Genetic architectures of postmating isolation and morphology of two highly diverged rockfishes (genus Sebastes)

Postmating isolation is thought to be an important driver of the late stages of speciation. However, relatively little is empirically known about the process compared to other isolating mechanisms that drive the early stages of speciation, especially in non-model organisms. We characterized the genetic architecture of postmating isolation between two rockfishes, Sebastes schlegelii and S. trivittatus, whose reproductive isolation is complete. We examined transmission ratio distortion (TRD) patterns of genetic markers in two reciprocal backcross populations. Markers showing either of the two types of TRD was widespread across the genome, with some of the distorted markers forming extensive clusters around the recombination coldspots. These suggest that the postmating isolation effectively prevents gene flow across the genome and the recombination landscape contributes to the genetic architecture. Comparisons between two backcross families and two developmental stages showed little similarity in the distorted markers, suggesting asymmetry and stage-specificity of the isolation. This may be due to hybrid incompatibility involving maternal factors or extrinsic selection. The lack of sex-ratio distortion in the mapping families suggested that Haldane's rule in terms of hybrid inviability does not hold. Additionally, QTL mapping detected significant QTLs for sex and the morphological traits relevant to speciation and convergence of rockfishes, including body coloration. Genes in the melanocortin system, including agouti-signaling protein 1 (asip1) and melanocortin 1 receptor (mc1r), might underlie the horizontal and vertical color patterns on the body, respectively. These findings constitute an essential step towards a comprehensive understanding of speciation and morphological diversification of rockfishes.

Methods
Linkage maps  were constructed using ddRAD-seq data of two backcross families of Sebastes trivittatus and S. schlegelii (femael F1 x male S. trivittatus and female F1 x male S. schlegelii). Phenotypic data were also obtained from the two families. More details are provided in the accompnying readme file and the manuscript.