A SEX-LINKED GENE CONTROLLING THE ONSET OF SEXUAL MATURITY IN FEMALE AND MALE PLATYFISH (XIPHOPHORUS MACULATUS), FECUNDITY IN FEMALES AND ADULT SIZE IN MALES

Abstract

A sex-linked gene, P, controls the onset of sexual maturity in the platyfish, Xiphophorus maculatus. The activity of this gene is correlated with the age and size at which the gonadotropic zone of the adenohypophysis differentiates and becomes physiologically active. Immature fish of all genotypes grow at the same rate; however, as adults, males with "early" genotypes are significantly smaller than males of "late" genotypes, since growth rate declines strongly under the influence of androgenic hormone. Five alleles, P¹… P⁵, have been identified from natural populations that under controlled conditions cause gonad maturation between eight and 73 weeks. P¹P¹ males become mature at eight weeks and 21 mm, P²P² and P³P³ males between eleven and 13.5 weeks and 25 to 29 mm, and P⁴P⁴ males at 25 weeks and 37 mm. Since P⁵ is X-linked, no males homozygous for P⁵ could be produced. The difference between P² and P³ is largely based upon their interaction with P⁵. P³P⁵ males mature at 17.5 weeks and 33.5 mm and P²P⁵ males at 28 weeks and 38 mm. The rate of transformation of the unmodified anal fin into a gonopodium, which is under androgenic control, is directly related to the age at initiation of sexual maturity, ranging from 3.2 weeks in P¹P¹ males to seven weeks in P²P⁵ males. These differences may reflect different levels of circulating gonadotropic and androgenic hormones.—In two genotypes of females, initiation of vitellogenesis was closely correlated with size and this critical size was independent of age (e.g., 21 mm for P¹P¹). In a third genotype (P¹P⁵) the minimum size for vitellogenesis decreased with increasing age, so that females would mature as early as eleven weeks, provided they had attained at least 29 mm, but at 25 weeks even females as small as 23 mm possessed ripe gonads. For P⁵P⁵ females, which become mature between 34 and 73 weeks of age, there is no correlation between size and initiation of vitellogenesis. In all four genotypes of females examined, egg number is strongly correlated with size, but the regression of egg number on standard length is distinct for each genotype. Late maturation of P⁵P⁵ females is not offset by an increased number of eggs; for this genotype there is a strong negative correlation between age and number of eggs. Heterozygous fish always mature later than those homozygous for the "earlier" allele. The site of action of the P locus could be the pituitary gland, the hypothalamus or higher centers of the brain where peripheral information is transduced into an appropriate signal required for the activation of the hypothalamus-pituitary-gonadal axis. The P gene could also control the peripheral information. The platyfish may be a useful model to test theories concerning the evolution of life history strategies.