Different genes are recruited during convergent evolution of pregnancy and the placenta

The repeated evolution of the same traits in distantly related groups
(convergent evolution) raises a key question in evolutionary biology: do
the same genes underpin convergent phenotypes? Here, we explore one such
trait, viviparity (live birth), which, qualitative studies suggest, may
indeed have evolved via genetic convergence. There are 150 independent
origins of live birth in vertebrates, providing a uniquely powerful system
to test the mechanisms underpinning convergence in morphology, physiology,
and/or gene recruitment during pregnancy. We compared transcriptomic data
from eight vertebrates (lizards, mammals, sharks) that gestate embryos
within the uterus. Since many previous studies detected qualitative
similarities in gene use during independent origins of pregnancy, we
expected to find significant overlap in gene use in viviparous taxa.
However, we found no more overlap in uterine gene expression associated
with viviparity than we would expect by chance alone. Each viviparous
lineage exhibits the same core set of uterine physiological functions.
Yet, contrary to prevailing assumptions about this trait, we find that
none of the same genes are differentially expressed in all viviparous
lineages, or even in all viviparous amniote lineages. Therefore, across
distantly related vertebrates, different genes have been recruited to
support the morphological and physiological changes required for
successful pregnancy. We conclude that redundancies in gene function have
enabled the repeated evolution of viviparity through recruitment of
different genes from genomic “toolboxes”, which are uniquely constrained
by the ancestries of each lineage.