Chromosome diversityA measure of diversity on X chromosomes compared with other chromosomes (Nx/Na) was higher than the expected value (black line).Hammer, et al. Men who have children with multiple women spread genetic
diversity along with their wild oats, a new study shows.
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 University of Arizona
in Tucson present
evidence in the Sept. 26 PLoS Genetics
that polygyny, the practice of men siring children from many different women,
accounts for the pattern of extra diversity.
“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 University
of California, Berkeley.
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
geneticist at Stanford
University..
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.
Found in: Genes & Cells
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It does stand to reason that unequally distributed polygyny -- some men's having more children at the expense of other men's having fewer -- would reduce overall diversity. But this isn't strictly the same question as that of polygyny itself; it's more to do with women's greater (on average) selectivity. Further, since the Y chromosome is much smaller, less would seem to be at stake in its reduced diversity.
This is the reverse of what they do.
What they do is cut diversity for other chromosomes, by over-contributing genes for the others. The net effect is that the chromosome they contribute half as much has more diversity.
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