In humans and most other mammals the new mutations that drive Darwinian evolution are inherited more from fathers than mothers. On average people acquire three or four times more genetic changes in the paternal than the maternal half of their genome – and in chimpanzees the paternal contribution is eight or nine times higher than the maternal, according to a study in the journal Science.
The reason for the uneven balance is that, while females are born with a lifetime supply of eggs, the male reproductive system keeps on producing new sperm through cell division. Errors in this process produce random mutations (usually harmful but occasionally beneficial). So older fathers tend to pass on more mutations; in men every year of age results in about two extra mutations in their children.
Although geneticists have long known about this mutational gender bias, the technology for measuring it by sequencing the DNA of successive generations is only now available. Scientists at the Wellcome Trust Centre for Human Genetics in Oxford and the Biomedical Primate Research Centre in the Netherlands analysed a family of nine chimps spanning three generations and compared the results with human studies.
“In chimpanzees the father’s age has a much stronger effect on mutation rate – about one-and-a-half times that in humans,” says Gil McVean, the study leader. “As a result, a greater fraction of new mutations enter the population through males, around 90 per cent, compared to humans, where fathers account for 75 per cent.”
The probable explanation lies in the chimpanzees’ mating system, involving larger testes and more plentiful sperm production, which means more cell division and therefore more opportunities for mutations to take place.
Because chimps mature, reproduce and die younger than people – living about half as long as humans – the overall mutation rate is similar in the two species. Both pass on about 70 genetic changes to the next generation on average but in chimps a much higher proportion of those come from the father.
Mutation rates can help to date important evolutionary events such as the divergence of species from a common ancestor. This study suggests that, assuming mutation rates in past primates were similar to those today, the lineages leading to the two species separated as long as 13 million years ago – twice as long as previously thought.
But McVean does not want to push these conclusions too hard until further research has been done. He is keen next to study gorillas, which have small testes and produce less sperm than other great apes. If the theory holds, their mutation rates should show a lower paternal bias.