News release

From: AAAS

Genomic adaptations to high sugar diets in sugar-consuming birds
Science

While high-sugar diets can trigger significant health issues for humans, some birds have independently evolved to flourish on sugar-rich nectar and fruit without ill effect. In a new study, researchers find that these bird species share convergent evolutionary changes in key physiological traits and metabolic genes that enable their high-sugar diets. High-sugar diets are harmful in humans and can lead to serious metabolic diseases like type 2 diabetes. However, several unrelated bird groups, such as hummingbirds, sunbirds, honeyeaters, and some parrots, have independently evolved to thrive on sugar-rich nectar and fruit. Birds differ from mammals in how they regulate blood sugar, maintaining naturally high glucose levels and relying less on insulin. Nectar-feeding species have further evolved specialized beaks, tongues, digestive enzymes, and water-balancing mechanisms to handle sugar-rich diets. Although previous studies, namely those in hummingbirds, have identified genetic changes linked to these traits, it remains unclear whether separate sugar-feeding lineages evolved similar adaptations through the same genes or via different molecular pathways.

To better understand how sugar-feeding bird species have genetically adapted to sweet diets, Ekaterina Osipova and colleagues generated high-quality reference genomes for five nectar- and fruit-eating species, alongside four related non-sugar feeding birds.  Comparative genomic screens across sugar-feeding species revealed convergent evolutionary changes in many of the same protein-coding and regulatory regions of genes involved in energy balance, sugar metabolism, insulin signaling, and even blood pressure regulation, suggesting that adaptation to sugar-loaded diets requires coordinated shifts across multiple physiological systems. According to Osipova et al., not all adaptations were unique to particular lineages, suggesting that while selection targeted similar biological systems, it did so via multiple genetic routes. Among the most prevalent genomic adaptations shared, which was observed across all studied sugar-consuming lineages, was repeated evolutionary change in MLCIPL, a transcription factor that regulates glucose metabolism and lipogenesis. Osipova et al. argue that alteration in this gene helps sugar-feeding birds efficiently convert excess glucose into fat for storage, then draw upon those reserves during fasting, which provides a specialized metabolic strategy that supports sugar-based diets.