Abstracts Biological Sciences

Tests for Positive Selection on Genes Encoding Heat Shock Proteins in the Marine Slipper Snail, Crepidula fornicata

by Matthew J Starr

Sedentary organisms that inhabit the marine intertidal zone must be adapted to withstand forms of environmental stress that are uncommon in other marine environments. Adaptations to these stressors are apparent in the morphologies, behaviors, life histories, and regulation and products of genes of sedentary intertidal organisms (Lent 1969; Garrity 1984; Schmidt et al 2008). However, for those species that have a planktonic larval stage, evolutionary specialization at the molecular level could be constrained by opposing requirements for larvae and adults that are adapted to very different environments. This constraint would have been removed in lineages that lost the ancestral larval stage, and reinstated in lineages that secondarily reacquired a larval stage. A comparative approach using a group in which the planktonic larval stage has been lost and reacquired could be used to test the hypothesis that adaptive evolution of stress-related genes accompanies these life-history transitions. Genes with known roles in physiological stress tolerance include those that encode the family of 70 kilodalton heat shock proteins (<i>hsp70s </i>) (Feder and Hofmann 1999). Statistical tests for departures from selective neutrality were applied to sequence variation in <i>hsp70 </i> genes of calyptraeid gastropods. The Calpytraeidae include both intertidal and subtidal species, as well as species that have lost or secondarily reacquired the ancestral planktonic larval stage (Collin 2003a; Collin et al. 2007). I focused on two species: <i>Crepidula fornicata</i> and <i>C. atrasolea</i>. Both species are found in low intertidal and subtidal habitats in the western tlantic region. However, whereas the eggs of <i>C. fornicata</i> hatch into planktonic veligers, the eggs of <i>C. atrasolea</i> develop directly into crawling juveniles. I tested for selection with statistics that characterize the distribution of sequence variation within and between species, comparing values for each <i> hsp70</i> gene fragment with expectations for selectively neutral loci.