Phenotypic plasticity allows organisms to adapt quickly to local environmental conditions and could facilitate adaptive radiations. Cichlids have recently undergone an adaptive radiation in Lake Malawi where they inhabit diverse light environments and tune their visual sensitivity through differences in cone opsin expression. While cichlid opsin expression is known to be plastic over development, whether adults remain plastic is unknown. Adult plasticity in visual tuning could play a role in cichlid radiations by enabling survival in changing environments and facilitating invasion into novel environments. Here we examine the existence of and temporal changes in adult visual plasticity of two closely related species. In complementary experiments, wild adult Metriaclima mbenji from Lake Malawi were moved to the lab under UV-deficient fluorescent lighting; while lab raised M. benetos were placed under UV-rich lighting designed to mimic light conditions in the wild. Surprisingly, adult cichlids in both experiments showed significant changes in the expression of the UV-sensitive single cone opsin, SWS1, in only three days. Modeling quantum catches in the light environments revealed a possible link between the light available to the SWS1 visual pigment and SWS1 expression. We conclude that adult cichlids can undergo rapid and significant changes in opsin expression in response to environmental light shifts that are relevant to their habitat and evolutionary history in Lake Malawi. This could have contributed to the rapid divergence characteristic of these fantastic fishes.

African cichlids are an exemplary system to study organismal diversity and rapid speciation. Species differ in external morphology including jaw shape and body coloration, but also differ in sensory systems including vision. All cichlids have 7 cone opsin genes with species differing broadly in which opsins are expressed. The differential opsin expression results in closely related species with substantial differences in spectral sensitivity of their photoreceptors. In this work, we take a first step in determining the genetic basis of opsin expression in cichlids. Using a second generation cross between 2 species with different opsin expression patterns, we make a conservative estimate that short wavelength opsin expression is regulated by a few loci. Genetic mapping in 96 F2 hybrids provides clear evidence of a cis-regulatory region for SWS1 opsin that explains 34% of the variation in expression between the 2 species. Additionally, in situ hybridization has shown that SWS1 and SWS2B opsins are coexpressed in individual single cones in the retinas of F2 progeny. Results from this work will contribute to a better understanding of the genetic architecture underlying opsin expression. This knowledge will help answer long-standing questions about the evolutionary processes fundamental to opsin expression variation and how this contributes to adaptive cichlid divergence.