X and Y chromosomes: Sex and death 9/10/03. By Ian Jones Photos of our chromosomes graphically illustrate the demise of the Y chromosome. Compared with monsters such as chromosome 1, it is tiny, dwarfed by its former partner, the X. Is the Y chromosome destined for oblivion?

The absence of recombination with a homologous partner means that it can never be 'repaired' by recombination. This has led to suggestions that the Y is destined for extinction – it will eventually dwindle to nothing. According to this model, its role in sex determination will eventually be taken on by genes elsewhere in the genome.

That may seem hard to imagine, but the XY method of sex determination is far from universal. In fruit flies the ratio of X chromosomes to autosomes is crucial. In crocodiles and other reptiles, chromosomes have a marginal role – the temperature at which eggs are incubated is key. Slipper limpets, which form stacks attached to seashore rocks, change sex according to their position in the stack. Even more bizarre is the marine worm, Bonellia viridis. Its larvae settle on the ocean floor and turn into 10-cm long females. Other larvae are attracted to the worm's proboscis, and when they land on it they are ingested and turn into tiny sperm-producing symbiotic males in the female gut.

Perhaps a more likely glimpse of the future of humans comes from obscure mammals in which SRY and the Y have already been superseded. The Armenian mole vole, for example, has diverged into two species, one of which survives without a Y or SRY. In some South American field mice and the Scandinavian wood lemming, XY females are commonplace. In the lemmings, a gene on the X chromosome can overpower SRY, when present, making embryos female. Professor Jennifer Marshall Graves, an expert in comparative genomics and mechanisms of sex determination at the Australian National University in Canberra, believes these oddities hint at the destiny of the human Y. "The rodents," she concludes, "are leading us into the new era of a Y-less existence."

A more optimistic interpretation is given by Professor David Page of the Whitehead Institute in Cambridge, USA, who led the team that sequenced the human Y chromosome and studies the genetic mechanisms underpinning human sex determination. Sequencing has revealed just what a mishmash of odd features the Y chromosome is, rich in repeats and with its DNA constantly shuffling around. Professor Page believes one oddity – a pair of large palindromes, identical repeats arranged in opposite orientations on the chromosome – may provide a mechanism to safeguard Y sequences.

Rather than swapping DNA with the X, the Y may exchange DNA with itself, in a recombination process mediated by the palindromes. The palindromes contain genes, so in effect the Y could be able to repair itself, replacing bad sequence with good. It's not all over for the Y chromosome just yet, suggests Professor Page.

Not everyone is convinced, however. As Mark Jobling points out, the evolutionary cost of this mechanism is high: bad sequence may overwrite good, spreading mutations that cause infertility. So, is the Y here to stay or are we going to go the way of the mole vole? Or, more intriguingly still, might both scenarios come to pass – could we end up going down divergent evolutionary pathways, with the creation of separate human species?

Further reading

Graves J A M (2002) Sex chromosomes and sex determination in weird mammals. Cytogenet. Genome Res, 96: 161–168. Abstract

Skaletsky H et al (2003) The male-specific region of the human Y chromosome is a mosaic of discrete sequence classes. Nature 423: 825–37. Abstract

Rozen S et al (2003) Abundant gene conversion between arms of palindromes in human and ape Y chromosomes. Nature 423: 873–76. Abstract