Monday, February 16, 2015

Sweet? Bitter? #Penguins Can't Tell, Research Suggests

Feb 16, 2015
                                               Wikimedia Commons/Tekken50

NEW YORK (GenomeWeb) – Penguins appear to lack taste receptor genes governing three of the five tastes, according to a genomic analysis conducted by a trio of researchers from the US and China.

The trio, led by Jianzhi Zhang at the University of Michigan, searched through the genomes of the Adélie penguin (Pygoscelis adeliae) and emperor penguin (Aptenodytes forsteri) and other birds for taste receptor genes. Penguins, they found, lacked receptors involved in perceiving sweet, bitter, and umami taste, but have retained salty and sour taste receptors, as they reported in Current Biology.

"Penguins eat fish, so you would guess that they need the umami receptor genes, but for some reason they don't have them," Michigan's Zhang said in a statement. "These findings are surprising and puzzling, and we do not have a good explanation for them. But we have a few ideas."

Zhang and his team searched for genes that encode taste receptors — sour's PKD2L1, salty's ENaC, umami's Tas1r1–Tas1r3 heterodimer, sweet's Tas1r2–Tas1r3 heterodimer, and bitter's Tas2r genes — in Adélie and emperor penguins, as well as in the little egret and a dozen or so other birds whose genomes were publicly available.

None of the birds had the Tas1r2 gene, which encodes part of the sweet taste receptor, though the researchers did find the gene in mammalian and reptile outgroups.

Tas1r3, which makes up the other part of the sweet taste receptor heterodimer as well as part of the umami taste receptor heterodimer, was also lacking in penguins. It was, the researchers noted, present in other birds.

The other half of the umami taste-specific receptor, Tas1r1, is actually a pseudogene in penguins, the researchers found, as it contains a two basepair deletion that leads to a premature stop codon.
Other penguin species shared this pseudogene, but other birds have working copies, Zhang and his team reported.

Similarly, the researchers identified three Tas2r pseudogenes in penguins while most other birds had working copies of the gene behind bitter taste. They noted, though, those three penguin pseudogenes were orthologous to the two working copies and one pseudogene version of Tas2r found in the little egret.

This, Zhang and his colleagues said, indicates that the common ancestor of all penguins lost the umami and bitter tastes, while the sweet taste was lost even earlier in the avian lineage.
The receptor gene for sour taste was present in all birds, including penguins, as were the genes encoding the subunits of the salty taste receptor, ENaC.

Zhang and his colleagues said they suspect that the penguin's ancestral stomping grounds of Antarctica might have had a role in this gene loss.

Trpm5, which is involved in transducing the sweet, umami, and bitter tastes, doesn't work well at lower temperatures. At freezing temperatures, the researchers suggested that it might not work at all, leading to the inability to taste sweet, umami, and bitter, and then to the loss of the genes linked to those tastes.

Journal Reference:
  1. Huabin Zhao, Jianwen Li, Jianzhi Zhang. Molecular evidence for the loss of three basic tastes in penguins. Current Biology, 2015; 25 (4): R141 DOI:10.1016/j.cub.2015.01.026

Tuesday, February 10, 2015

#Penguin change stuns scientists

Yellow-eyed penguin. Adult standing showing wing 'flippers'. Otago Peninsula, January 2006. Image © Craig McKenzie

When yellow-eyed penguins arrived in New Zealand just decades after the country's native waitaha penguin became extinct, it became one of the most rapid prehistoric animal turnovers ever found, University of Otago researchers say.

The team of researchers used carbon dating and DNA analysis of penguin remains from coastal New Zealand to establish the timing of the waitaha's extinction and the colonisation by yellow-eyed penguins from the subantarctic.

University of Otago postdoctoral research fellow Dr Nic Rawlence, who carried out the study, said the combination of ecology, archaeology and DNA in this way was new and was also being used to investigate if similar patterns exist with New Zealand sea lions, Stewart Island shags, elephant seals and fur seals.

Previous research had shown at the time of human arrival, New Zealand was inhabited by the waitaha penguin. ''Hunting and habitat change apparently caused the extinction of this unique mainland penguin, before the yellow-eyed penguin later arrived here from the subantarctic,'' Dr Rawlence said.

The new dating study showed waitaha went extinct around the same time as the giant flightless moa, within 200 years of Polynesian settlement of New Zealand - before 1500 AD. The yellow-eyed penguin then replaced the extinct penguin within about 20 to 30 years, in the early 1500s. ''It's one of the most rapid biological turnovers ever documented.''

Associate Prof Ian Smith, who was also involved in the study, said the very rapid biological shift implied a substantial change in human pressure around that time. ''Interestingly, recent archaeological studies similarly suggest that the Maori population in southern New Zealand declined around 1500 AD, and coincided with a major dietary shift.''

Dr Rawlence said if there had not been the dietary shift from large animals to fish and shellfish, yellow-eyed penguins would not have been able to fill the niche left by the waitaha. ''Yellow-eyeds would have arrived and then become extinct if there hadn't been that change.''

The near absence of the yellow-eyed penguin from the mainland before the extinction of the waitaha was similar to what happened to New Zealand's sea lions.

A University of Otago study published last year found today's sea lions replaced an extinct prehistoric New Zealand sea lion. The Marsden and Allan Wilson Centre-funded research on penguins included team members from the Universities of Auckland, Otago, Adelaide and Oslo, as well as Canterbury Museum and Te Papa.

The team's findings were published this week in the leading international journal Quaternary Science Reviews.