be erroneous.(3) At a glance the karyotype of the Muscovy duck assumes a close resemblance to that of the domestic duck, but a close analysis of the complement reveals some significant differences existing between the two species. Remarkable differences are seen in the largest V-shaped chromosom