COMPARATIVE NUCLEAR AND MITOCHONDRIAL GENOME DIVERSITY IN HUMANS AND CHIMPANZEES

Restriction mapping and sequencing have shown that humans have substan tially lower levels of mitochondrial genome diversity (d) than chimpan zees. In contrast, humans have substantially higher levels of heterozy gosity (H) at protein-coding loci, suggesting a higher level of divers ity in the nuclear genome. To investigate the discrepancy further, we sequenced a segment of the mitochondrial genome control region (CR) fr om 49 chimpanzees. The majority of these were from the Part troglodyte s verus subspecies, which was underrepresented in previous studies. We also estimated the average heterozygosity at 60 short tandem repeat ( STR) loci in both species. For a total sample of 115 chimpanzees, d = 0.075 +/- 0.037, compared to 0.020 +/- 0.011 for a sample of 1,554 hum ans. The heterozygosity of human STR loci is significantly higher than that of chimpanzees. Thus, the higher level of nuclear genome diversi ty relative to mitochondrial genome diversity in humans is not restric ted to protein-coding loci. It seems that humans, not chimpanzees, hav e an unusual d/H ratio, since the ratio in chimpanzees is similar to t hat in other catarrhines. This discrepancy in the relative levels of n uclear and mitochondrial genome diversity in the two species cannot be explained by differences in mutation rate. However, it may result fro m a combination of factors such as a difference in the extent of sex r atio disparity, the greater effect of population subdivision on mitoch ondrial than on nuclear genome diversity, a difference in the relative levels of male and female migration among subpopulations, diversifyin g selection acting to increase variation in the nuclear genome, and/or directional selection acting to reduce variation in the mitochondrial genome.