Visual Tuning May Boost African Cichlid Diversity

Abstract

African cichlid fish form new species faster than any other vertebrates, with hundreds of species evolving within the last 2 million years in Lake Malawi and within the last 120,000 years in Lake Victoria. This rapid speciation makes cichlids good models for elucidating the genetic mechanisms behind biodiversity. Vision may play a key role in cichlid evolution, adapting them to forage for new foods or colonize new habitats. Vertebrate retinas have two groups of light-sensitive proteins called opsins: those in rod photoreceptors, which are sensitive to dim light, and those in cone photoreceptors, which are sensitive to color. Changes in the visual system could be due to differences either in the expression of opsin genes or in their DNA sequences. A Research Article in this issue of PLoS Biology by Christopher Hofmann and colleagues suggests that both mechanisms underlie changes in visual sensitivity in cichlids. To investigate differences in cichlids’ visual sensitivity, the researchers compared 56 types of fish from Lake Malawi, where the water is spectacularly clear, and 11 types of fish from Lake Victoria, where the water is turbid. First, they used messenger RNA to compare the expression of opsin genes in the Malawi and Victoria fish. Cichlids have one rod opsin and six cone opsins, which range in sensitivity from ultraviolet to red wavelengths (roughly 300 to 700 nanometers). In addition, cichlids have two kinds of cones: single cones, which contain the three opsins that are sensitive to shorter wavelengths (ultraviolet, violet, and blue); and double cones, which contain the three opsins that are sensitive to longer wavelengths (blue-green, green, and red). The results showed that Malawi cichlids collectively expressed all six cone opsins and so were sensitive to wavelengths across the spectrum. The researchers then used opsin expression patterns in Malawi cichlids to estimate the sensitivities of their single cones, which contain the shorter wavelength opsins, and double cones, which contain the longer wavelength opsins. They found that Malawi cichlids clustered into three distinct groups that are sensitive to short, middle, and long wavelengths. This opsin diversity is likely driven by foraging. For example, Malawi cichlids that eat other fish generally expressed more long-wavelength opsins, while those that eat plankton and algae generally expressed more short-wavelength opsins. Ultraviolet-sensitivity is known to increase fishes’ foraging efficiency on zooplankton and other microorganisms, suggesting that these differences in opsin expression may be adaptive. In contrast, while all of the Victoria cichlids expressed the red-sensitive opsin, none of them expressed the ultravioletsensitive opsin. This makes sense because little ultraviolet light penetrates turbid water. However, expression of the violetsensitive opsin varied amongst Victoria cichlids. The diversity of this opsin is likely driven by light levels, which vary by nearly four orders of magnitude in Lake Victori-