Is the post-Neolithic Y chromosome bottleneck necessarily explained by violent competition between lineages?
Résumé
Most modern human populations are structured in lineages and/or clans (54%) and display unilineal descent rules, with individuals affiliated to the kinship group of their father (patrilineality, 40%) or of their mother (matrilineality, 14%) only. Moreover, residence rules – defining the place where a couple lives after marriage – such as patrilocality (64%) or matrilocality (20%), involving a differential migration of individuals between populations according to their sex, are also over-represented compared to residence rules that do not take into account the sex of individuals. Therefore, kinship systems often cause sex-biased demographic processes, prone to leave a genetic signature. Around 7.000 y.a., human populations from the entire world went through a strong decrease of Y chromosome diversity compared to mtDNA. This event has been interpreted by Zeng et al. (2018, Nature Communication) as the result of a violent competition between patrilineal lineages. Indeed, because related men are gathered in a lineage, Y chromosome is expected to be very homogeneous within a lineage and differentiated between two lineages. The massacre of some lineages by other lineages would then produce a major decrease of Y chromosome diversity within the population. In order to confront this scenario to other hypotheses, we developed a model of populations displaying different descent and residence rules, with or without competition between lineages, and for each scenario, we computed diversity estimators on the Y chromosome and mtDNA. Obtained results suggest that if violent competition between lineages strongly decreases diversity on the Y chromosome, patrilineality alone generates a major bottleneck on the Y chromosome with respect to mtDNA.