Human population structure and its effects on sampling Y chromosome sequence variation.

The excess of rare variants in global sequencing studies of the nonrecombining portion of the Y chromosome (NRY) has been interpreted as evidence for the effects of human demographic expansion. However, many NRY polymorphisms are geographically localized and the effect of different geographical sampling on patterns of NRY variation is unknown. We use two sampling designs to detect population structure and its effects on patterns of human NRY polymorphism. First, we sequence 26.5 kb of noncoding Y chromosome DNA from 92 globally distributed males representing 35 populations. We find that the number of polymorphisms with singleton variants is positively correlated with the number of populations sampled and that there is a significant negative correlation of Tajima's D (TD) and Fu and Li's D (FD) statistics with the number of pooled populations. We then sequence the same region in a total of 73 males sampled from 3 distinct populations and find that TD and FD values for the 3 pooled and individual population samples were much less negative than those in the aforementioned global sample. Coalescent simulations show that a simple splitting model of population structure, with no changes in population size, is sufficient to produce the negative values of TD seen in our pooled samples. These empirical and simulation results suggest that observed levels of NRY population structure may lead to an upward bias in the number of singleton variants in global surveys and call into question inferences of population expansion based on global sampling strategies.