The role of Pleistocene refugia and rivers in shaping gorilla genetic diversity in central Africa

Abstract

The role of Pleistocene forest refugia and rivers in the evolutionary diversification of tropical biota has been the subject of considerable debate. A range-wide analysis of gorilla mitochondrial and nuclear variation was used to test the potential role of both refugia and rivers in shaping genetic diversity in current populations. Results reveal strong patterns of regional differentiation that are consistent with refugial hypotheses for central Africa. Four major mitochondrial haplogroups are evident with the greatest divergence between eastern (A, B) and western (C, D) gorillas. Coalescent simulations reject a model of recent east-west separation during the last glacial maximum but are consistent with a divergence time within the Pleistocene. Microsatellite data also support a similar regional pattern of population genetic structure. Signatures of demographic expansion were detected in eastern lowland (B) and Gabon/Congo (D3) mitochondrial haplogroups and are consistent with a history of postglacial expansion from formerly isolated refugia. Although most mitochondrial haplogroups are regionally defined, limited admixture is evident between neighboring haplogroups. Mantel tests reveal a significant isolation-by-distance effect among western lowland gorilla populations. However, mitochondrial genetic distances also correlate with the distance required to circumnavigate intervening rivers, indicating a possible role for rivers in partitioning gorilla genetic diversity. Comparative data are needed to evaluate the importance of both mechanisms of vicariance in other African rainforest taxa.

Fig. 1.

Minimum spanning tree of mitochondrial HV1 haplotypes identified in all three major gorilla subspecies. The mutational steps separating haplotypes are indicated by using cross bars or by number. Major mitochondrial haplogroups (A–D) and their respective subdivisions (C1–2, D1–3) are based on haplogroups recovered in the phylogeny (see SI Figs. 4 and 5).

Fig. 2.

Geographic distribution of major mitochondrial haplogroups across central Africa. Important rivers are also indicated with an arrow. CR, Cross River; SA, Sanaga River; OG, Ogooué River; IV, Ivindo River; SG, Sangha River; UB, Ubangui River; CG, Congo River. The locations of two published museum samples (22, 57) are indicated by “?.”

Fig. 3.

Multidimensional scaling plot of microsatellite distances between populations from all major gorilla haplogroups (A–D). In each case, the mitochondrial haplogroup affiliation is indicated.