The onset of sexual maturity in fishes of the genus Xiphophorus depends upon a multiple allelic series at the P locus. X . helleri of genotype P h P h initiate sexual maturity at 10.4 weeks and X. maculatus of genotypes P3P3 or PP at 8.7 or 20 weeks, respectively. Hybrids between the two species of genotype P h P h and possessing some autosomes of X . maculatus in homozygous condition exhibit precocious maturation at 5.3 weeks and are correspondingly small. PhPh hybrids of the backcross generations to X . helleri mature at the same age and size as X . helleri. The P 3 P h and P P h backcross hybrids, however, exhibit greatly delayed maturation, and the delay becomes more severe as the number of X . helleri autosomes increases. The phenotype results from the interaction of the P locus with a system of regulatory genes. Both the P factors and the regulatory systems are speciesspecific.Synthetic luteinizing hormone-releasing hormone (LH-RH) was effective in inducing proliferation and functional activity in the gonadotrops and ultimately sexual maturity in all genotypes, including PhP3 hybrids. Whereas the response of different genotypes to gonadotropin was essentially the same, their response to LH-RH was significantly different. A positive correlation exists between the rapidity of the response to LH-RH and the genetically determined age at which spontaneous maturation takes place. The maturity induced by LH-RH was not permanent. After cessation of LH-RH administration the gonadotrops became inactive and regressed. These observations suggest that the Plocus is not concerned with the production of LH-RH. Rather, the P locus appears to control the physiological function or the fate of LH-RH.

The sex-determining mechanism of swordtails (Xiphophorus, Poeciliidae, Pisces) is poorly understood, because many laboratory strains of these species are characterized by biased sex ratios in either the male or female direction. Because of these ratios sex determination is said to be controlled by numerous male and female factors scattered over many chromosomes. However, direct experimental evidence for the polygenic theory is absent. Three strains of X. alvarezi were derived from natural populations in Guatemala, two of which (Dolores, Candelaria) always exhibit a ratio of 1:1, whereas the third one (San Ramón) shows a preponderance of males. Males of X. alvarezi, like those of other species of swordtails, develop a caudal appendage or sword at maturity. Chromatophores with carotenoid pigment (orange) are present in the sword of Dolores males and pigment cells containing sepiapterin (green) are present in the swords of males and the other two stocks. F1 and backcross hybrids were produced between the Dolores and Candelaria stocks. The sex ratios of all hybrid pedigrees was always in statistical agreement with unity. The pattern of inheritance of the two pigmentary traits demonstrated sex-linkage with female heterogamety (WY female female-YY male male).