THE GENETIC AND ENDOCRINOLOGICAL BASIS OF ADULT SIZE POLYMORPHISM IN XIPHOPHORUS PYGMAEUS (TELEOSTEI: POECILIIDAE)

In the pygmy swordtail, Xiphophorus pygmaeus nigrensis, three alleles (S, I, and L) exist at the P locus which control the growth rate of immature fish and the size at which maturation is initiated. Immature males and females of the genotype SS have a slower growth rate and mature at a smaller size than SL males. SI males have an intermediate growth rate and mature at an intermediate size.;The larger active somatotropic zone of SL males presumably corresponds to a higher level of somatotropin which would account for their faster growth rate.;The gonadotrops develop and become active in the SS fish at a smaller size than in SL males. In both genotypes the gonadotropic zone proliferates prior to becoming active. Once active the gonadotropic zones of mature SS and SL males were indistinguishable.;Concurrent with the development and differentiation of the gonadotropic zone are changes within the gonads of the SS and SL fish. In both male morphs the structure of the testes was virtually the same throughout development. The immature testes contained germ cells that developed up to the spermatogonial type II stage. As maturation proceeded the testes were indistinguishable although somewhat larger in the SL males. The development of the pituitary-gonadal axis, however, was delayed in the SL males. In the SS females, vitellogenesis occurred only in the presence of a fully developed and functional gonadotropic zone.;SL males have larger somatotropic zones than the SS males or females at three to eleven weeks. The somatotrops are derived from chromophobes which become active before birth in both genotypes. However, by three weeks after birth up to thirty per cent of the somatotropic area in the SS fish becomes chromophobic again.;Male and female embryos were identified prior to germ cell differentiation. Males were characterized by a hilar stroma, in at least one of their gonads. In females a hilar stroma was absent.;The result of this study demonstrates that the P locus controls both rate of growth and onset of sexual maturation through its effect on the development and activity of the somatotrops and gonadotrops.