Spatial and temporal population genetic structure of five northeastern Pacific littorinid gastropod species
This thesis is an investigation of the spatial and temporal population genetic structure of five northeastern Pacific littorinid gastropods that comprise three species with, and two species without a free-swimming and feeding larval phase. These 'Littorina' species share similar ecological niches and thus offer an ideal system to evaluate the effect of the presence or absence of a planktotrophic larval phase on their spatial and temporal genetic structure. Although many studies of spatial genetic structure exist, this thesis is the first to compare temporal genetic structure between direct-developing and planktotrophic marine benthic invertebrate species. The first chapter is an analysis of mitochondrial DNA variation in the planktotrophic species, ' Littorina keenae'. The analysis revealed no significant spatial population structure along approximately 2,000 km of California coastline, but two examples of a significant temporal structure between samples that were 2-3 years and 10 years apart. These data suggest that gene flow among the populations is high probably because they have a planktotrophic larval stage, while their effective population size ('Ne') is small enough to be strongly affected by genetic drift. This high level of on-going gene flow might prevent 'L. keenae' populations from becoming adapted to thermally varying local habitats. The second chapter is an ecophysiological study that found a non-linear negative relationship between thermal tolerance and degrees of latitude of the collecting sites for 'L. keenae'. The third chapter is an investigation of mitochondrial DNA variation in four ' Littorina' species using 414 bp of cytochrome 'b' gene. The analysis suggests that two planktotrophic species, 'L. scutulata ' and 'L. plena' have significantly higher levels of temporal variation, but lower levels of spatial variation in their genetic structure than does the direct-developing species, 'L. subrotundata '. To resolve the spatial and temporal genetic structure of the four species more conclusively, the fourth and final chapter extends the mitochondrial DNA data set by adding two single-copy nuclear DNA markers. Both marker sets yield similar patterns of genetic structure: the two direct-developing species have more spatial structure than the two planktotrophic species, whereas the latter species have more temporal variation in genetic structure.