Genetic variation in maternal investment patterns in platyfish Xiphophorus maculatus.

Maternal investment in offspring is constrained by a variety of trade-offs, and the investment pattern expressed by an individual can have important fitness consequences. The authors investigate two maternal investment tradeoffs in the southern platyfish, Xiphophorus maculatus: investment in offspring size versus number, and investment in sons versus daughters. Variation in maternal age and size often confound measurements of investment patterns. Because allelic variation at a single locus in platyfish has a major effect on age and size at sexual maturation, investment patterns within a single age and size genotype could be examined, minimizing the influence of these factors on investment patterns. In addition, the platyfish sex determination system allows experimental investigation of relative investment in sons and daughters because different mating types produce different brood sex ratios. Total investment in broods did not differ among sibships, but the investment pattern did; some produced larger but fewer offspring, whereas others produced smaller but more offspring. In addition, there was no evidence of differential investment in sons and daughters; broods with all sons did not differ in size or weight from broods with an equal proportion of the sexes. This result provides direct support for the assumption of equal investment in sons and daughters that is common in discussions of sex-ratio evolution and sex allocation.