Dynamics of vertebrate sex chromosome evolution: from equal size to giants and dwarfs

Abstract

The Y and W chromosomes of mammals and birds are known to be small because most of their genetic content degenerated and were lost due to absence of recombination with the X or Z, respectively. Thus, a picture has emerged of ever-shrinking Ys and Ws that may finally even fade into disappearance. We review here the large amount of literature on sex chromosomes in vertebrate species and find by taking a closer look, particularly at the sex chromosomes of fishes, amphibians and reptiles where several groups have evolutionary younger chromosomes than those of mammals and birds, that the perception of sex chromosomes being doomed to size reduction is incomplete. Here, sex-determining mechanisms show a high turnover and new sex chromosomes appear repeatedly. In many species, Ys and Ws are larger than their X and Z counterparts. This brings up intriguing perspectives regarding the evolutionary dynamics of sex chromosomes. It can be concluded that, due to accumulation of repetitive DNA and transposons, the Y and W chromosomes can increase in size during the initial phase of their differentiation.

Cytogenetic Characterization of a Population of <b><i>Acanthodactylus lineomaculatus</i></b> Duméril and Bibron, 1839 (Reptilia, Lacertidae), from Southwestern Morocco and Insights into Sex Chromosome Evolution

“…In this context, the presence of het-ITSs within the long arm of the W chromosome of A . lineomaculatus could be linked to structural changes that probably occurred in the early stages of sex chromosome differentiation [see Schartl et al, 2016] in this taxon. The fact that the W chromosome of A .…”

Genomic Organization and Physical Mapping of Tandemly Arranged Repetitive DNAs in Sterlet<b> </b>(<b><i>Acipenser ruthenus</i></b>)

“…According to bioinformatics data, these repeats together with satellite ArutF167A are overrepresented in the female genome in comparison with the male one. Differences between homologous chromosomes are observed not only for these chromosomes and can be explained by intraspecific size variation of heterochromatic blocks not connected with sex, but the unusually high diversity of repetitive DNAs in these chromosomes is also characteristic for sex chromosomes in general [Schartl et al, 2016]. Currently there are many examples of rRNA genes present on sex chromosomes, for example 18S rDNA cistrons on the W chromosome of Triportheus species [Yano et al, 2014a] and 5S rDNA on the Y chromosome of some Antarctic fishes [Ghigliotti et al, 2013].…”

“…The small chromosomes entirely marked by repetitive DNAs are the most plausible candidates. It is well-known that due to suppression of recombination the proto-sex chromosome (either Y or W) accumulates a multitude of mutations and repetitive DNAs and gradually degenerates, reducing in size [Schartl et al, 2016]. For example, the comparison of the sex-determining regions shared by 3 salmonids revealed a minimal region (4.1 kb) triggering masculinization, which contains all elements necessary for transposition [Faber-Hammond et al, 2015].…”

“…Ubiquitous homomorphy of the sex chromosomes can be explained by their recent origin in the evolution, as the differentiation by repeat accumulation and gene degeneration can take a substantial amount of time [Schartl et al, 2016]. Sex-determining mechanisms of fish, contrary to birds and mammals, showed a rapid turnover, and new sex chromosomes appeared repeatedly [Ellegren, 2011].…”

“…There are many examples of fish sex chromosomes carrying an evident C-block [Cioffi and Bertollo, 2012]. It was proposed that in the evolutionarily young sex chromosomes of fish both Y and W chromosomes can increase in size during the initial phase of their differentiation due to the accumulation of repetitive DNA and transposons [Schartl et al, 2016].…”