Tuesday, March 13, 2007

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]

7 comments :

  1. How did they measure the genome size of extinct dinosaurs?

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  2. They measured the size of holes left in fossilized bone by osteocytes. Knowing that cell size correlates with the size of the genome allows them to estimate genome size.

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  3. what about bats? they should have smaller genomes too if it is a mere adaption for flight.

    Does someone know the genome size of bats?

    wicker

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  4. A neat story and thanks for the link to the Gregory lab.

    “It has been known for some time that birds have smaller genomes than reptiles and mammals. This has naturally given rise to an adaptationist 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.”

    Yes, the data of Organ et al., lends support to the view that the smaller genome of birds didn’t initially evolve because of the metabolic demands of flight (just as feathers appear to have first evolved for reasons other that flight).

    On the other hand, the ‘adaptationist explanation’ mentioned above may still be important in modern birds. If correct for example, then we expect to see a correlation between genome size and flightlessness in birds.

    From the Gregory website:

    “Average genome size versus relative flight ability in birds. There is a significant negative correlation between genome size and metabolic rate in birds (and mammals), with strong-flying birds (and bats) having especially small genomes.”

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  5. This was a typical aha! experience for me. You start out with the layman's lazy assumption that genome size isn't especially interesting. But after reading (and assimilating the beautiful graphics of) the post and Gregory's description of the research area this naive view is completely transformed.

    So thanks for another great post!

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  6. Note that the correlation within birds is between genome size and metabolic rate--not "flight ability." As I understand it, higher rates of cellular energy expenditure select for higher surface area-to-volume ratios (for a cell, this ratio is roughly supply-to-demand), thus smaller cells, thus smaller genome. Not the same thing as a reduction of weight for flight just-so story.

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  7. I've just started my PhD with T.R. Gregory, and this is a really great post about some of the stuff I'm somewhat involved in (I'll mostly be working on inverts rather than birds or mammals). Thanks for linking to us!

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