Is the Y chromosome disappearing?—Both sides of the argument

Abstract

On August 31, 2011 at the 18th International Chromosome Conference in Manchester, Jenny Graves took on Jenn Hughes to debate the demise (or otherwise) of the mammalian Y chromosome. Sex chromosome evolution is an example of convergence; there are numerous examples of XY and ZW systems with varying degrees of differentiation and isolated examples of the Y disappearing in some lineages. It is agreed that the Y was once genetically identical to its partner and that the present-day human sex chromosomes retain only traces of their shared ancestry. The euchromatic portion of the male-specific region of the Y is~1/6 of the size of the X and has only~1/12 the number of genes. The big question however is whether this degradation will continue or whether it has reached a point of equilibrium. Jenny Graves argued that the Y chromosome is subject to higher rates of variation and inefficient selection and that Ys (and Ws) degrade inexorably. She argued that there is evidence that the Y in other mammals has undergone lineage-specific degradation and already disappeared in some rodent lineages. She also pointed out that there is practically nothing left of the original human Y and the added part of the human Y is degrading rapidly. Jenn Hughes on the other hand argued that the Y has not disappeared yet and it has been around for hundreds of millions of years. She stated that it has shown that it can outsmart genetic decay in the absence of "normal" recombination and that most of its genes on the human Y exhibit signs of purifying selection. She noted that it has added at least eight different genes, many of which have subsequently expanded in copy number, and that it has not lost any genes since the human and chimpanzee diverged~6 million years ago. The issue was put to the vote with an exact 50/50 split among the opinion of the audience; an interesting (though perhaps not entirely unexpected) skew however was noted in the sex ratio of those for and against the notion.

RNA sequencing reveals sexually dimorphic gene expression before gonadal differentiation in chicken and allows comprehensive annotation of the W-chromosome

“…Mammals have an XX/XY sex chromosome system, characterised by male heterogamety (XY), while birds have a ZZ/ZW system and female heterogamety (ZW). In eutherian mammals, almost all genes on the Y chromosome are associated with male development and fitness, namely the testis-determining SRY gene and genes required for spermatogenesis [2]. The Z and W sex chromosomes of birds evolved independently of the eutherian X and Y, and hence birds lack the SRY gene [3].…”

Tracking the evolution of sex chromosome systems in Melanoplinae grasshoppers through chromosomal mapping of repetitive DNA sequences

“…Non-recombining regions of the Y chromosome containing accumulated repetitive DNAs have been well documented in, for example, mammalian species [11,26] and Drosophila melanogaster [27], in which the sex chromosome systems are evolutionarily ancient [8,28]. The accumulation of repetitive sequences, even in young sex chromosomes, has also been observed in other organisms, such as Drosophila miranda [19], Silene latifolia [29-31] and Rumex acetosa [23].…”

Contribution of mobile elements to the uniqueness of human genome with more than 15,000 human-specific insertions

“…Assuming this bias is valid, how would it affect chromosome Y? Recently, there has been a heated debate among the science community about whether chromosome Y is disappearing or not [70]. Our result could serve as evidence that "chromosome Y has not disappeared yet" and HS-MEs may have contributed to the observed fast evolving pattern on chromosome Y after the human and chimpanzee divergence [48].…”