Vision, like other biological attributes, isshaped by evolution through environmental pressures and demands, andeven closely-related species that are in other ways very similar mightrespond to their particular environments by interpreting the visualworld slightly differently, using photoreceptors that are attuned toparticular wavelengths of light. By studying a special group ofclosely-related fish species inhabiting the Great Lakes of Africa,researchers have uncovered clues to understanding how the components ofcolor vision can undergo change over a relatively short period ofevolutionary time.
The work is reported by James K. Bowmaker of University CollegeLondon, Karen L. Carleton of the University of New Hampshire, and theircolleagues.
Cichlid fish of the East African Rift Lakes are renowned fortheir diversity: Owing to migrations of ancestor species out of LakeTanganyika and into other lakes, such as Lake Malawi, it has beenestimated that hundreds of new cichlid species have arisen in theselakes in the last 100,000 years. Thanks to the relatively recentcolonization by these fish of different ecological niches, as well asthe prominent role of nuptual coloring in the mating preferences ofthese species, the cichlids offer a unique opportunity to study howcolor vision can undergo change in rapidly evolving species. Forexample, because color plays a significant role in mate choice,differences in color vision could greatly influence and even drivecichlid speciation.
In the new work, the researchers performed physiological andmolecular genetic analyses of color vision in cichlid fish from LakeMalawi and demonstrated that differences in color vision betweenclosely related species arise from individual species' using differentsubsets of distinct visual pigments. The scientists showed thatalthough an unexpectedly large group of these visual pigments areavailable to all the species, each expresses the pigments selectively,and in an individual way, resulting in differences in how the visualworld is sensed.
The researchers identified a total of seven "cone"(color-sensing) visual pigments underlying color vision in thesecichlids. They have measured the sensitivities of the cones todifferent wavelengths of light and isolated the seven genes that giverise to the pigment proteins. The seven cone types have maximumsensitivities ranging from the red end of the spectrum right through tothe ultraviolet--light outside the range of human sensitivity. Theresearchers showed that in order to tune its color vision, each cichlidspecies primarily expresses three of the seven cone pigment genesencoded by their genomes.
It is not clear why such closely related cichlid species haveevolved such different visual sensitivities, but the sensitivities mostlikely relate to such selective forces as foraging specializations andsubtle differences in the underwater light environment. Evolutionarycomparison of pigment genes suggests that other groups of fish may usea similar strategy for shaping their color vision.
The researchers included Juliet W.L. Parry, Aba Carboo, David M.Hunt, and James K. Bowmaker of University College in London, UnitedKingdom; Karen L. Carleton and Tyrone Spady of the University of NewHampshire, Durham, New Hampshire. This work was supported by theLeverhulme Trust and by the National Science Foundation.
Parry et al.: "Mix and match colour vision: tuning spectralsensitivity by differential opsin gene expression in Lake Malawicichlids." Publishing in Current Biology, Vol. 15, pages 1734-1739,October 11, 2005. DOI 10.1016/j.cub.2005.08.010 www.current-biology.com
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