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Genetic control of the hypothalamo pituitary axis and the effect of hybridization on sexual maturation xiphophorus pisces poeciliidae


, : Genetic control of the hypothalamo pituitary axis and the effect of hybridization on sexual maturation xiphophorus pisces poeciliidae. Journal of Experimental Zoology 220(3): 297-310

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 PhPh initiate sexual maturity at 10.4 wk and X. maculatus of genotypes P3P3 or P4P4 at 8.7 or 20 wk, respectively. Hybrids between the 2 species of genotype PhPh and possessing some autosomes of X. maculatus in homozygous condition exhibit precocious maturation at 5.3 wk and are correspondingly small. PhPh hybrids of the backcross generations to X. helleri mature at the same age and size as X. helleri. The P3Ph and P4 P4 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 species-specific. Synthetic LHRH 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 LHRH was significantly different. A positive correlation exists between the rapidity of the response to LHRH and the genetically determined age at which spontaneous maturation takes place. The majority induced by LHRH was not permanent. After cessation of LHRH administration the gonadotrops became inactive and regressed. The P locus is not concerned with the production of LHRH. Rather, the P locus appears to control the physiological function or the fate of LHRH.

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