Genetic structure of men and women
in Central Asia
Laure
1
Ségurel , Evelyne
1
Heyer &
Renaud
1
Vitalis
1
Unit of Eco-anthropology and ethnobiology
UMR5145 CNRS – MNHN – Université Paris 7, Musée de l’Homme, Paris
Background:
In the last two decades, the mitochondrial DNA (mtDNA) and the non-recombining portion of the Y chromosome
(NRY) have been extensively used in order to measure the maternally- and paternally-inherited genetic structure of
human populations. Most studies converge towards the notion that among populations, women are genetically less
structured than men. This could be due to a higher migration rate of women, or to their higher effective number.
Since most human populations are known to be patrilocal (a tendency for men to stay in their birthplace while
women move to their husband’s house), this has been mainly explained by a higher mobility of women. The
alternative hypothesis of their higher effective number, due for example to polygyny (multiple mating for men but not
women) or to a higher mortality rate in men, has often been neglected. Yet, since population structure (as reflected by
the F-st value) depends upon the product of the effective number of individuals and the migration rate, both forces
have confounding effects.
More, inferences based on the NRY and mtDNA are limited by the fact that each of them is a single genetic locus,
and is then subject to stochastic variability and potentially to selection side effects.
1
Fst (mtDNA) 
1  2N f m f
>
1
Fst ( NRY ) 
1  2 N m mm
Poorly structured
populations of women
Goal:
In order to test the robustness of this sex-specific genetic structure, we developed a multi-locus approach
based on the analysis of the rest of the genome: autosomal and X-linked markers. Our main question was : Is this
Highly structured
populations of men
multi-locus approach consistent with the sex-specific structure as measured with uniparentally markers ?
N.m (women) > N.m (men)
N.m : Number of migrant per generation
N : Effective number of individuals ; m : Migration rate
Sampling informations:
We sampled 780 healthy adult men of 21 populations with contrasted lifestyle and social
organizations, from West Uzbekistan to East Kyrgyzstan, (see Figure 1) :
- 10 populations of bilineal agriculturalists (10 Tajiks), i.e. organized into nuclear or extended
families where blood links and rights of inheritance through both male and female ancestors are of
equal importance. They preferentially establish endogamous marriages with cousins.
- 11 populations of patrilineal herders (6 Kyrgyz, 2 Karakalpaks, 2 Kazaks, and 1 Turkmen), i.e.
they are organized into paternal descent groups (tribes, clans, lineages). They practice exogamous
marriages, in which a man chooses a bride from a different clan.
We sequenced on these individuals 367bp of the first hypervariable segment of the mtDNA
control region (HVS-1) and we genotyped 11 Y-linked STRs markers, 27 autosomal STRs markers
and 9 X-linked STRs markers.
Figure 1: Geographic repartition of the 21 sampled populations.
mtDNA / Y chromosome :
Autosomes & the X chromosome :
Fst (mtDNA) = 0.034 and Fst (NRY)=0.069 in the bilineal populations and
Fst (mtDNA) = 0.010 and Fst (NRY)=0.177 in the patrilineal populations.
From these values, we found that the number of migrant per generation (Nm) is 20 times
higher in women compared to men in patrilineal populations, while it is only twice time
higher in women for bilineal groups. Therefore, both populations show signs of sexspecific structure, but patrilineal populations exhibit a more contrasted one. However, we
don’t know if this is due to the effect of migration rate or to effective number of individuals.
* In patrilineal populations, the Fst value on the autosomes [Fst (Auto) = 0.008] is
significantly higher than that of X chromosome [Fst (X) = 0.003], while there is no such
significant differences for bilineal populations [Fst (Auto) = 0.014 and Fst (X) = 0.013].
Thus, N (women) > N (men) in patrilineal populations.
* To have a better picture of the sex-specific demography in these two groups, we
compared our observed Fst (Auto) and Fst (X) with the expected values under an island
model of population, for each set of (Nf, Nm, mf, mm). Rejected set of (Nf, Nm, mf, mm), i.e.
a significant difference between the observed and expected values of Fst, are depicted
in dark grey on figure 2a and 2b. Thus the light grey area correspond to non rejected
sets of (Nf, Nm, mf, mm).
For patrilineal populations (fig. 2a), we can see that compatible scenarios correspond
to higher effective number for women, and for the vast majority, to higher migration rates
for women. On the contrary, for bilineal populations (fig. 2b), all combinations are
possible, including equal effective number and migration rate between men and women.
Figure 2a : Patrilineal populations
Proportion of
the women migration rate
* If women and men have the same demographic history, the theory predicts that, in an
island model of population structure, Fst (X) > Fst (Auto), because of a different intensity
of drift on these markers. If Fst (Auto) > Fst (X), then the model is only compatible with a
number of women higher than that of men, whatever the migration rates are.
Figure 2b : Bilineal populations
mf >mm
mf >mm
Non rejected
scenarios
Rejected
scenarios
Nf >Nm
Nf >Nm
Proportion of the effective number of women
Discussion:
mtDNA / NRY versus X chromosome / Autosomes : The information given by mtDNA and the NRY is represented by dotted lines in figure 2a and 2b. First of all, our study clearly shows
that all genetic systems (mtDNA, NRY, X-linked and autosomal markers) are consistent, as they converge toward the notion that patrilineal groups, in contrast to bilineal, have a strong sexspecific genetic structure. Yet, our multi-locus method is substantial, since we show that this is likely due to both higher migration rates and larger effective numbers for women than for men,
which result could not be obtained by the uni-parentally makers alone. Importantly, this implies that differences in effective numbers can shape the sex-specific genetic structure.
Impact of social organization : As our result does not hold for bilineal agriculturalist populations, we interpret the higher effective number of women as the consequence of the patrilineal
organization of herders. These populations are indeed organized in patrilineal descent groups (tribes, clans, lineages), and children are affiliated with the descent groups of the father. Chaix et
al. (2007) showed that the average number of individuals carrying the same Y chromosome haplotype was much higher in patrilineal populations than in bilineal populations (where children
are affiliated both to the mother and the father). Indeed, the descent groups are not formed randomly and related men tend to cluster together. This particular dynamics tends to increase
relatedness among men, and may therefore reduce the effective number of men, as compared to women.
However, our results could also be due for example to polygyny, or to a higher variance of reproductive success in men.
Finally, different cultural features can shape the sex-specific genetic structure in humans. Further study of populations with different behaviors will enable us to understand them.