Admixture between historically isolated mitochondrial lineages in captive Western gorillas: recommendations for future management.

Although captive populations of western gorilla have been maintained in the United States for over a century, little is known about the geographic origins and genetic composition of the current zoo population. Furthermore, although previous mitochondrial analyses have shown that free-range gorilla populations exhibit substantial regional differentiation, nothing is known of the extent to which this variation has been preserved in captive populations. To address these questions, we combined 379 pedigree records with data from 52 mitochondrial sequences to infer individual haplogroup affiliations, geographical origin of wild founders and instances of inter-breeding between haplogroups in the United States captive gorilla population. We show that the current captive population contains all major mitochondrial lineages found within wild western lowland gorillas. Levels of haplotype diversity are also comparable to those found in wild populations. However, the majority of captive gorilla matings have occurred between individuals with different haplogroup affiliations. Although restricting crosses to individuals within the same haplogroup would preserve the phylogeographic structure present in the wild, careful management of captive populations is required to minimize the risk of drift and inbreeding. However, when captive animals are released back into the wild, we recommend that efforts should be made to preserve natural phylogeographic structure.

Complete mitochondrial genome sequence of the Eastern gorilla (Gorilla beringei) and implications for african ape biogeography.

The Western and Eastern species of gorillas (Gorilla gorilla and Gorilla beringei) began diverging in the mid-Pleistocene, but in a complex pattern with ongoing gene flow following their initial split. We sequenced the complete mitochondrial genomes of 1 Eastern and 1 Western gorilla to provide the most accurate date for their mitochondrial divergence, and to analyze patterns of nucleotide substitutions. The most recent common ancestor of these genomes existed about 1.9 million years ago, slightly more recent than that of chimpanzee and bonobo. We in turn use this date as a calibration to reanalyze sequences from the Eastern lowland and mountain gorilla subspecies to estimate their mitochondrial divergence at approximately 380000 years ago. These dates help frame a hypothesis whereby populations became isolated nearly 2 million years ago with restricted maternal gene flow, followed by ongoing male migration until the recent past. This process of divergence with prolonged hybridization occurred against the backdrop of the African Pleistocene, characterized by intense fluctuations in temperature and aridity, while at the same time experiencing tectonic uplifting and consequent shifts in the drainage of major river systems. Interestingly, this same pattern of introgression following divergence and discrepancies between mitochondrial and nuclear loci is seen in fossil hominins from Eurasia, suggesting that such processes may be common in hominids and that living gorillas may provide a useful model for understanding isolation and migration in our extinct relatives.