Recent evolution of large offspring size and post-fertilization nutrient provisioning in swordtails.

Organisms have evolved diverse reproductive strategies that impact the probability that their offspring survive to adulthood. Using morphological measurements in embryos and fry, gene expression analysis and hybrid crosses, we describe divergence in reproductive strategy between two closely related species of swordtail fish (Xiphophorus), which have internal fertilization and give birth to free-swimming fry. We find that one species, Xiphophorus malinche, has evolved larger offspring than its closest relative Xiphophorus birchmanni and dwarfs the offspring size of other species in the genus. The fry of X. malinche are more resilient to starvation than X. birchmanni, hinting that the evolution of large offspring size could be an adaptation to the challenging environments in which X. malinche are born. We also find evidence that X. malinche mothers provision nutrients to their offspring during embryonic development, the first time this process has been documented in the Xiphophorus genus. Moreover, in the ovary, we observe differential regulation of genes associated with maternal nutrient provisioning in other groups that use this reproductive strategy. Finally, we generated hybrid crosses between X. malinche and X. birchmanni to explore the impact of genetics and maternal environment on offspring size, finding that offspring size is at least in part genetically determined. Intriguingly, we find a low rate of survival in one cross direction and investigate the links between reproductive strategy and this asymmetric hybrid incompatibility.