Seeing in the Deep: Evolution of the Opsin Gene Expression in Bermin Crater Lake Cichlids.

Cichlid visual systems can evolve rapidly during adaptive radiations. The Bermin crater lake species flock in Cameroon, comprising 13 (nine valid and four undescribed) Coptodon species, offers an ideal model to investigate visual adaptation to the deep-water light environments. Here, we examine visual opsin genes sequences and expression using 109 retina transcriptomes, focusing on interspecific variation with habitat depth, as well as on seasonal changes in the migratory species between depths. All species possess a multichromatic system with at least five cone opsins. While opsin coding sequences show limited divergence-consistent with the flock's recent origin-opsin expression profiles vary substantially. Deep-water species showed reduced sws1 and sws2b expression, in line with lower UV and violet light availability in deeper waters. Unexpectedly, proportional expression of the red-sensitive lws opsin gene increases with depth, contrasting with patterns in other lacustrine cichlids. In the seasonally migrating species Coptodon imbrifernus, opsin expression is plastic, with decreased sws2b levels in deeper-dwelling dry-season individuals. To contextualize our findings, we compared Bermin cichlids to the older Barombi Mbo crater lake radiation. While single cone adaptations to the depth were convergent (loss of UV/violet sensitivity, enhanced blue sensitivity), double cone response diverged: lws expression was lost in Barombi Mbo while increased in Bermin deep-water species. Our findings suggest that plasticity in opsin expression plays a crucial role at the onset of sensory evolution, potentially paving the way for future genetic change. This study underscores the power of young systems like Bermin for uncovering the mechanisms driving early visual system diversification.