Journal
PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY B-BIOLOGICAL SCIENCES
Volume 376, Issue 1833, Pages -Publisher
ROYAL SOC
DOI: 10.1098/rstb.2020.0186
Keywords
transposable element; endogenous retrovirus; transcriptome; sex chromosome; W chromosome; Haldane's rule
Categories
Funding
- Swedish Research Council Formas [2017-01597, 2018-01008]
- Swedish Research Council Vetenskapsradet [2016-05139, 2018-03017, 621-2014-5113, 2019-03900]
- SciLife-Lab Swedish Biodiversity Program [2015-R14]
- Villum Foundation [15560]
- Carlsberg Foundation [CF17-0248]
- National Geographic Research and Exploration grant [8853-10]
- Dybron Hoffs Foundation
- Corrit Foundation
- Formas [2018-01008] Funding Source: Formas
- Swedish Research Council [2016-05139, 2019-03900, 2018-03017] Funding Source: Swedish Research Council
- Vinnova [2019-03900, 2018-01008, 2016-05139] Funding Source: Vinnova
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Studies show that in birds with highly heteromorphic sex chromosomes, the W chromosome has a high density of transposable elements and contains a large number of potentially active endogenous retroviruses, which may lead to a higher mutational load in females. This could have implications for genetic incompatibilities between species through TE/repressor mismatches in hybrids.
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)'.
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