They whiz from flower to flower while hovering – researchers have now determined which genetic basis enables the hummingbirds to perform this extremely energy-intensive mode of locomotion. In the case of the tiny aerial acrobats, less was obviously more: the loss of the function of a special metabolic gene means that hummingbirds can use more energy than other birds and thus achieve their high flight performance, the study shows.
Their wings beat up to eighty times a second: hummingbirds have developed one of the most amazing modes of locomotion in the animal kingdom. The extremely high frequency of flapping of their wings allows the representatives of this tropical group of birds to hover in the air like helicopters. In contrast to other birds, the tiny creatures, which are often only the size of a thumb, can not only fly forwards, but also backwards and sideways. In this way, they were able to find a special source of food in the best possible way: floating, they soak up nectar from flowers with their long beaks and swish nimbly from one source to the next. The hummingbirds can only afford their amazing flight performance thanks to the sweet fuel. Because their rapid flapping of wings requires extremely high energy.
On the trail of genetic recipes
Hummingbirds are known to have the highest metabolic rate of any vertebrate to release the energy from the sugars in flower nectar throughout the body. Biochemical adaptations that make this possible are already known, but the underlying genomic changes are largely unknown. The scientists led by Ekaterina Osipova from the Max Planck Institute for Molecular Cell Biology and Genetics in Dresden have now devoted themselves to this research topic. For their study, they sequenced the genome of the long-tailed shadowy hummingbird (Phaethornis superciliosus). In combination with genetic information from other hummingbird species, they then compared it with the genomes of 45 "normal" birds, including chickens, pigeons and eagles.
As the scientists report, they came across a distinctive feature in the hummingbird's genome: this group of birds apparently lacks a functional gene for the muscle enzyme fructose bisphosphate 2 (FBP2). Analyzes of the birds' family relationships showed that this gene had apparently already been lost in the common ancestor of all hummingbirds. This happened around 48 to 30 million years ago - in the phase in which the hovering flight of the hummingbird ancestors probably developed as well as their specialization in feeding on flower nectar. Thus, it was suggested that the loss of FBP2 may have played an evolutionary role in metabolic adaptations to hover flight. But what exactly does the missing enzyme do?
Optimally fueled metabolism
The researchers investigated this question by examining muscle cell lines from quail in the laboratory. Using CRISPR/Cas gene scissors, they deactivated the FBP2 gene, which is naturally functional in these birds, in one group, meaning that the enzyme was not produced. "Through these experiments, we were able to show that switching off the FBP2 gene in a targeted manner increases sugar metabolism. Furthermore, our analyzes showed that the number and activity of mitochondria, which are important for energy production, also increase at the same time,” reports Osipova. For quail and co, these effects do not make sense energetically. However, these effects of the missing enzyme apparently enable the hummingbirds to have an optimized supply of energy for their laborious flapping of wings.
"Since the gene encoding FBP2 is only active in muscle cells, our results suggest that the loss of this gene in hummingbird ancestors may have represented an important step in the adaptations of muscle metabolism required for hovering flight," concludes the senior author Michael Hiller from the Senckenberg Nature Research Society in Frankfurt am Main presented the results of the study. In addition to this genetic peculiarity, the investigations revealed even more evidence of genomic changes in the hummingbirds, the scientists report. It is therefore becoming apparent that certain other selection processes have led to changes in hereditary factors that are also associated with sugar metabolism. "Further studies and experiments can certainly clarify the significance of these gene changes for the evolutionary adaptations in the metabolism of hummingbirds," Hiller concludes.
Source: Senckenberg Society for Natural Research, specialist article: Science, doi: 10.1126/science.abn7050