No signature of selection on the C-terminal region of glucose transporter 2 with the evolution of avian nectarivory

Background  Flying birds, especially those that hover, need to meet high energetic demands. Birds that meet this demand through nectarivory face the added challenges of maintaining homeostasis in the face of spikes in blood sugar associated with nectar meals, as well as transporting that sugar to energetically demanding tissues. Nectarivory has evolved many times in birds and we hypothesized thatthe challenges of this dietary strategy would exert selective pressure on key aspects of metabolic physiology. Specifically, we hypothesized we would find convergent or parallel amino acid substitutions among different nectarivorous lineages in a protein important to sensing, regulating, and transporting glucose, glucose transporter 2 (GLUT2).

Methods  Genetic sequences for GLUT2 were obtained from ten pairs of nectarivorous and non-nectarivorous sister taxa. We performed PCR amplification of the intracellular C-terminal domain of GLUT2 and adjacent protein domains due to the role of this region in determination of transport rate, substrate specificity and glucosensing.

Results  Our findings have ruled out the C-terminal regulatory region of GLUT2 as a target for selection by sugar-rich diet among avian nectarivores, though selection among hummingbirds, the oldest avian nectarivores, cannot be discounted.

Conclusion  Our results indicate future studies should examine down-stream targets of GLUT2-mediated glucosensing and insulin secretion, such as insulin receptors and their targets, as potential sites of selection by nectarivory in birds.