An Investigation of Population Genetic Structure and Natural Selection in the Blue Crab, Callinectes sapidus, Using Nuclear Gene Sequences

Research on the population genetics of marine invertebrates has long been grounded on the assumptions that planktonic larvae disperse widely and do not experience significant levels of selection. These assumptions were bolstered by studies that found widespread genetic homogeneity in some species. However, an increasing number of studies show significant genetic differentiation can occur across a range of geographic scales. Recent evidence also suggests that larvae seldom reach their dispersal potential. Oceanographic and physical barriers, variation in pelagic larval duration, and larval behavior can all reduce dispersal distance. In addition, some studies also suggest gene flow can be limited by selection acting on dispersing larvae or on early life stages following recruitment. The spatial and temporal genetic population structure of blue crabs, Callinectes sapidus, was examined across nine locations on the Louisiana Coast using sequences of five nuclear protein-coding genes. Selection was also investigated in blue crabs with environmental association analyses and neutrality tests using both population and interspecific divergence data. Significant genetic differentiation was found across locations in 2010, but not between a subset of locations that were sampled in 2011. Temporal genetic differentiation was found among years for all locations combined. Despite the evidence for genetic structure, megalopae collected in 2010 did not differ significantly from the adults and juveniles collected at the same locations in either 2010 or 2011, indicating blue crabs along the Louisiana Coast experience high levels of larval mixing and gene flow. Alleles at three loci were significantly associated with water temperature across locations in 2010. One allele for hsp was also significantly associated with a combined water temperature and salinity variable. These results demonstrate that despite having a life history expected to ensure widespread larval dispersal, slight but significant genetic differentiation can develop among populations and may be the result of selection acting on certain alleles. Interspecific neutrality tests using data from C. sapidus and its congener C. similis also provide strong evidence that tps may be under selection. Further research into the functional differences of the alleles at these loci may improve our understanding of adaptation in C. sapidus..