Non-protein coding regions of the Y-chromosome have been shown to influence the expression of hundreds of autosomal and X-linked genes in multiple species. This Y-linked regulatory variation (YRV) may provide the sex-specific variation in gene expression required for the adaptive evolution of sexually dimorphic traits. This requires that YRV exist within natural populations, influence fitness related traits, and be at least in part comprised of additive genetic variation (i.e. selection must be able to shape it). In this study we investigated the ability for selection to shape YRV using wild caught Drosophila melanogaster.
We collected wild D. melanogaster from a single locality to investigate the potential for selection to shape YRV. To determine if YRV exists within a population, we introgressed 30 Y-chromosomes into a single genetic background and assayed for variation in immune gene expression. To determine if YRV has any fitness consequences, we then tested these same Y-chromosome lines for differential responses to a real immune challenge. Lastly, we crossed 4 Y-chromosomes into 4 different genetic backgrounds to determine if intra-population YRV was comprised of additive genetic variation.
Our data suggest that intra-population YRV does exist and that this variation is correlated with survival to a bacterial pathogen. Furthermore, the magnitude of the YRV effect was quite large (~42.6% of genetic variation in our experimental lines). However, to our surprise, this variation was comprised entirely of non-additive genetic variation, suggesting that selection may not be able to act on immune-related YRV and may instead act as a significant constraint to the adaptive evolution of insect immune function. The extent to which this non-additive variation is constraining the adaptive evolution of immune response is currently being tested using experimental evolution on more realistic population level variation. Additional traits are also being investigated as it is possible for YRV to manifest differently for different YRV sensitive traits.