Phylogeny and vicariant speciation of the Grey Rhebok, Pelea capreolus

Abstract

A South African endemic antelope, the Grey Rhebok (Pelea capreolus), has long been an evolutionary enigma in bovid systematics-its phylogenetic intractability attributed to its curious combination of derived and primitive morphological attributes and the consequences of a rapid radiation. By using a combination of DNA sequences, chromosomal characteristics and quantitative and qualitative morphological features we show that the species is a sister taxon to a clade that comprises the waterbuck, reedbuck and allies. Our finding of few unambiguous synapomorphies reinforces suggestions of a rapid radiation and highlights the effects of incomplete lineage sorting, including the hemiplasic nature of several chromosomal rearrangements. We investigate these data to address the general question of what may have led to Pelea being both genetically and ecologically distinct from the Reduncini. We argue that its adaptation to exposed habitats, free of standing water, arose by vicariance prompted by increasing aridity of the extreme south/southwestern region of the African continent in the Miocene. Ancestral lineages leading to the extant Redunca and Kobus, on the other hand, retreated to water-abundant refugia in the north during these mostly globally cool phases. The mosaic of water-rich environments provided by the Okavango and the drainage systems in the southwestern extension of the East African Rift system are considered to have facilitated speciation and chromosomal evolution within these antelope.

Figure 1

(a) G-banded karyotype of Pelea capreolus (2n=56). The Robertsonian fusion chromosomes (pairs 1 and 2) correspond to unfused acrocentric chromosomes in cattle (BTA). (b) Confirmation by cross-species FISH showing the cattle orthologues to pair 1 of the Pelea karyotype using cattle painting probes BTA1 and BTA11 and (c) BTA2 and BTA25 to pair 2 of the Pelea karyotype.

Figure 2

C-banded metaphase spreads of (a) P. capreolus, (b) R. fulvorufula and (c) K. ellipsiprymnus. The X and Y chromosomes are arrowed in each instance.

Figure 3

Cross-species FISH with a microdissected R. fulvorufula Y-chromosome painting probe showing hybridization patterns on the X and Y chromosomes for (a) P. capreolus, (b) R. fulvorufula and typically for the species within Kobus, (c) K. ellipsiprymnus.

Figure 4

(a) Phylogenetic tree based on DNA sequences from two mitochondrial DNA (Cyt-B and COI) and four nuclear gene fragments (B-Spectrin nonerythrocytic 1, PRKC1, Stem cell factor and MC1R). Bootstrap values are presented above each node, while Bayesian posterior probability values are presented below. Prior model specification for the Bayesian analyses were as follows: B-Spectrin nonerythrocytic 1 and MC1R: nst=2, rates=gamma; PRKC1: nst=2, rates propinv; Stem cell factor: nst=6, rates propinv; Cyt-B: nst=6, rates=invgamma; COI: nst=6, rates=gamma, (b) Supermatrix parsimony phylogeny based on the mtDNA and nuclear sequences presented in (a) above, together with Robertsonian fusion chromosomal data and FISH patterns from a microdissected R. fulvorufula Y-chromosome painting probe. Bootstrap values are presented above each node. (c) Supermatrix parsimony phylogeny based on the complete data set (i.e., sequences, chromosomes and morphological characters). Bootstrap values are presented above each node.