An international group of genetic scientists led by Prof Jianzhi Zhang from the University of Michigan has found that penguins lost three of the five basic vertebrate tastes – sweet, bitter and the savory, meaty taste known as umami – more than 20 million years ago.
Gentoo penguin (Pygoscelis papua) swimming underwater at Nagasaki Penguin Aquarium, Nagasaki, Japan. Image credit: Ken Funakoshi / CC BY-SA 2.0.
Vertebrates typically possess five basic tastes: sweet, sour, salty, bitter and umami.
Over the past fifteen years, remarkable progress in understanding the molecular basis of taste has opened the door to inferring taste abilities from genetic data through the examination of taste receptor genes.
Compared with mammals, birds are thought to be poor tasters, due in part to the observations that 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.
In a new study published in the journal Current Biology, Prof Zhang and his colleagues found that all penguin species lack functional genes for the receptors of sweet, umami and bitter tastes.
“Because penguins are fish eaters, the loss of the umami taste is especially perplexing. Penguins eat fish, so you would guess that they need the umami receptor genes, but for some reason they don’t have them. These findings are surprising and puzzling, and we do not have a good explanation for them. But we have a few ideas,” said Prof Zhang, who is the senior author on the study.
In the genomes of Adelie and emperor penguins, the umami and bitter taste receptor genes have become so-called pseudogenes – genetic sequences resembling a gene but lacking the ability to encode proteins. Pseudogenes often result from the accumulation of multiple mutations over time.
“Our results strongly suggest that the umami and bitter tastes were lost in the common ancestor of all penguins, whereas the sweet taste was lost earlier,” the scientists said.
Penguins originated in Antarctica after their separation from tubenose seabirds around 60 million years ago, and the major penguin groups separated from one another about 23 million years ago.
The taste loss likely occurred during that 37-million-year span, which included periods of dramatic climate cooling in Antarctica.
Prof Zhang’s leading suspect is the protein Trpm5, which is required for the transduction of sweet, umami and bitter taste signals to the nervous system in all vertebrates. Previous studies on mice showed that Trpm5 does not function well at cold temperatures.
“This give us a hint, perhaps, that this loss of taste genes has something to do with the inability of this protein to work at lower temperatures,” Prof Zhang.
“In mice, the protein Trpm5 is also involved in insulin secretion and the detection of pheromones. If the same is true in penguins, then Trpm5 is essentially being asked to work simultaneously at a warm body temperature and at the frigid ambient temperature, which may not be possible. When such a dilemma arises, the more important function is retained by natural selection, while the less important one is sacrificed.”
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Huabin Zhao et al. 2015. Molecular evidence for the loss of three basic tastes in penguins. Current Biology, vol. 25, no. 4, pp. R141–R142; doi: 10.1016/j.cub.2015.01.026
