A comprehensive overview of the developmental basis and adaptive significance of a textbook polymorphism: head asymmetry in the cichlid fish Perissodus microlepis

Identifying the evolutionary and developmental bases of adaptive phenotypes is of central interest in evolutionary biology. Cichlid fishes have been a useful research model due to their extraordinary phenotypic diversity reflecting adaptations to often very narrow niches. Among them, the scale-eating Perissodus microlepis is considered to be a textbook example for balanced polymorphism: its asymmetric head and handed behavior is thought to be maintained by negative frequency-dependent selection via prey–predator interactions. However, several contradictory findings and open questions have emerged in recent years, challenging our understanding of this model. Here, we review existing evidence for both genetic and non-genetic effects influencing head asymmetry, the association between morphological asymmetry and behavioral laterality, and the identification of signatures of balancing selection. Recent technological and theoretical developments have opened new exciting research avenues that can help identifying the drivers of adaptive traits in P. microlepis and other nonmodel organisms, and we discuss promising directions worth exploring. We highlight the importance of using integrative approaches that analyze genetic, environmental, and epigenetic variation in natural populations to aid a comprehensive understanding of why cichlids are so diverse and how evolution has produced and continues to generate such a vibrant and often complex phenotypic diversity.