Evolution of maternal egg size effects in sister salamander species.

Egg size varies genetically and with the maternal environment. It is correlated with and can act as a resource fueling variation in many other key life history traits. This study examined hypotheses about how plastic responses of offspring to yolk variation evolve (and contribute to phenotypic evolution) when maternal investment in egg size evolves. I used a split-clutch, controlled, surgical experiment with a longitudinal (repeated-measures) design to examine the effects of yolk removal on sister salamander species with distinct egg and larval phenotypes. Yolk removal had large effects in the derived larger-egged species, A. barbouri, and greatly reduced effects in A. texanum. Early hatching and smaller larval body size was only found in A. barbouri and survival rates decreased more in A. barbouri. These results provide strong experimental evidence that as female salamanders evolve greater yolk investment in each egg, offspring coevolve an increased magnitude of phenotypic plasticity in response to yolk variation across a suite of life history traits. Yolk therefore acts as an integrator of phenotypes that allows females to modify modules of life history traits together (facilitating adaptation). When organisms invade new environments, complex integrated phenotypes may evolve via correlated responses to increased maternal investment, yet individual traits can be coupled or decoupled to yolk quantity variation in different species.