The unexpected diversity comes courtesy of randy males
DNA analysis of nonfunctional regions on the X chromosome and on the non-sex chromosomes (called autosomes) in six different groups of people from three continents reveals that the X chromosome is more genetically diverse than would be expected if men and women were equally successful at passing along their genes.
A group of researchers led by Michael Hammer at the
“Overall, these results underscore the importance of
sex-specific demographic factors in understanding the history of human
populations and patterns of diversity in our genome,” says John Pool, a
population geneticist at the
In humans, women carry two copies of the X chromosome while men have one X and one Y chromosome. Both men and women have two copies of each of the other 22 chromosomes. Since there are fewer X chromosomes (three X chromosomes for every four autosomes) spread throughout the population, scientists also expected the X chromosome to be less diverse than the autosomes (about 25 percent less diverse).
Instead, an examination of DNA from six populations of people — including the Biaka from the Central African Republic, Mandenka of Senegal, San from Namibia, Melanesians in Papua New Guinea, French Basque and Han Chinese — revealed that X chromosomes contain as many DNA variations as the autosomes, and sometimes more.
Hammer and his colleagues sampled DNA from parts of the chromosomes that are far from known genes and that don’t appear to have any function. That reduces the chances that natural selection acts to increase diversity in those regions and allows the team to look at how social behavior might affect the genome.
“The regions we chose, we presume, are nonfunctional, so diversity should reflect history and not selection,” Hammer says.
Other explanations, including selection weeding out deleterious mutations, migration patterns and population bottlenecks, were also considered as the source of X chromosome diversity. The researchers rejected each of these alternatives.
“It’s undoubtable that some of these other forces are
playing a role in generating diversity, but they aren’t sufficient by
themselves to create the pattern we see,” says Dmitri Petrov, a population
Men like Clint Eastwood, who has at least seven children with five different women, or Genghis Khan, whose genetic legacy is found in about 8 percent of men in a region stretching from northeast China to Uzbekistan, have successfully passed along their genes. But one man’s success comes at the expense of other men’s, barring them from contributing to the gene pool, Petrov says. Over many generations, such skewing of reproductive success led to increased X chromosome diversity and decreased diversity on the Y chromosome, a phenomenon Hammer and others demonstrated in previous studies.
But that’s not an entirely bad thing, Petrov says. “I would say that the health of the genes on the X chromosome is better than it would be without polygyny.”
It remains to be seen whether current human mating practices will change the amount of diversity on the X chromosome in the future.
Hammer, M.F., Mendez, F.L., Cox, M.P., Woerner, A.E. and Wall, J.D. 2008. Sex-Biased Evolutionary Forces Shape Genomic Patterns of Human Diversity. PLoS Genetics 4(Sept. 26): e1000202. doi:10.1371/journal.pgen.1000202