Facultative Transitions Have Trouble Committing, But Stable Life Cycles Predict Salamander Genome Size Evolution

Abstract

Facultative traits can provide phenotypic lability in dynamic environments, but it is unclear how weaving between disparate habitats impacts non-facultative traits that are carried along the way. The life cycles of salamanders are associated with distinct ontogenies, aquatic-to-terrestrial (biphasic), completely-terrestrial (direct development), and completely-aquatic (larval form paedomorphic). Salamanders have some of the largest genomes among vertebrates, and the most extreme expansions have been attributed to paedomorphosis and life cycle simplification. Recent analyses of genome size evolution across amphibians have rejected this hypothesis for salamanders. Our analyses show that treatment of facultatively paedomorphic salamanders, which are alternatively biphasic, in part explains this discrepancy. Nearly all of the facultatively paedomorphic species analyzed have genome sizes that overlap with the optimum of biphasic species. We found that obligate paedomorphs, alone and when combined with direct developers, have significantly larger genome sizes than biphasics plus facultative paedomorphs. In general, salamander genome size variation fits life cycle models better than those for larval ecology, adult ecology, or aquatic habitat stability. Obligate transitions to a simple life cycle appear to have been an important route for lineages to evolve significant divergence in genome size from biphasic ancestors. Our analyses support the classic association between genome size variation and life cycle complexity in salamanders, which may ultimately reflect patterns of time limited development.