The avian W chromosome is a refugium for endogenous retroviruses with likely effects on female-biased mutational load and genetic incompatibilities.
It is a broadly observed pattern that the non-recombining regions of sex-limited chromosomes (Y and W) accumulate more repeats than the rest of the genome, even in species like birds with a low genome-wide repeat content. Here, we show that in birds with highly heteromorphic sex chromosomes, the W chromosome has a transposable element (TE) density of greater than 55% compared to the genome-wide density of less than 10%, and contains over half of all full-length (thus potentially active) endogenous retroviruses (ERVs) of the entire genome. Using RNA-seq and protein mass spectrometry data, we were able to detect signatures of female-specific ERV expression. We hypothesize that the avian W chromosome acts as a refugium for active ERVs, probably leading to female-biased mutational load that may influence female physiology similar to the 'toxic-Y' effect in Drosophila males. Furthermore, Haldane's rule predicts that the heterogametic sex has reduced fertility in hybrids. We propose that the excess of W-linked active ERVs over the rest of the genome may be an additional explanatory variable for Haldane's rule, with consequences for genetic incompatibilities between species through TE/repressor mismatches in hybrids. Together, our results suggest that the sequence content of female-specific W chromosomes can have effects far beyond sex determination and gene dosage. This article is part of the theme issue 'Challenging the paradigm in sex chromosome evolution: empirical and theoretical insights with a focus on vertebrates (Part II)'.
['Peona|Valentina|V|', 'Palacios-Gimenez|Octavio M|OM|', 'Blommaert|Julie|J|', 'Liu|Jing|J|', 'Haryoko|Tri|T|', 'Jønsson|Knud A|KA|', 'Irestedt|Martin|M|', 'Zhou|Qi|Q|', 'Jern|Patric|P|', 'Suh|Alexander|A|']
D000818:Animals D001717:Birds D020077:Endogenous Retroviruses D005260:Female D005298:Fertility D008297:Male D059645:Mutation Rate D012730:Sex Chromosomes D012737:Sex Factors D013045:Species Specificity
W chromosome sex chromosome transposable element Haldane's rule endogenous retrovirus transcriptome