Abstract
When closely related individuals mate, they produce inbred offspring, which often have lower fitness than outbred ones. Geographical exogamy, by favouring matings between distant individuals, is thought to be an inbreeding avoidance mechanism; however, no data has clearly tested this prediction. Here, we took advantage of the diversity of matrimonial systems in humans to explore the impact of geographical exogamy on genetic diversity and inbreeding. We collected ethno-demographic data for 1,344 individuals in 16 populations from two Inner Asian cultural groups with contrasting dispersal behaviours (Turko-Mongols and Indo-Iranians) and genotyped genome-wide single nucleotide polymorphisms in 503 individuals. We estimated the population exogamy rate and confirmed the expected dispersal differences: Turko-Mongols are geographically more exogamous than Indo-Iranians. Unexpectedly, across populations, exogamy patterns correlated neither with the proportion of inbred individuals nor with their genetic diversity. Even more surprisingly, among Turko-Mongols, descendants from exogamous couples were significantly more inbred than descendants from endogamous couples, except for large distances (>40 km). Overall, 37% of the descendants from exogamous couples were closely inbred. This suggests that in Inner Asia, geographical exogamy is neither efficient in increasing genetic diversity nor in avoiding inbreeding, which might be due to kinship endogamy despite the occurrence of dispersal.
Highlights
At least 10% of humans descend from parents that are related as second cousins or closer, based on registers as well as civil and medical surveys[1]
To explore the impact of geographical exogamy on genetic diversity at both the population and individual scales, we jointly analysed ethnological, geographical and genome-wide data in 16 populations corresponding to 11 distinct ethnic groups from Inner Asia, i.e., in Uzbekistan, Kyrgyzstan, Tajikistan, West Mongolia and South Siberia
Based on the local minimum of the distance densities, we set the limit for geographical exogamy at 4 km and found the percentage of exogamous couples per population to be significantly higher in the 12 Turko-Mongol populations (60% on average, from 33% to 84%) than in the four Indo-Iranians (28% on average, from 6% to 60%) (Supplementary Fig. 1A, Supplementary Table 1, one-tailed MWU test p-value = 0.021)
Summary
At least 10% of humans descend from parents that are related as second cousins or closer, based on registers as well as civil and medical surveys[1]. Their genomes contain multiple chromosomal segments that are identical by descent[6] (i.e., that are inherited from a common ancestor) These segments, called runs of homozygosity (ROHs), consist of a succession of homozygous sites[7] that subsequently increase the risk for deleterious recessive mutations to be phenotypically expressed[8] and reduce the fitness at loci under heterozygote advantage[9]. One line of evidence supporting this hypothesis comes from the negative relationship observed between exogamy and individual inbreeding in inhabitants from the Orkney Islands of Scotland[43] Despite this observation, there is a lack of data, whether in humans or in other species, to better understand the general impact of geographical distance between mates on both mating choice inbreeding and drift inbreeding patterns, as well as on population genetic diversity. This is the first time that such quantitative data have been combined in humans or in any other species
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