The dinosaur ancestors of birds had evolved small, more efficient cells and genomes 230 million years ago, long before archeopteryx took to the air.

Credit: Wikipedia

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SYDNEY: The dinosaur ancestors of birds had small genomes that may have been an essential adaptation in the evolution of flight, research published today in the British journal Nature suggests.

So-called 'genome reduction' is one of the lesser-known and mysterious correlates of flight in vertebrates; previous studies have shown that birds have remarkably small genomes compared to other vertebrates, while bats tend to have smaller genomes than those of non-flying mammals.

According to one unusual theory, flying vertebrates' small genomes were an important step that led to efficient flight. The idea is that small genomes can exist in smaller cells than can large ones - and small cells themselves are more efficient at gas exchange than bigger cells because of their high surface area to volume ratio.

Supporters of the theory argue that improving efficiency by reducing genome (and hence cell) size could have made the evolutionary difference for an energy-intensive activity like flying.

Evidence to back up the idea came from another recent University of South Carolina study. These U.S. researchers analysed a particular class of repetitive DNA in chickens and found that they were consistently shorter than the equivalent sequences in humans. Over time, the loss of these DNA repeats in chickens could have played a significant role in reducing genome size.

"Our results are consistent with the hypothesis that genome size reduction in birds is adaptive … in view of the energetic demands of flight," said Austin Hughes and Helen Piontkivska, the authors of that previous study.

Following that, Harvard University geneticist Chris Organ and his colleagues became interested in how the unique genome architecture of birds evolved. But the absence of genetic information for dinosaurs - the long-extinct ancestors of bird - was a significant challenge to their research.

"Roughly 99% of all animals that have ever lived are now extinct, leaving us with scant amounts of evidence from the entirety of the living world", Organ told Cosmos Online. "It's important to consider extinct animals as living, breathing creatures because this helps us figure out how organisms evolve over long periods of time."

To overcome the lack of dinosaur DNA, Organ and colleagues took advantage of a well-known correlation between genome size and cell size - and looked for patterns in dinosaur cells preserved since the Mesozoic era as fossils.

"We calculated osteocyte [bone cell] size directly from histological sections of bone by measuring the small pockets … in the mineral matrix in which the bone cells resided during life," said the authors in their paper.

Cross-checking on modern day birds backed up the evidence that their method gave a good idea of genome size, so they applied it to 31 species of dinosaurs, including several extinct direct ancestors of birds.

Their results suggest that the small genomes typically associated with avian flight were also present in Saurischian dinosaurs - the ancestral group to birds - between 230 and 250 million years ago. This was long before the first birds spread their wings.

The team found that, in contrast, Ornithischian dinosaurs - a different lineage that gave rise only to terrestrial dinosaurs - had larger genomes.

There is now a long list of adaptations once considered to have evolved in birds, but now thought to have arisen in dinosaurs. These adaptations include feathers, nesting and parental care. Small genomes should now be added to that list, said the researchers.

Organ and colleagues point out that despite having small genomes from the get-go, birds have been under evolutionary pressure to maintain or even shrink them further.

"The secondary expansion of genome size in flightless birds suggests that, even though flight and genome size may not have arisen together, the two may be functionally related, perhaps at a physiological level," they said.