Genome evolution and introgression in the New Zealand mud snails Potamopyrgus estuarinus and Potamopyrgus kaitunuparaoa

<p>We have sequenced, assembled, and analyzed the nuclear and mitochondrial genomes and transcriptomes of <i>Potamopyrgus estuarinus</i> and <i>Potamopyrgus kaitunuparaoa</i>, two prosobranch snail species native to New Zealand that together span the continuum from estuary to freshwater.<i> </i>These two species are the closest known relatives of the freshwater species <i>P. antipodarum—</i>a model for studying the evolution of sex, host-parasite coevolution, and biological invasiveness—and thus provide key evolutionary context for understanding its unusual biology. The <i>P. estuarinus</i> and <i>P. kaitunuparaoa </i>genomes are very similar in size and overall gene content. Comparative analyses of genome content indicate that these two species harbor a near-identical set of genes involved in meiosis and sperm functions, including seven genes with meiosis-specific functions. These results are consistent with obligate sexual reproduction in these two species and provide a framework for future analyses of <i>P. antipodarum—</i>a species comprising both obligately sexual and obligately asexual lineages, each separately derived from a sexual ancestor. Genome-wide multigene phylogenetic analyses indicate that <i>P. kaitunuparaoa</i> is likely the closest relative to <i>P. antipodarum. </i>We nevertheless show that there has been considerable introgression between <i>P. estuarinus</i> and <i>P. kaitunuparaoa.</i> That introgression does not extend to the mitochondrial genome, which appears to serve as a barrier to hybridization between <i>P. estuarinus </i>and <i>P. kaitunuparaoa.</i> Nuclear-encoded genes whose products function in joint mitochondrial-nuclear enzyme complexes exhibit similar patterns of non-introgression, indicating that incompatibilities between the mitochondrial and the nuclear genome may have prevented more extensive gene flow between these two species.<i>&nbsp;</i>&nbsp;</p>