Ontogenetic mechanisms of explosive morphological divergence in the Lake Tana (Ethiopia) species flock of large African barbs (Labeobarbus; Cyprinidae; Teleostei)

Species flock of Lake Tana (Ethiopia) large African barbs (Labeobarbus; Cyprinidae; Teleostei), the largest recent cyprinid species flock, was studied as a model system for investigating ontogenetic mechanisms of the explosive morphological divergence often accompanying sympatric speciation in bony fishes. Comparative morphological analysis carried out with the use of geometric morphometric techniques revealed quantitative differences in the head shapes of species under study. Comparative analysis of skull development revealed significant interspecies differences in the temporal characteristics of craniogenesis in these species. These two lines of evidence suggest that heterochronies in craniogenesis underlie divergence in the head shapes of adult Tana barbs. This prediction was verified via experimental changes of temporal characteristics of craniogenesis in L. intermedius, a putative ancestor for the Lake Tana species flock. For this aim, timing and rate of skull development were changed by artificial manipulation of thyroid hormone levels. This experiment verified, that it is heterochronies that underlie an explosive morphological divergence of the Lake Tana barbs species flock. Our findings together with those reported in the literature suggest variability in the activity of the hypothalamic-pituitary-thyroid axis to contribute to these heterochronies.