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Abstract:
The human and chimpanzee X chromosomes are less divergent than expected based on
autosomal divergence. We study incomplete lineage sorting patterns between humans,
chimpanzees and gorillas to show that this low divergence can be entirely explained by
megabase-sized regions comprising one-third of the X chromosome, where polymorphism
in the human-chimpanzee ancestral species was severely reduced. We show that background
selection can explain at most 10% of this reduction of diversity in the ancestor.
Instead, we show that several strong selective sweeps in the ancestral species can explain
it. We also report evidence of population specific sweeps in extant humans that overlap the
regions of low diversity in the ancestral species. These regions further correspond to chromosomal
sections shown to be devoid of Neanderthal introgression into modern humans.
This suggests that the same X-linked regions that undergo selective sweeps are among the
first to form reproductive barriers between diverging species. We hypothesize that meiotic
drive is the underlying mechanism causing these two observations.
