Meeting Abstract
Among benthic marine invertebrates, some species make small eggs that develop into complex planktotrophic larvae, while others make large eggs that develop into lecithotrophic larvae able to metamorphose into benthic juveniles without ever feeding. Transitions between these two resource-allocation strategies are common in evolutionary history. Streblospio benedicti is one of a handful of species in which both strategies occur. Developmental mode is highly heritable in S. benedicti, and we have used experimental crosses to dissect the genetic basis for differences between planktotrophs and lecithotrophs. By genetically mapping different aspects of larval development to the S. benedicti genome, we have discovered that, larval size, which is governed by maternal genetic effects, varies due to loci unlinked to loci that act zygotically to shape larval morphology. This genetic architecture of larval development — joint determination by unlinked maternal- and zygotic-effect loci — shapes the possibilities for phenotypic evolution. Zygotic effect alleles find themselves in maternal-effect backgrounds according to the local maternal-effect allele frequencies, creating positive frequency-dependent selection. In other words, egg-size allele frequencies are the selective environment for larval morphology alleles.
