Selection of the sex-linked in mountain pine beetle () driven by enhanced expression during early overwintering.

The mountain pine beetle () is an insect native to western North America; however, its geographical range has recently expanded north in BC and east into Alberta. To understand the population structure in the areas of expansion, 16 gene-linked microsatellites were screened and compared to neutral microsatellites using outlier analyses of and values. One sex-linked gene, (), showed a strong signature of positive selection for neo-X alleles and was analyzed for evidence of adaptive variation. Alleles of were sequenced, and differences between the neo-X and neo-Y alleles were consistent with neutral evolution suggesting that the neo-Y allele may not be under functional constraints. Neo-Y alleles were amplified from gDNA, but not effectively from cDNA, suggesting that there was little expression from neo-Y alleles. There were no differences in overall expression between males and females with the common northern neo-X allele suggesting that the neo-X allele in males compensates for the reduced expression of neo-Y alleles. However, males lacking the most common northern neo-X allele thought to be selected for in northern populations had reduced overall expression in early October-at a time when beetles are preparing for overwintering. This suggests that the most common allele may have more rapid upregulation. The reduced function of neo-Y alleles of suggested by both sequence differences and lower levels of expression may foster a highly selective environment for neo-X alleles such as the common northern allele with more efficient upregulation.