Genome Size in Birds

 
The Animal Genome Size Database is maintained by T. Ryan Gregory of the University of Guelph in Ontario. Gregory has collected data on genome size in animals from the scientific literature and from work in his own lab. He is interested in several projects on genome evolution.

There are several ways of reporting genome size. The most common is to give the C-value (haploid genome size) in picograms (pg) because a lot of the data simply measures the amount of DNA in a nucleus using a DNA-specific stain. The range of C-values for different groups of organisms is shown in the figure (above right). As the legend states, there's no special significance to the order of the groups (from top to bottom) other than the fact that it's easy to understand if mammals are at the top.

One of the things that Gregory works on is the correlation between cell size and genome size. It turns out that the size of the nucleus is related to the size of the cell, such that large genomes give rise to large nuclei and large cells. This is particularly evident when you look at red blood cells and Gregory has a remarkable image showing this correlation on his website [Gregory Lab].

It has been known for some time that birds have smaller genomes than reptiles and mammals. This has natually given rise to an adaptionist explanation;namely, that the small genome is due to selection for small cells in birds because they exert a lot of energy in flight. In other words, small genomes are an adaption for flight.

A recent News article on the Nature website raises an important question concerning this adaptionist explanation. If birds have smaller genomes than other vertebrates then is that a derived trait or did birds inherit a small genome from their dinosaur ancestors? [Did a 'light' genome help birds take flight?].

A study of dinosaur genomes hints that the early evolution of a smaller genome might have been necessary for later vertebrates to take to the skies.

Birds have long been known to have much smaller genomes than mammals and reptiles living on the ground. And a small genome has been linked to both small cell size and high metabolic rate: the lower volume-to-surface ratio of small cells, which don't have much DNA to pack inside, can allow for faster transport of nutrients and signals across the membrane. Thus, some suggest that the energetic demands of flight require birds to have a 'light' genome.

But which came first: flying birds or the smaller genome?

The paper by Organ et al. (2007) looked at genome size in extinct dinosaurs with a view to discovering whether the bird ancestors had large or small genomes. Obviously, they couldn't measure genome size directly in fossils. What they did was measure the size of fossilized cells, having previously established that there's a correlation between the size of cells and the size of the nucleus. The size of the nucleus, in turn, depends on the amount of DNA in the genome.

The result is shown below. Red and purple lines indicate species with small genomes. You can see that the bird lineages (Aves) all have smallish genomes. So do the theropods that cluster with the birds on the right-hand group within Dinosauria. What this means is that the entire group of dinosaurs that descended from theropods had small genomes. It means that birds, which didn't arise until later, inherited their small genomes from ancestral theropods.

The result indicates that small genome size in birds is not an adaptation for flight. Perhaps it is not an adaptation at all but simply an accident due to the fact that the ancestor of sauropods just happened to have a reduced genome.

Before I had a chance to prepare this article, Carl Zimmer had not only done the work and interviewed Gregory, but he had published the review on the Science website [Jurassic Genome]!!! Please read Zimmer's excellent article for a more complete story.

Organ, C.L., Shedlock, A.M., Meade, A., Pagel, M., and Edwards, S.V. (2007) Origin of avian genome size and structure in non-avian dinosaurs. Nature 446:180-184. [PubMed]