Evidence for the existence of transformer genes for sex in the teleost Xiphophorus maculatus.

sex-determining mechanism in Poeciliid fishes of the genus Xiphophorus ' F : been studied for more than forty years (BELLAMY 1928; GORDON 1927), yet many problems remain unsolved. In the southern platyfish, Xiphophorus maculatus, native to Mexico and adjacent parts of Central America, there are three types of females (XX, WX, WY) and two types of males (XY and YY) (GORDON 1947, 1954; KALLMAN 1965a). The W chromosome carries essentially a stronger female determining potency than the X, because the WY constitution gives rise to females ancl the XY constitution to males. Two other species, X . variatus (BELLAMY 1936; KOSSWIG 1937; RUST 1939) and x. milleri (KALLMAN 1965b) possess an XX o o -XY 8 8 system. Their sex chromosomes are homologous with those of X . maculatus (KALLMAN and ATZ 1967). In X . hellerii, the swordtail, no sex-linked characters have been discovered, but because of widely fluctuating sex ratios between and even within stocks (BREIDER 1935; PETERS 1964) this species is thought to possess a polygenic mode of sex determination (KOSSWIG 1964; KOSSWIG and OKTAY 1955). Occasionally, in X . maculatus, X . variatus, and X . miZleri genetic sex reversals arise: individuals that have either the WY or XX genotypes, but differentiate into functional males (BELLAMY and QUEAL 1951; BREIDER 1942; OKTAY 1959) and XY fish that are fertile females (ANDERS and ANDERS 1963; MACINTYRE 1961). The overall incidence of such genetic sex reversals in .X. maculatus is less than one percent, but their occurrence is not a random event (KALLMAN 1965a). Similar sex reversals have been discovered in the guppy, Poecilia reticulata (Poeciliidae) (WINGE 1934) and in the medaka, Oryzias Zatipes (Oryziatidae) ( AIDA 1936, YAMAMOTO 1963), species that are normally XX (2 o and XY 8 8 , The occurrence of sex reversals is thus not limited to one species of fish, but may be of general significance. WINGE proposed that in addition to the sex chromosomes there are also autosomal genes with maleand female-determining potencies (WINGE 1934; WINGE and DITLEVSEN 1948). Usually the additive effect of these autosomal factors is not sufficient to upset the switch mechanism inherent in the XX-XY system of the guppy. Sex reversals are individuals with rare fortuitous combinations of autosomal male or female determining genes whose combined strength overrides the action of the sex chromosomes. Similar explanations have been suggested for