This morning Dienekes pointed out a new paper in the open access journal PLoS Genetics on polygyny and its impact on human genetic variation. Razib followed suite, providing a more in depth review of the study. I recommend you check out both. In this post, I’m also gonna have a stab at reviewing the paper since it has an important anthropological impact.
The paper, “Sex-Biased Evolutionary Forces Shape Genomic Patterns of Human Diversity,” is authored by some people you may have heard of, such as Michael Hammer and Jeffrey Wall among others. Like many population geneticists, they isolated the problems and limitations of previous studies which investigated genetic diversity of humans from only markers on the mitochondrial DNA and non-recombining portion of the Y chromosome genome. They proposed that markers on the autosomal genome, including the X sex chromosome, will provide a more insightful understanding.
So they compared the genetic variation among 40 independent loci on the X chromosome and autosomes in 90 individuals from six different populations. 20 loci on the X chromosome and 20 on the autosomes were picked from non-coding regions of the genome.
Why wasn’t there a more even distribution of sites across the whole genome? Well, the authors specifically sought to seek out the impact of sex-specific processes, such as mating patterns, in shaping genomic patterns of variability. Both Razib and Dienekes do an excellent job in explaining this, but I’ll snip what Razib wrote since it is more clear in my mind:
“Assuming equal numbers of males and females in any given generation you expected a ratio of diversity of 0.75 between the X and the autosomes; remember that the number of copies of X circulating within the population are reduced by 25% because males carry only one copy, while women carry two.”
In other words, the X chromosome is present in two copies in females and a single copy in males. We all know that. We expect that the other chromosomes will show more genetic diversity than the X chromosome in a population with an equal number of breeding males and females because they are inherited equally by both sexes from each parent. In a populations with an unequal number of breeding males to females, we should see something different. Actually, we expect to see more genetic diversity on the X chromosome than on the other chromosomes in areas where men don’t get to pass on their genes, while most women do.
The authors’ samples included individuals from Africa, such as the Biaka of the Central African Republic, the Mandenka from Senegal, and the San from Namibia were included. Outside of Africa, the French Basque, the Han Chinese and Melanesians were also sampled. Roughly 210kb of DNA was sequenced from each of these individuals, and a basic statistical summary of the nucleotide diversity in six human populations was conducted. Comparing the observed nucleotide diversity on the X chromosome to the chromosomes showed that there was more genetic differences in the X chromosome than would be expected if equal numbers of males and females tended to mate.
Even though I explained this in two paragraphs above, polygyny could be the only reason why we see such results. Some men just didn’t get a chance to pass on their genes. The authors even made sure to rule out background selection, changes in population size and sex-specific migration in their conclusion. Only the process of polygyny could account for the sex ratio skew and resulting patterns of genomic variation. By this process, fewer unique male genes are being passed into the next generation.
In the same issue, a very similar paper was also published that I don’t think many other people noticed. A separate team of academics applied this multilocus approach to the genetic diversity of Central Asia. It is published under the title, “Sex-Specific Genetic Structure and Social Organization in Central Asia: Insights from a Multi-Locus Study.” Their sample included 10 populations of bilineal agriculturalists and 11 populations of patrilineal herders from West Uzbekistan to East Kyrgyzstan. Bilineal means that there’s an even migration of men and women while patrilineal means there’s an uneven migration of women to their husband’s location. In total, their sample size represents 780 healthy adult men from 5 ethnic groups: Tajiks, Kyrgyz, Karakalpaks, Kazaks, and Turkmen. They conclude that the number of reproductive individuals is likely to be higher for women among patrilineal populations.
Both these studies show that the organization and structure of patrilineal populations is the likely cause of the observed genetic patterns, where men tend to father children with more females than females do with males despite institutionalized monogamy.
- Michael F. Hammer, Fernando L. Mendez, Murray P. Cox, August E. Woerner, Jeffrey D. Wall, Dmitri A. Petrov (2008). Sex-Biased Evolutionary Forces Shape Genomic Patterns of Human Diversity PLoS Genetics, 4 (9) DOI: 10.1371/journal.pgen.1000202
- Laure Ségurel, Begoña Martínez-Cruz, Lluis Quintana-Murci, Patricia Balaresque, Myriam Georges, Tatiana Hegay, Almaz Aldashev, Firuza Nasyrova, Mark A. Jobling, Evelyne Heyer, Renaud Vitalis, Molly Przeworski (2008). Sex-Specific Genetic Structure and Social Organization in Central Asia: Insights from a Multi-Locus Study PLoS Genetics, 4 (9) DOI: 10.1371/journal.pgen.1000200