The genome of the endangered dryas monkey provides new insights into the evolutionary history of the vervets

Genomic data can be a powerful tool for inferring ecology, behaviour and conservation needs of highly elusive species, particularly when other sources of information are hard to come by. Here we focus on the dryas monkey, an endangered primate endemic to the Congo Basin with cryptic behaviour and possibly less than 250 remaining individuals. Using whole genome data we show that the dryas monkey represents a sister lineage to the vervet monkeys and has diverged from them at least 1 million years ago with additional bi-directional gene flow 590,000 - 360,000 years ago. After bonobo-chimpanzee admixture, this is the second reported case of gene flow that most likely involved crossing the Congo River, a strong dispersal barrier. As the demographic history of bonobos and dryas monkey shows similar patterns of population increase during this time period, we hypothesise that the fluvial topology of the Congo River might have been more dynamic than previously recognised. As a result of dryas monkey - vervet admixture, genes involved in resistance to the simian immunodeficiency virus (SIV) have been exchanged, possibly indicating adaptive introgression. Despite the presence of several homozygous loss-of-function mutations in genes associated with reduced sperm mobility and immunity, we find high genetic diversity and low levels of inbreeding and genetic load in the studied dryas monkey individual. This suggests that the current population carries sufficient genetic variability for the long-term survival of this species. We thus provide an example of how genomic data can directly improve our understanding of elusive species.