Insights into the genetic basis of bilateral head asymmetry in a scale-eating cichlid fish.

Almost all animals are bilaterally symmetrical, and only rarely is this universality broken and maintained as a stable polymorphism. One such case is Perissodus microlepis, a scale-eating cichlid fish endemic to Lake Tanganyika. This species is polymorphic with left- or right-headed morphs, each specialized for scraping scales from the opposite side of their prey. This polymorphism is stably maintained around a 50:50 ratio and is likely driven by negative frequency-dependent selection, a classic form of balancing selection. Using a genome-wide dataset, including a de novo haplotype-resolved reference genome, whole-genome sequences, and microcomputed tomography scans of 102 individuals, we identified 72 genetic loci linked to head asymmetry. Nearby genes are enriched for roles in symmetry establishment, facial morphology, and nervous system function, with evidence suggesting that balancing selection maintains this diversity. Our study expands our understanding of the genetic basis of head asymmetry and provides candidate genes for further research.