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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.

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Abstract:We present the first report to our knowledge of an alien ascidian Molgula manhattensis DeKay, 1843 from Akkeshi-ko Estuary in Hokkaido. M. manhattensis was identified through molecular analysis using about 840 bp of nuclear 18S rRNA and about 546 bp of mitochondrial cytochrome C oxidase subunit 1 mt-CO1 genes, and by evaluating its morphological and biological characteristics. The 18S rRNA gene analysis and evaluations of the morphological and biological features are considered more preferable than mt-CO1 gene analysis for the identification of M. manhattensis, because the mt-CO1 genes of M. manhattensis and the congener M. provisionalis in the GenBank at National Center for Biotechnology Information NCBI are identical. Akkeshi-ko Estuary is a known production area of cultured oyster Crassostrea gigas ; the introduction of M. manhattensis possibly occurred during oyster cultch transfer from Honshu. Our findings show that the distribution of M. manhattensis has expanded over a wide range in Japan from Kyushu to Hokkaido during the 40 years since it was first reported in Japan.

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