Riddle on the riffle: Miocene diversification and biogeography of endemic mountain loaches in the Western Ghats Biodiversity Hotspot

Abstract

Aim : The Western Ghats Hotspot in peninsular India harbours remarkable diversity and endemism of freshwater fish. However, the ichthyofauna's evolutionary histories and biogeography are poorly known. Here, we investigate (a) the diversity, evolutionary history and biogeography of endemic mountain loaches and (b) the potential influence of the physiography of hill ranges, geological barriers and river systems on the diversification and cladogenesis of loaches, in the Western Ghats Biodiversity Hotspot. Location : Southern Western Ghats mountain ranges (8–13°N latitudes), Western Ghats†Sri Lanka Biodiversity Hotspot. Taxa : Mountain loaches Bhavania annandalei and B. australis (Cypriniformes: Balitoridae). Methods : We carried out a multigene phylogenetic analysis with mitochondrial and nuclear markers using Bhavania specimens collected throughout the genus' range. The Automated Barcode Gap Analysis, Poisson Tree Process and Generalized Mixed Yule†Coalescent Model were used to delimit species. A Bayesian chronogram was constructed to estimate the time elapsed since the most recent common ancestor of the distinct lineages of Bhavania. Ancestral ranges of distinct lineages of Bhavania were reconstructed using the dispersal–extinction–cladogenesis model. Results : Phylogenetic analysis of combined mitochondrial and nuclear data, as well species delimitation using the Poisson Tree Process and Generalized Mixed Yule†Coalescent Model analyses supported eight distinct lineages, which included the narrowly distributed B. annandalei and widely distributed B. australis. The Barcode Gap Analysis, however, supported only seven lineages. Bayesian divergence time dating suggests that the genus originated early in the Neogene and diversified in the Miocene. Ancestral state reconstruction indicated Bhavania diversifed as a result of sympatric, subset and vicariant speciation with five dispersal and one vicariant events across biogeographic barriers and river systems. Main conclusions : Bhavania australis is a ‘species complex’. Miocene†associated climatic changes including intensification of the south†west monsoon likely triggered dispersal and range expansion; subsequent aridification would have led to drying up of riverine connections, formation of land barriers and fragmentation of streams, resulting in cladogenesis. Our results also provide preliminary evidence that Cauvery, one of the largest east flowing rivers of Western Ghats, facilitates an east†west pathway for dispersal and diversification of endemic lineages of the region.