Diversifying Selection Underlies the Origin of Allozyme Polymorphism at the Phosphoglucose Isomerase Locus in Tigriopus californicus

The marine copepod Tigriopus californicus lives in intertidal rock pools along the Pacific coast, where it exhibits strong, temporally stable population genetic structure. Previous allozyme surveys have found high frequency private alleles among neighboring subpopulations, indicating that there is limited genetic exchange between populations. Here we evaluate the factors responsible for the diversification and maintenance of alleles at the phosphoglucose isomerase (Pgi) locus by evaluating patterns of nucleotide variation underlying previously identified allozyme polymorphism. Copepods were sampled from eleven sites throughout California and Baja California, revealing deep genetic structure among populations as well as genetic variability within populations. Evidence of recombination is limited to the sample from Pescadero and there is no support for linkage disequilibrium across the Pgi locus. Neutrality tests and codon-based models of substitution suggest the action of natural selection due to elevated non-synonymous substitutions at a small number of sites in Pgi. Two sites are identified as the charge-changing residues underlying allozyme polymorphisms in T. californicus. A reanalysis of allozyme variation at several focal populations, spanning a period of 26 years and over 200 generations, shows that Pgi alleles are maintained without notable frequency changes. Our data suggest that diversifying selection accounted for the origin of Pgi allozymes, while McDonald-Kreitman tests and the temporal stability of private allozyme alleles suggests that balancing selection may be involved in the maintenance of amino acid polymorphisms within populations

Seascape Genomics: Contextualizing Adaptive and Neutral Genomic Variation in the Ocean Environment

Seventy-one per cent of the earth’s surface is covered by ocean which contains almost 80% of the world’s phyla – “seascape genomics” is the study of how spatial dependence and environmental features in the ocean influence the geographic structure of genomic patterns in marine organisms. The field extends from seascape genetics where the study of small numbers of neutral loci predominates, to additionally consider larger numbers of loci from throughout the genome that may be of some functional or adaptive significance and are subject to selection. Seascape genomics is conceptually similar to landscape genomics; the disciplines share theoretical underpinnings, and the genetic measures and analytical methods are often the same. However, the spatio-temporal variability of the physical ocean environment and the biological characteristics of marine organisms (e.g. large population sizes and high dispersal ability) present some characteristic challenges and opportunities for spatial population genomics studies. This chapter provides an overview of the field of seascape genomics, outlines concepts and methods to consider when conducting seascape genomics studies, and highlights future research avenues and opportunities for the application of seascape genomics to global issues affecting our marine environment