In clonal unisexual vertebrates, the genes specifying the males become dispensable. To study the fate of such genes the gynogenetic all-female fish Poeciiaformosa was treated with androgens. Phenotypic males were obtained that exhibited the complete set of male characteristics of closely related gonochoristic species, including body proportions, pigmentation, the extremely complex insemination apparatus of poeciliid fish, sexual behavior, and spermatogenesis. The apparent stability of such genic structures, including those involved in androgen regulation, is contrasted by high instability of noncoding sequences. Frequent mutations, their donal transmission, and at least two truly hypervariable loci leading to individual differences between these otherwise donal organisms were detected by DNA fingerprinting. These observations substantiate the concept that also in "ameiotic" vertebrates certain compartments of the genome are more prone to mutational alterations than others.Clonal organisms occur in natural populations of multicellular animals at all levels of organismic evolution, but within vertebrates they are confined to teleost fish, amphibians, and reptiles (1). They exist as unisexual lineages that exclude effective genetic recombination. In such animals the genetic information specifying the other sex has apparently become dispensable. These organisms are therefore uniquely suited to investigate questions centering on problems of "dispensable" genes (2). Moreover, the existence of naturally occurring clonal animals poses questions about their origin and abilities to evolve in the absence of meiotic recombination.The amazon molly, Poecilia formosa, is an all-female poeciliid fish that inhabits freshwater streams and brackish coastal lagoons over a broad geographic range from southeastern Texas to northeastern Mexico. The reproductive mechanism in P. formosa is gynogenesis, a modified form of parthenogenesis. Sperm do not participate in syngamy but are required to trigger the onset of embryogenesis in diploid eggs, containing only the maternal genetic information (3, 4). Except for rare cases (5) the offspring are clonal replicates of their mothers (6-8). In natural populations sperm is contributed by males of two closely related bisexual species that occur sympatric with P. formosa. In the northern range this is Poecilia latipinna (5), whereas in the mexican habitats it is Poecilia mexicana (10). Biochemical and cytological data (8) supported the initial hypothesis (11) that the amazon molly is a hybrid species between P. latipinna and P. mexicana. The question arose if the genes determining male sex, phenotype, and behavior are absent or not functional in P. formosa.Preliminary studies (12-15) reported that treatment with androgenic steroids induces several phenotypic changes that resembled the male characteristics of related species. Androgen-induced masculinization, therefore, appeared to be a useful approach to investigate if the genes instrumental in determining the male phenotype and behavio...