Evidence of disassortative mating in a Tanganyikan cichlid fish and its role in the maintenance of intrapopulation dimorphism.

Morphological dimorphism in the mouth-opening direction ('lefty' versus 'righty') has been documented in several fish species. It has been suggested that this deflection is heritable in a Mendelian one-locus, two-allele fashion. Several population models have demonstrated that lateral dimorphism is maintained by negative frequency-dependent selection, resulting from interactions between predator and prey species. However, other mechanisms for the maintenance of lateral dimorphism have not yet been tested. Here, we found that the scale-eating cichlid fish Perissodus microlepis exhibited disassortative mating, in which reproductive pairings between lefties and righties occurred at higher than expected frequency (p<0.001). A previous study reported that a lefty-righty pairing produces a 1:1 ratio of lefty:righty young, suggesting that disassortative mating contributes to the maintenance of lateral dimorphism. A combination of disassortative mating and negative frequency-dependent selection may stabilize lateral dimorphism more than would a single mechanism.