Supplementary material from "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, X. malinche, which lives at high-elevations, has evolved larger offspring than its closest relative X. 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 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 Xiphophorus. 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, finding that offspring size is in part genetically determined and document a strong incompatibility in the other cross direction.