The evolutionary implications of alternative larval development in ascidians

Abstract

Considering traits that facilitate and inhibit evolutionary diversification can best be done with traits where the genetic and developmental basis is known. My PhD. research has focused on the evolutionary implications of larval development in molgulid ascidians. Ascidians or sea squirts, are a group of tunicates, which phylogenomic evidence suggests is sister-group to the vertebrates. Most ascidians develop as tadpole larvae that have an eyespot, gravity-sensing otolith, and a notochord, which converges and extends to form a tail that is used for dispersal. In molgulid ascidians, there have been 15 species described from at least four separate evolutionary origins that have lost the eyespot, otolith, and tail in their larval stage. Using transcriptome data for a species with normal tadpole larval development, a species with tailless larval development, and their hybrid, I have found that molgulid ascidians express genes normally expressed at metamorphosis early in development, and hypothesize this early expression is driving the tailless phenotype. Additionally, using molecular phylogenetic comparative methods in ascidians, I found species with the derived, tailless, less-dispersive larvae have lower speciation rates and smaller geographic ranges. Lower speciation rates in lineages with the derived, less dispersive larval development is in contrast to speciation rates estimated from molecular phylogenies in gastropods, and from fossil gastropods and echinoderms, and this difference suggests that the demography of species with different larval development may be important to consider.