Adaptive radiation of barbs of the genus Labeobarbus (Cyprinidae) in the East African river

Abstract

Large African barbs of the genus Labeobarbus are widely distributed in African freshwaters, and exhibit profound phenotypic plasticity that could be a prerequisite for adaptive radiation. Using morphological, molecular, and stable isotope analyses, we investigated whether an adaptive radiation has occurred in a riverine assemblage of the L. gananensis complex. This complex is composed of six phenotypically distinct sympatric forms inhabiting the Genale River (Ethiopian highlands, East Africa). Of the six forms, five were divergent in their mouth morphology, corresponding to generalized, lipped, scraping (two forms) and large-mouthed phenotypes. Stable isotope analysis revealed differences in 15N and 13C among these forms, representing different foraging strategies (omnivorous, scraping and piscivorous). Phylogenetic analysis of two mtDNA markers confirmed the monophyly of L. gananensis, suggesting an intra-riverine radiation. However, the Genale assemblage appears to have originated through a combination of allopatric and sympatric events. Some of the specialized forms within this drainage originated independently from the local generalized forms in three different river regions within local mini-flocks composed of two to three sympatric forms. Our study shows that adaptive radiation in rivers can be enhanced by a combination of sympatric speciation and temporal geographic isolation, leading to local sympatric speciation followed by migration.