Last year, researchers raised the possibility that Mexican blind cavefish once could see but traded in their vision for bigger jaws and teeth (SN: 8/23/03, p. 126: Available to subscribers at Did cavefish trade eyes for good taste?). Those same scientists now report genetic evidence bolstering their theory.
William Jeffery of the University of Maryland at College Park and his colleagues had previously reported that the gene sonic hedgehog controls eye and mouth formation in the freshwater fish Astyanax mexicanus. This single species has both a sighted form, which swims in surface waters, and a blind form, which lives in cave ponds.
By examining gene expression in embryos of both fish types, Jeffery’s team found that the blind fish express sonic hedgehog and a second gene, tiggy-winkle hedgehog, in a larger region of the body than sighted fish do. This expanded expression prevents normal eye development.
When the researchers injected sighted fish embryos with messenger RNA corresponding to either gene, the fish developed with the sightless features of their blind counterparts. Treating blind fish embryos with a drug that inhibits both the hedgehog proteins partially restored eye development.
Jeffery suggests that the wider expression pattern of these two genes in the blind fish indicates a survival-conferring trait retained during the course of evolution, rather than a detrimental mutation. Because the genes orchestrate both mouth and eye development, the blind fish may have lost their eyes as they gained a more effective mouth—a useful feature for catching food in the dark. The researchers report their findings in the Oct. 14 Nature.