This study illustrates that non-hominoid primate genomics are readily available to provide a useful genomic model system complementary to hominoids. By comparing evolutionary rate variation in Old World monkeys with those from hominoids, we can distinguish common patterns of primate genome evolution from those specific to certain lineages.
Male-driven evolution can reduce the divergence of the X-chromosomes compared with autosomes. The male-to-female mutation ratio (αm
) in primates is considered to be approximately 5, mostly based upon comparisons between humans and chimpanzees (Shimmin et al. 1993
; Li et al. 2002
). We showed that between humans and Old Word monkeys, αm
is lower than earlier estimates. A previous study also noted that αm
between humans and macaques is lower than 5 (RMGSAC 2007
). Therefore, the degree of male mutation bias appears to be variable, even within primates. Male mutation bias may be correlated with generation time, because the relative contributions of male germlines towards the mutational pool should increase with generation times (Bartosch-Harald et al. 2003
). Our observation is in accord with this view. However, the extent of variation of male mutation bias and causative mechanisms underlying the observed variation should be investigated further.
Evolutionary rates vary greatly between different Old World monkeys. Human substitution rates are known to be lower than chimpanzee rates (Elango et al. 2006
). We observe a comparable phenomenon between baboons and macaques. In fact, baboons appear to evolve more slowly than the other three branches in Old World monkeys. Such observations pose at least two immediate questions. First, what caused the reported substitution rate differences? Life-history traits may influence evolutionary rates, but it is not clear which are the main determinants of rate variation between the Old World monkeys. Detailed analyses of evolutionary rates between lineages, while controlling for specific life-history traits, will allow us to evaluate the role of different life-history variables on genome evolution. Second, the fact that substitution rates vary substantially and significantly between even closely related lineages make us ask whether we can confidently infer divergence times and other evolutionary events from molecular data. On the one hand, methods incorporating rate changes need to be developed urgently. On the other hand, the observation that CpG sites exhibit relatively constant molecular evolution compared with non-CpG sites (Kim et al. 2006
, this study) may provide an alternative means to infer past evolutionary events.