Scientists have found that the Y chromosome is evolving more quickly than any other part of the human genetic code. In the first comparison of human and chimpanzee Y chromosomes, a team from the Whitehead Institute, Massachusetts, US, found that the two differ dramatically in structure and gene content. The finding was published in the journal Nature.
Chimpanzee and human lineages shared a common ancestor six million years ago and overall the DNA of the two species is very similar, only differing by around one per cent. The Whitehead Institute team found, however, that the genes on the Y chromosomes differed by more than 30 per cent. The chimp's Y chromosome had only two thirds as many distinct genes and only 47 per cent as many protein coding elements as the human Y chromosome. Even the genes that do match up have undergone rapid relocation.
The Y chromosome occurs on its own and has no partner during cell division. This had led scientists to believe that it would slowly waste away. But the findings indicate the Y chromosomes are changing more quickly than the rest of their respective genomes. The authors David Page and Jennifer Hughes suggest the divergent evolution may be due to several factors, including differences in mating behaviours and traits specific to Y chromosomes.
With chimps, several males may mate with a single female, setting up competition between the sperm of different males. Because of the important role of the Y chromosome in sperm production any genetic variation that improves a chimp's chances of reproducing will be favoured and quickly spread through the population. 'The region of the Y that is evolving the fastest is the part that plays a role in sperm production,' commented Hughes.
If a Y chromosome with advantageous sperm production genes also carries mutations that affect a gene not related to sperm production, they may also be passed on. This is able to happen because the Y chromosome has no partner to swap with in cell division. The Y chromosome is therefore treated by natural selection as one unit.
In chimps, improved sperm production may outweigh the disadvantages of an altered or deleted unrelated gene. This could result in a Y chromosome with fewer genes than the human Y chromosome.