Abstract
The W/Y chromosome is unique among chromosomes as it does not recombine in its mature form. The main side effect of cessation of recombination is evolutionary instability and degeneration of the W/Y chromosome, or frequent W/Y chromosome turnovers. Another important feature of W/Y chromosome degeneration is transposable element (TEs) accumulation. Transposon accumulation has been confirmed for all W/Y chromosomes that have been sequenced so far. Models of W/Y chromosome instability include the assemblage of deleterious mutations in protein coding genes, but do not include the influence of transposable elements that are accumulated gradually in the non-recombining genome. The multiple roles of genomic TEs, and the interactions between retrotransposons and genome defense proteins are currently being studied intensively. Small RNAs originating from retrotransposon transcripts appear to be, in some cases, the only mediators of W/Y chromosome function. Based on the review of the most recent publications, we present knowledge on W/Y evolution in relation to retrotransposable element accumulation.
Highlights
The W and Y chromosomes differ from the other chromosomes, mainly as they do not have homologous partners and do not recombine in their mature form (Charlesworth 1996; Rice 1996)
Once genetic sex determination is located on a heteromorphic chromosomal system, selection for alleles that are advantageous in males but disadvantageous to females can lead to further genetic differentiation between the two sex chromosomes at other loci
By reviewing the recent discoveries in the areas of nonrecombining chromosome evolution, dynamics and evolution of transposable elements (TEs), as well as the genome defense system, we have attempted to show the interactions between these three molecular fields
Summary
The W and Y chromosomes differ from the other chromosomes, mainly as they do not have homologous partners and do not recombine in their mature form (Charlesworth 1996; Rice 1996). The beginning of transposable element accumulation on the W/Y chromosome (Fig. 1) may start from several insertions in the close neighborhood of crucial heterogametic sex-linked genes This position guarantees that functional TEs can escape the removal from the population due to stochastic population processes, methylation/heterochromatization (Hollister and Gaut 2009), or Y chromosome contraction. TEs linked to the heterogametic sex-determining genes should be the most active and successful because they possess the ability to produce functional transcripts over long evolutionary time scales (reflecting the view of the genome as an ecosystem with TEs as individual members of a species, reviewed in Hua-Van et al 2011) Such processes may result from slower pseudogenisation and heterochromatization of functional TEs located in the neighborhood of functionally important genes in the condition of no-recombination (Mourier and Willerslev 2010). Degeneration of Y chromosome is prevented by rare recombination with X in phenotypic females
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