Estrone-induced white YY females and mass production of white YY males in the medaka, Oryzias latipes.

N the medaka (Oryzius Zutipes) , an oviparous cyprinodont fish, the normal I sex-chromosome constitution is XX for female and XY for male (AIDA 1921). A series o€ our previous experiments have convincingly demonstrated that a heterotypic (atypical) sex hormone administered prior to and passing through the stage of gonadal sex differcntiation induces a complete reversal in sex differentiation. In other 'words, a functional sex phenotype opposite to the sex genotype can be achieved at will in either direction (YAMAMOTO 1953,1958). In progeny tests of estrogen-induced XY females by XY males we have been confronted with an intriguing problem of viability of YY zygotes. The problem has been fully discussed on the basis of experimental results (YAMAMOTO 1964a). While viable YRYR zygotes(R is a dominant gene responsible for deposition of a carotenoid) produced by mating estrogen-induced XrYR females with normal X'Y" males are very rare, YRYr zygotes yielded by mating estrogen-induced XrYR females by white X'Y' males are, unexpectedly, all viable. At first sight, it may seem curious, but when we take into consideration the origin of the Y' chromosome, these facts are cogently rationalized. The inference is that the regular YR has an inert section (-) and the ordinary X' and XR have a positive segment (+) essential for viability. The r of the Y' is known to have derived from X' by crossing over. This being so, it is quite possible that Y' is Yr,+ in constitution and Y R V is YR,-Yr,+, so that, the majority of YRYR zygotes have double inert sections, YR,-YR,-, and are lethal. Presence of the inert section (-) in the regular YR chromosome has been inferred and experimentally proven not only in the d-rR strain but in other strains, including the wild type (YAMAMOTO 1965). However, a few viable YRYR zygotes are considered to be the recombinant YRY: having the constitution YRj-Ytj+, the (+) element having been transferred from X'J+ by crossing over. (The subscript c denotes a recombinant). In short, lethal YRYR and viable YRY" zygotes are radically different in genetic make-up. Experiments in conformity with this conclusion were presented earlier (YAMAMOTO 1964a, b) . This being so, it is hardly surprising that practically all Y'Y' zygotes having the (+) segment in duplicate, namely Yr,+Yr,+, are viable. In €act, we were able IO detect two Y'Yr sons among 12 F, offspring of estrone-induced X'Y' females mated with X'Y' males (YAMAMOTO 1964a). This is in strong contrast to detecting only two YRYR males out of 57 F, offspring derived from matings of estrogen-induced X7YR females with normal X'Y" males (YAMAMOTO 1959).