Morphological and genetic correlates in the left-right asymmetric scale-eating cichlid fish of Lake Tanganyika

The stable polymorphism in mouth asymmetry in the cichlid fish Perissodus microlepis is a textbook example of adaptive evolution accomplished by functionally relevant morphological changes, ecological specialization and negative frequency-dependent selection. Knowledge about the morphological and developmental basis of this stable polymorphism and the mechanisms driving intraspecific variation in this fish remains largely incomplete. Here, we focus on often-neglected but potentially important aspects of the biology of this fish. In particular, we explore patterns of body shape variation, neutral genome-wide genetic diversity across its geographical distribution, and the presence of asymmetry in eyes centroid size in relationship to mouth bending angle. Geographical space (or a factor associated with geography) has a significant effect on both morphological and genetic diversity, suggesting restricted gene flow across the range of this species. We discuss potential implications of these observed patterns, including the possibility that the genetic basis of asymmetry could vary among locations. A significant association between eye and mouth laterality suggests that the entire head might be involved in the asymmetry. These findings highlight that head asymmetry in P. microlepis is a complex polymorphism involving the interaction of the genetic basis of multiple potentially independent traits and the environment.