Extreme genomic variation in a natural population

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Abstract
Whole-genome sequence data from samples of natural populations provide fertile grounds for analyses of intraspecific variation and tests of population genetic theory. We show that the urochordate Ciona savignyi , one of the species of ocean-dwelling broadcast spawners commonly known as sea squirts, exhibits the highest rates of single-nucleotide and structural polymorphism ever comprehensively quantified in a multicellular organism. We demonstrate that the cause for the extreme heterozygosity is a large effective population size, and, consistent with prediction by the neutral theory, we find evidence of strong purifying selection. These results constitute in-depth insight into the dynamics of highly polymorphic genomes and provide important empirical support of population genetic theory as it pertains to population size, heterozygosity, and natural selection.