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

Large African barbs of the genus Labeobarbus constitute a hexaploid lineage (2n = 150). This group is widely distributed in African freshwaters, and exhibits profound phenotypic variation 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 Labeobarbus gananensis complex. This complex is composed of six phenotypically distinct sympatric forms inhabiting the Genale River (Ethiopian highlands, East Africa) in the Juba and Wabe-Shebelle drainages (Indian Ocean basin). Of the six forms, five were divergent in their mouth morphology, corresponding to generalised, lipped, scraping (two forms), and large-mouthed phenotypes. One form had no mouth specialisation, but differed from the others in body shape (short and deep body; short form). Stable isotope analysis revealed differences in N-15 among these forms, representing different foraging strategies (omnivorous, scraping, and piscivorous). Phylogenetic analysis of two mitochondrial DNA 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 specialised forms within the Juba and Wabe-Shebelle drainage originated independently from the local generalised forms in three different river regions. Each of these cases could be considered as a small species flock composed of two or 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.