Genomic landscape of reproductive isolation in Lucania killifish: The role of sex loci and salinity

<p>Adaptation to different environments can directly and indirectly generate reproductive isolation between species. Bluefin killifish (<i>Lucania goodei</i>) and rainwater killifish (<i>L. parva</i>) are sister species that have diverged across a salinity gradient and are reproductively isolated by habitat, behavioral, extrinsic, and intrinsic postzygotic ­­isolation. We asked if salinity adaptation contributes indirectly to other forms of reproductive isolation via linked selection and hypothesized that low recombination regions, such as sex chromosomes or chromosomal rearrangements, might facilitate this process. We conducted QTL mapping in backcrosses between <i>L. parva </i>and <i>L. goodei</i> to explore the genetic architecture of salinity tolerance, behavioral isolation, and intrinsic isolation. We mapped traits relative to a chromosome that has undergone a centric fusion in <i>L. parva</i> (relative to <i>L. goodei</i>). We found that the sex locus appears to be male determining (XX-XY), was located on the fused chromosome, and was implicated in intrinsic isolation. QTL associated with salinity tolerance were spread across the genome and did not overly co-localize with regions associated with behavioral or intrinsic isolation. This preliminary analysis of the genetic architecture of reproductive isolation between <i>Lucania</i> species does not support the hypothesis that divergent natural selection for salinity tolerance led to behavioral and intrinsic isolation as a byproduct. Combined with previous studies in this system, our work suggests that adaptation as a function of salinity contributes to habitat isolation and that reinforcement may have contributed to the evolution of behavioral isolation instead, possibly facilitated by linkage between behavioral isolation and intrinsic isolation loci on the fused chromosome.</p>