The avian W chromosome is a refugium for endogenous retroviruses with likely effects on female-biased mutational load and genetic incompatibilities

Valentina Peona, Octavio M. Palacios-Gimenez, Julie Blommaert, Jing Liu, Tri Haryoko, Knud A. Jønsson, Martin Irestedt, Qi Zhou, Patric Jern, Alexander Suh

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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.
Original languageEnglish
Article number20200186
JournalPhilosophical Transactions of the Royal Society B: Biological Sciences
Issue number1833
Early online date26 Jul 2021
Publication statusPublished - 13 Sep 2021


  • Haldane's rule
  • W chromosome
  • endogenous retrovirus
  • sex chromosome
  • transcriptome
  • transposable element

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