17 February 2015

Genetic evidence leaves sour and salty taste in penguins' mouths

King penguin.
Credit: Jianzhi "George" Zhang
Penguins apparently can't enjoy or even detect the savoury taste of the fish they eat or the sweet taste of fruit. 

A new analysis of genetic evidence reported in the Cell Press journal Current Biology this week suggests that penguins have lost three of the five basic tastes over evolutionary time. For them, it appears, food comes in only two flavours: salty and sour.

Many other birds can't taste sweet things either. But they do have receptors for detecting bitter and umami (or meaty) flavours.

"Penguins eat fish, so you would guess that they need the umami receptor genes, but for some reason they don't have them," said Jianzhi "George" Zhang of the University of Michigan, one of the authors of the study.

"These findings are surprising and puzzling, and we do not have a good explanation for them. But we have a few ideas." 

It was Zhang's colleagues in China who led him to this discovery after they realised that they couldn't find some of the taste genes in their newly sequenced genomes of Adelie and emperor penguins. They wanted Zhang to help determine whether the absent genes were the result of incomplete sequencing – or a true evolutionary deletion.

Zhang and his colleagues took a closer look at the Adelie and emperor data. They also analysed bird tissue samples (chinstrap, rockhopper and king penguins, plus eight other closely related non-penguin bird species) and publicly available genomes for 14 other non-penguin bird species.

They found that all five penguin species lack functional genes for the receptors of sweet, umami and bitter tastes. The genomes of all non-penguin birds studied contain the genes for the umami and bitter tastes but, as expected, lack receptors for the sweet taste. The researchers concluded that all penguins have lost three of the five vertebrate senses.

"Taken together, 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 authors wrote.

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, Zhang said.

In this gustatory whodunit, Zhang's leading suspect is the protein Trpm5, which all vertebrates require for the transduction of sweet, umami and bitter taste signals to the nervous system. 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," said Zhang.

So, the researchers suggest, the genes encoding those taste receptors may have been lost in penguins not because they weren't useful, but rather because of the extremely cold environments in which the birds originated.

Penguin tongues are also unusual in other ways, the researchers note. Anatomical studies have suggested that some penguins lack taste buds – the primary location for taste receptors – altogether. Their tongues are instead covered with stiff, sharp papillae covered by a thick, horny layer. It seems that penguin tongues are used not so much to taste food, but rather to catch and hold onto it.

Penguins also have a habit of swallowing their food whole, which might leave them less concerned about what their food actually tastes like. Or could it be the other way around?

"Their behaviour of swallowing food whole, and their tongue structure and function, suggest that penguins need no taste perception, although it is unclear whether these traits are a cause or a consequence of their major taste loss," Zhang said.


Current Biology citation
Zhao, H., Li, J., & Zhang, J. (2015). Molecular evidence for the loss of three basic tastes in penguins. Current Biology, 25(4): R141. doi:10.1016/j.cub.2015.01.026

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