Research article

Visual sensitivities tuned by heterochronic shifts in opsin gene expression

Karen L Carleton¹ , Tyrone C Spady² , J Todd Streelman³ , Michael R Kidd⁴ , William N McFarland^{^ 5} and Ellis R Loew⁶

¹Department of Biology, University of Maryland, College Park, MD 20742, USA

²National Human Genome Research Institute, National Institutes of Health, Bethesda, MD 20892, USA

³School of Biology, Georgia Institute of Technology, Atlanta, GA 30332, USA

⁴Section of Integrative Biology, University of Texas at Austin, Austin, TX 78712, USA

⁵Friday Harbor Laboratories, University of Washington, Friday Harbor, WA 98250, USA

⁶Department of Biomedical Sciences, Cornell University, Ithaca, NY 14853, USA

author email corresponding author email^Deceased

BMC Biology 2008, 6:22doi:10.1186/1741-7007-6-22

Published:	23 May 2008

Abstract

Background

Cichlid fishes have radiated into hundreds of species in the Great Lakes of Africa. Brightly colored males display on leks and vie to be chosen by females as mates. Strong discrimination by females causes differential male mating success, rapid evolution of male color patterns and, possibly, speciation. In addition to differences in color pattern, Lake Malawi cichlids also show some of the largest known shifts in visual sensitivity among closely related species. These shifts result from modulated expression of seven cone opsin genes. However, the mechanisms for this modulated expression are unknown.

Results

In this work, we ask whether these differences might result from changes in developmental patterning of cone opsin genes. To test this, we compared the developmental pattern of cone opsin gene expression of the Nile tilapia, Oreochromis niloticus, with that of several cichlid species from Lake Malawi. In tilapia, quantitative polymerase chain reaction showed that opsin gene expression changes dynamically from a larval gene set through a juvenile set to a final adult set. In contrast, Lake Malawi species showed one of two developmental patterns. In some species, the expressed gene set changes slowly, either retaining the larval pattern or progressing only from larval to juvenile gene sets (neoteny). In the other species, the same genes are expressed in both larvae and adults but correspond to the tilapia adult genes (direct development).

Conclusion

Differences in visual sensitivities among species of Lake Malawi cichlids arise through heterochronic shifts relative to the ontogenetic pattern of the tilapia outgroup. Heterochrony has previously been shown to be a powerful mechanism for change in morphological evolution. We found that altering developmental expression patterns is also an important mechanism for altering sensory systems. These resulting sensory shifts will have major impacts on visual communication and could help drive cichlid speciation.