Map of Life
Convergent Evolution Online

The ability to taste is obviously an essential component in the life of any animal, both to assess the potential quality of food, its nutrient capacities and also to detect toxins or other dangers. Like other sensory modalities it depends on transduction, and in some cases involves the employment of transmembrane proteins with seven helices of amino-acids. Whilst the capacity to taste is clearly very ancient and so at first sight would not seem to be a likely example of convergence, in point of fact we find several interesting examples.

Gustatory (and olfactory) proteins in arthropods

Perhaps what is most astonishing is that it is now clear that although the proteins involved in transduction of taste molecules in arthropods have effectively the identical structure to other animals (i.e. seven-helical transmembrane proteins) they clearly have a completely different origin. This is evident not only from the lack of sequence similarity, but more importantly the respective amino (N) and carboxy (C) terminations are reversed. Because the gustatory proteins of arthropods are evolutionarily related to the olfactory proteins, this convergence also extends to the latter modality. This is, therefore, a striking example of molecular convergence, and it is all the more puzzling because typically the gustatory/olfactory receptor proteins are understandably related to the visual proteins, the opsins. The arthropods have the same opsins as other animals, so why recruit completely new proteins to taste and smell?

Gustation in fruit-loving omnivores

Specific capacities for taste vary widely; cats, for example, are entirely uninterested in sweet foods, and not surprisingly lack the capacity to recognize them (or interestingly the receptor required is still present but “turned-off” as a pseudogene). There is, however, a particularly striking convergence in taste modalities between ourselves and the fruitfly, Drosophila. This includes both receptors for bitter compounds (e.g. quinine), and even more strikingly for sweet compounds. Just how precise this modality turns out to be is evident from the fact that Drosophila can detect a whole range of synthetic non-nutritive sweeteners, which in close relatives such as the New World monkeys elicits no response. Why? Oddly enough we and Drosophila are much more similar than might appear: we were both brought up developing a taste for African fruit for which we retain a shared avidity, and we are both omnivorous. No wonder some species of Drosophila are amongst the most successful of our commensals. More examples of convergence in taste are also known, including the independent evolution of a bitter modality in ourselves and chimpanzees.