Molecular and morphological evidence for a Pleistocene radiation of laminate-toothed rats (Otomys: Rodentia) across a volcanic archipelago in equatorial Africa

Abstract

West African Mountains of the Cameroon Volcanic Line harbour two montane-endemic species of laminated-toothed rats (Otomys), which represent the most westerly occurrence of the genus. We explore here through mtDNA sequencing and cranial morphometrics the taxonomic status and phylogenetic relationships of O. burtoni (Mt Cameroon) and O. occidentalis (Mts Oku and Gotel). We conclude that both species are valid and can be discriminated by molecular data, as well as quantitative and qualitative cranial characters. From molecular data, O. occidentalis and O. burtoni are closest neighbours (p-distance = 7.5–8.5%) and weakly associated sister species (suggesting a single West African radiation) and both are sister clades to a well supported clade of central, East and northeast African members of the O. typus s.l. and O. tropicalis s.l. species complexes from mountain ranges comprising the East African ‘Montane Circle’ and Ethiopian Highlands. Re-evaluation of the evolutionary origins of the allopatric Otomys populations in equatorial Africa is undertaken in light of fossil evidence of a southern African origin of the genus. We can conclude that Otomys reached the Cameroon Volcanic Line via corridors of temperate grasslands during the Late Pliocene. Our data support the hypothesis that, following major peripatric speciation events at around 2.3 to 2.03 Ma (from East Africa into West and North Africa respectively), further speciation occurred across neighbouring mountain ranges in West, Central-East and North-East Africa. Estimated molecular dates of speciation events in Otomys reveal close congruence with well-constrained geochronological estimates, pertinently the uplift of the Albertine Rift in the Early Pleistocene. These regional analyses reveal how peripatric speciation events established narrow-range endemics of Otomys on principal stratovolcanoes across the East African plateau and Cameroon.