Natural Selection Reduced Diversity on Human Y Chromosomes

The human y chromosome exhibits surprisingly low levels of genetic diversity. this could result from neutral processes if the effective population size of males is reduced relative to females due to a higher variance in the number of offspring from males than from females. alternatively,selection acting on new mutations,and affecting linked neutral sites,could reduce variability on the y chromosome. here,using genome-wide analyses of x,y,autosomal and mitochondrial dna,in combination with extensive population genetic simulations,we show that low observed y chromosome variability is not consistent with a purely neutral model. instead,we show that models of purifying selection are consistent with observed y diversity. further,the number of sites estimated to be under purifying selection greatly exceeds the number of y-linked coding sites,suggesting the importance of the highly repetitive ampliconic regions. while we show that purifying selection removing deleterious mutations can explain the low diversity on the y chromosome,we cannot exclude the possibility that positive selection acting on beneficial mutations could have also reduced diversity in linked neutral regions,and may have contributed to lowering human y chromosome diversity. because the functional significance of the ampliconic regions is poorly understood,our findings should motivate future research in this area. © 2014 wilson sayres et al.