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 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 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.