Genomic landscape of reproductive isolation inLucaniakillifish: The role of sex chromosomes and salinity

Abstract

1 Understanding how speciation occurs and how reproductive barriers contribute to population 2 structure at a genomic scale requires elucidating the genetic architecture of reproductive isolating 3 barriers. In particular, it is crucial to determine if loci underlying reproductive isolation are 4 genetically linked or if they are located on sex chromosomes, which have unique inheritance and 5 population genetic properties. Bluefin killifish (Lucania goodei) and rainwater killifish (L. 6 parva) are closely related species that have diverged across a salinity gradient and are 7reproductively isolated by assortative mating, hybrid male infertility, viability of hybrid 8offspring at high salinities, as well as reduced overall fitness of F2 offspring and backcrosses to 9 L. goodei. We conducted QTL mapping in backcrosses between L. parva and L. goodei to 10 determine the genetic architecture of sex determination, mate attractiveness, fertility, and salinity 11 tolerance. We find that the sex locus appears to be male determining and located on a 12 chromosome that has undergone a Robertsonian fusion in L. parva relative to L. goodei. We find 13 that the sex locus on the fused chromosome is involved in several genomic incompatibilities, 14which affect the survival of backcrossed offspring. Among the backcrossed offspring that 15 survived to adulthood, we find that one QTL for male attractiveness to L. goodei females is 16 closely linked to this sex locus on chromosome 1. Males homozygous for L. goodei alleles at the 17 sex locus laid more eggs with L. goodei females. QTL associated with salinity tolerance were 18 spread across the genome but did not tend to co-localize with reproductive isolation. Thus, 19 speciation in this system appears to be driven by reinforcement and indirect selection against 20 hybrids rather than direct natural selection for salinity tolerance. Our work adds to growing 21 evidence that sex chromosome evolution may contribute to speciation. 22 23