One of the most fascinating things about birds is their ability to fly. When birds first arose from theropod dinosaurs, a group that includes the well-known and terrifying carnivore Tyrannosaurus rex, they were the first vertebrates showing powered flight. The origin of flight was a huge evolutionary transition and probably depended on several avian traits. One such trait is believed be the exceptionally small genome size of birds. Genome size has a significant influence on cell size because a smaller genome can be contained within a smaller nucleus, and a smaller nucleus leads to a smaller cell, which is less costly to maintain. Some researchers believe that the small genomes of birds created energy-saving conditions beneficial for flight. However, unlike more familiar avian characteristics such as feathers, wings,efficient lungs and high body temperatures, the evolutionary history of genome size in birds was unknown. To investigate this area further, Chris Organ from Harvard University and his colleagues decided to explore the genome sizes of the dinosaur ancestors of birds.
But how can genome size be determined in extinct dinosaurs, whose only earthly remains are fossils? Fossils usually preserve only the mineralized parts of an organism, and small cellular details are typically lost. However,cells in bone called osteocytes are an exception. In living animals,individual osteocytes reside in small mineralized pockets called lacunae, and lacunae size can be measured from fossils. Because Organ and colleagues knew that cell size is related to genome size, they worked out the correlation between osteocyte size and genome size across living vertebrates. By using this correlation they could calculate the genome size of both theropod and non-theropod dinosaurs by simply measuring the size of their lacunae.
Interestingly, the researchers found that theropod dinosaurs had genome sizes as small as birds. On the other hand, ornithischian dinosaurs such as Triceratops, which didn't give rise to birds, had much larger genomes, similar to those of living reptiles. From these observations, the authors concluded that an abrupt reduction in genome size occurred in early theropods, well before the origin of birds.
Organ and colleagues were also interested in which parts of the genome had been reduced. In addition to the small portion of our genome made up of protein-coding genes, there is a large portion made up of so-called interspersed repetitive elements, which do not code for proteins or regulate gene expression. The authors knew that genome size is related to the number of interspersed repetitive elements in the genome, since larger genomes have a higher proportion of these elements. By correlating genome size to the percentage of interspersed repetitive elements in the genomes of living vertebrates, Organ and colleagues inferred the percentage of these elements in theropod dinosaurs, finding that the genomes of theropods had fewer interspersed repetitive elements than those of ornithischians.
The decrease in genome size in the close ancestors of birds was probably caused by reductions in the non-protein coding regions of the genome. This change likely occurred before the origins of flight, along with the appearance of other pre-flight features such as an efficient respiratory system and high body temperature. So, despite millions of years of evolution, it appears that T. rex and turkeys aren't that different!
