How birds achieve sweet success

(Press-News.org) Anyone who has seen a hummingbird poking her beak deep into a trumpet creeper blossom, or a honeyeater using its brush-tipped tongue to extract nectar from eucalyptus flowers, has witnessed something that from a human perspective is rather remarkable. Although many bird species avoid sugar-rich foods, others survive almost entirely on sugar-rich nectar or fruit, processing massive sugar loads without developing the diseases that such diets cause in people and other animals.

Groups including hummingbirds, sunbirds, and some honeyeaters and parrots independently evolved the ability to thrive on extreme sugar diets on different continents, separated by millions of years of evolutionary history. Researchers at Harvard University, the Max Planck Institute for Biological Intelligence, and the Senckenberg Research Institute and Natural History Museum Frankfurt set out to answer a major puzzle: did these birds all arrive at the same genetic changes, or did each group chart its own path?

The answer, it turns out, is both.

Building on studies that showed how these birds evolved to taste sugar and handle extreme energy demands, like the hovering flight of hummingbirds, the new work published in Science investigates the genetic changes behind the birds’ extreme metabolic adaptations.

“A diet heavy in nectar or sweet fruits presents unique physiological challenges,” says Ekaterina Osipova, a postdoc at the Senckenberg Research Institute and Natural History Museum Frankfurt and Harvard University and co-first author. “These birds must process huge amounts of sugar without overwhelming their systems, and must manage enormous fluid volumes while maintaining proper blood pressure and salt balance. The genetic patterns we have found start to reveal a bigger picture of how these birds can take in huge amounts of sugar in ways we cannot and help answer fundamental questions about repeatability in evolution.”

A shared toolkit and unique changes

The researchers studied whole genome sequences of sugar-consuming birds from across the Americas, Australia, Africa, and Asia, comparing sugar-feeding birds with their closely related non-sugar-feeding relatives and conducting laboratory experiments to confirm some of the findings.

Some genetic changes identified were unique to individual groups, whereas others were shared across two or more groups. Changes affected genes controlling how the body coordinates blood pressure with water balance, as well as genes regulating heart rhythm and kidney ion transport – reflecting the challenges birds face managing both high sugar concentrations and large fluid volumes from their food sources. The researchers also found repeated changes associated with insulin signaling genes across all sugar-feeding groups.

Out of thousands of genes examined, only one – MLXIPL, a master regulator of sugar metabolism – was modified by members of all four sugar-feeding groups and none of their non-sugar-eating relatives. Laboratory tests confirmed the hummingbird version of this gene was far more active than the same gene from swifts, close relatives that don't eat sugar, suggesting evolution has fine-tuned it for high-sugar diets. That birds in all four groups independently changed this same gene, despite evolving on different continents over millions of years, points to it being essential for handling extreme sugar loads. This gene is also important in human metabolism, making it a potentially valuable target for understanding disease.

“What I find particularly exciting is that our findings open up new questions about metabolism, physiology, and how other animals handle extreme diets,” says Meng-Ching Ko, a postdoc at the Max Planck Institute for Biological Intelligence and co-first author. “Our ancestors evolved on low-sugar diets, but many of us now consume far more sugar than our bodies can handle. Understanding how these birds adapted may ultimately help identify new therapeutic targets for diabetes and other metabolic diseases.”

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