メダカの性分化の人爲的転換

概要

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The sex-differentiation of the gonad in a cyprinodont fish (the medaka, Oryzias latipes) indicates that it is a “differentiated” species. There is no evidence of juvenile hermaphroditism in normal development. The effects of estrogens (estrone and stilbestrol) and an androgen (methyl testosterone) on gonadal sex-differentiation were studied in a genetically analyzed breed (d-rR strain). The sex-linked characters R (orange-red) and r (white) were used as markers of the sex-chromosomes in order to distinguish the sex genotype. The offspring of white females (XrXr) mated en masse with heterozygous orange-red males (XrYR) are usually white females and orange-red males since the R (dominant in effect) is resident only in the Y chromosome. Peroral administration of either estrone (1, 250 i.u./g-diet) or stilbestrol (5, 000 i.u./g-diet) from the time of hatching up to the age of several months results in reversal of sex differentiation in genetic males (XrYR). Fully grown sex-reversed females of the male genotype have reproductive capacity, yielding offspring in mating with normal males.