The genetic control of the pituitary‐gonadal axis in the platyfish, Xiphophorus maculatus

Abstract

AbstractA sex‐linked gene controls the age at which the gonadotropic zone of the pituitary gland develops and becomes physiologically active. The alleles for early (Pe) and late(Pl) differentiation are linked to pigment genes that serve as genetic markers. Pe may be marked by Sp, Dr, Sr, Ir and + and Pl by Br and N. Ir females (PePe) matured between 10 to 15 weeks of age and Br (PePl) females between 16 to 20 weeks. Dr (PePe) females had ripe ovaries six to eight weeks earlier than N Dr (PePl) females (8 to 12 weeks versus 16 to 20 weeks). The average age of sexual maturation was 12.5 weeks for Ir Ir (PePe) males, 20 weeks for Ir Br (PePl) and 26.5 weeks for Br Br (PlPl) males. Dr Sr (PePe) males reached maturity at 13.5 weeks and N Sr (PePl) males at 27.5 weeks. The transformation of the anal fin into a gonopodium which is under androgenic control, takes longest in the late maturing fish (Ir Ir, 3.8 weeks; Ir Br, 4.8; Br Br, 6.5; Dr Sr, 4.2; N Sr, 6.4). Similarly, the rate of development of the gonadotropic zone and of the gonad is protracted in the late genotypes. Eventually, the gonadotropic zone of early and late maturing fish are indistinguishable. The Pl factor linked to N delays sexual maturity to a greater extent than the one linked to Br suggesting that not all Pl factors are identical and that important allelic interactions may exist. The growth rate of immature males and females is the same, however it declines sharply in males (but not females) at the time of sexual maturity. This leads to significant differences in adult size between male genotypes. Female genotypes do not exhibit size differences and eventually become larger than all males. The development of the gonadotropic zone precedes, and is essential for, complete gonadal maturation. In the absence of a functional gonadotropic zone, oocytes developed through the oil droplet stage and were surrounded by active follicle cells and a prominent zona pellucida. Spermatogenesis proceeded up to the spermatocyte stage and the cells lining the efferent ducts displayed a modicum of activity. Some of our data suggest that low levels of androgens are present in all males as early as five weeks of age prior to the development of gonadotrops. The genes regulating the development of the pituitary‐gonadal axis may be an important factor in explaining the evolution of poeciliid fish.