Because penguins are fish eaters, the loss of umami flavor is especially puzzling. "Penguins eat fish, so I imagine they need umami receptor genes, but for some reason they don't have them," says Jianzhi "George" Zhang, a professor in Michigan's Department of Ecology and Evolutionary Biology who is responsible for the genetic study. . "These results are surprising and we don't have a good explanation for them. But we do have some ideas."
Zhang suspects that the sensory changes are linked to ancient cooling weather phenomena in Antarctica, where the penguins originated. Their main hypothesis, published in Current Biology, is that genes were lost after cold Antarctic temperatures interfered with taste perception.
Vertebrates typically have five basic tastes: sweet, sour, salty, bitter, and umami. Over the past 15 years, remarkable progress in understanding the molecular basis of flavor has opened the door to inferring the ability to taste through examination of the flavor receptor genes.
Compared to mammals, birds are thought to be poor tasters, in part because they have fewer taste buds on their tongues and lack teeth for chewing food. Previous genetic studies showed that the sweet taste receptor gene is absent from the genomes of all birds examined to date.
Interested in the case of penguins, Zhang found that all five species lack functional genes for sweet, umami, and bitter taste receptors. In the Adelie and Emperor genomes, the bitter taste receptor and umami genes have become "pseudogenes," genetic sequences that resemble a gene but lack the ability to encode proteins. Pseudogenes are often the result of the accumulation of multiple mutations over time.
The genomes of all the nonpenguin birds studied - including herons, finches, flycatchers, parrots, macaws, hawks, chickens, and wild ducks - contain the genes for umami and bitter flavors, but unsurprisingly lack the receptors for the sweet taste. The researchers concluded that all penguins have lost three of the five vertebrate tastes.
"Taken together, our results strongly suggest that the umami and bitter tastes were lost in the common ancestor of all penguins, while the sweet taste was lost earlier," the authors wrote.
Penguins originated in Antarctica about 60 million years ago, and the major groups of penguins separated from each other about 23 million years ago. The loss of taste likely occurred during that 37-million-year span, which included periods of drastic cooling in Antarctica, Zhang said.
In his opinion, the culprit is the TRPM5 protein, which is required for the transduction of bitter, sweet, umami taste signals and for the nervous system in all vertebrates. Previous studies in mice showed that TRPM5 does not work well in cold temperatures. "This will give us a clue, perhaps, that this loss of flavor genes has something to do with the inability of this protein to work at lower temperatures," says Zhang.
Taste in vertebrates is mediated by taste receptors normally found on taste buds located on the upper surface of the tongue and other parts of the mouth. The human tongue has several thousand taste buds. (ABC - Spain)