Otomys irroratus (Brants, 1827)

Otomys irroratus View in CoL – O. auratus clade

Phylogenetic analysis of Cytb gene sequences reveal the existence of two major evolutionary lineages with no shared haplotypes, consistent with the results of Taylor et al. (2009a) and Engelbrecht et al. (2011). These two lineages, which diverged 1.1 Mya ( Taylor et al., 2009a; Engelbrecht et al., 2011), coincide with different biomes and mountain ranges ( O. irroratus with the Cape Fold Belt and the fynbos biome and O. auratus with the Great Escarpment and the grassland biome). A contact zone between the two lineages was identified at Alice in the Eastern Cape by Engelbrecht et al. (2011). The pattern we observed is mirrored exactly in the case of another montane rodent, the four-striped mouse Rhabdomys pumilio (Sparrman, 1784) s.l., whose range corresponds closely to that of O. irroratus s.l., where disjunct genetic lineages coincided with grassland and fynbos biomes and also showed a contact zone in the region of Alice ( Du Toit et al., 2012). Likewise, Willows-Munro & Matthee (2011) identified distinct lineages of montane forest shrews from the fynbos and grassland biomes of South Africa. Together with the fact that the O. karoensis s.l. complex shows exactly the same pattern of vicariance (see above), this striking congruence across several distinct montaneadapted rodent and shrew taxa points to a common evolutionary cause, as detailed below.

Our new data from AS allow us to extend the western limit of O. auratus in the Eastern Cape from ~ 27°E (Hogsback) to 25°E ( AS). The origin of the two major clades within O. irroratus s.l. (and O. karoensis s.l.) might be attributable to vicariance processes caused by aridification. Historically,Africa has experienced periods of high-latitude glaciation cycles, which influenced African climates from the Late Pliocene, causing periods of aridification ( Lawes et al., 2007; Taylor et al., 2009a). In southern Africa, this climate change is thought to have caused the onset of drier and/or warmer conditions that led to shrinking of grasslands and fragmentation of biomes owing to increased temperature and decreased rainfall ( deMenocal, 2004; Taylor et al., 2009 a, 2015). Aridification of the biomes might have caused the preferred habitat of the species to shrink in size away from other suitable habitats, thereby separating the species distribution into two or more isolated ranges and preventing gene flow between them. A genetic study of snails occurring in the Southern Escarpment demonstrated the importance of aridification in the vicariance of lineages ( Barker et al., 2013). Temperateadapted species, such as O. irroratus and O auratus , would have been displaced to higher elevations in both the Drakensberg and the Cape Fold Mountain Ranges, effectively isolating populations to these two distinct mountain ranges, whose higher elevations retained temperate climates. Therefore, climatic change linked to elevational heterogeneity is the most likely cause for the distinct difference in the two clades of O. irroratus s.l. Specifically, the dated split between O. irroratus and O. auratus of 1.1 Mya corresponds closely to a period of aridification ~1 Mya ( deMenocal, 2004; Taylor et al., 2009a). The fact that the range of O. irroratus corresponds closely to both the Cape Fold Belt geology and the fynbos biome, whereas the range of O. auratus corresponds to both the grassland biome and the Great Escarpment mountain system, suggests that although geology combined with climate might have been the cause of the initial divergence of lineages, subsequent speciation and evolutionary divergence might have been aided by ecological coadaptation to the different vegetation biomes.

From mismatch coefficients from mtDNA sequences, Engelbrecht et al. (2011) showed that O. irroratus had a unimodal distribution indicative of a recent population expansion, possibly explaining the eastern expansion of O. irroratus populations away from the Cape Fold Mountains in the Eastern Cape Province to form a secondary contact zone with O. auratus at Alice on the Great Escarpment. In contrast, O. auratus has a multimodal mismatch indicative of a stable (and older) population. As also found by Taylor et al. (2009a) and Engelbrecht et al. (2011), our data reveal two distinct subclades in O. auratus , which are distributed west (central plateau of South Africa and the eastern highlands of Zimbabwe) and east (eastern coastal escarpment populations extending from Alice and Hogsback in the Western Cape to populations from the Drakensberg foothills and midlands of KwaZulu-Natal) of the Great Escarpment. This event was dated at 0.66 Mya by Engelbrecht et al. (2011) and could be explained by a period of warming and/or aridification that resulted in populations becoming fragmented and shrinking downslope on the foothills and escarpments west and east of the high Drakensberg Mountains. Significantly, populations from the Sneeuberg Range are affiliated with the western clade from the Central Plateau rather than with the eastern coastal clade, although populations from Mt Zebra (western clade) and Hogsback (eastern clade) are separated by only 100 km.