First non-avian reptile sequence helps explain vertebrate evolution.
Publication of the genome of the North American green anole lizard has filled a yawning genome-sequence gap in the animal lineage. The paper, which appears today in Nature1, is the first to sequence the genome of a non-avian reptile. "This fills out a clade that has been completely ignored before," says lead author Jessica Alföldi of the Broad Institute of MIT and Harvard in Cambridge, Massachusetts.
Amniotes, the first truly terrestrial vertebrates, diverged from other animals some 320 million years ago to form the mammalian and reptilian lineages. Until now, however, the only representatives of the reptile branch to be sequenced were birds — the chicken, the turkey and the zebra finch.
But "birds have rather odd genomes that may not be totally representative", says Alföldi, who collaborated with more than 20 different groups on the project. Molecular ecologist Anita Malhotra of Bangor University, UK, who was not involved in the study, agrees. "One can't possibly expect to comprehend the whole of amniote biology by comparison to the chicken," she says.
The genome of the green anole, Anolis carolinensis, has already offered up evidence to support this point, including the revelation — through the sequence of a previously unknown X chromosome — of a sex-determination system similar to that of humans. This finding makes birds, with their 'reverse' ZW system — in which females, not males, have two different sex chromosomes — the odd ones out among the amniotes.
Cracking the egg
The A. carolinensis sequence also provides clues about the development of the amniotic egg, a major evolutionary innovation that allowed animals to breed out of water by protecting the developing embryo from drying out.
Using proteomics — large-scale identification of proteins by mass spectrometry — facilitated by the genome sequence, the researchers were able to hunt down and investigate egg-protein genes in the lizard. They found that, among amniotes, egg proteins seem to have evolved more rapidly than other proteins. The high mutation rate that this would have entailed may have been instrumental in the evolution of the egg.
A wealth of information
The Anolis genus is considered a textbook example of evolutionary adaptation, owing to the independent evolution of hundreds of sister species on islands throughout the Caribbean. Alföldi and her colleagues knew that the lizards offered "a rather neat system to study evolution", but until now, "we didn't really know how they all fitted together".
By comparing the A. carolinensis genome to shorter sequences from 92 other Anolis species, the authors uncovered evidence that similar features seen in different species resulted from convergent evolution rather than inter-island migration.
Their work underscores the importance of learning as much about related species' genomes as possible. "To me, this paper highlights the fact that just taking one representative of a major lineage is not enough," says Malhotra.
Vertebrate biologist and palaeontologist Susan Evans at University College London agrees, but thinks that even more amniote sequences are needed. "It is worth bearing in mind that the very reasons that made Anolis an obvious choice may also make it atypical," she says. "It will be illuminating to compare it with more conservative lizards - not to mention representatives of a wider range of reptiles such as snakes, tuatara, crocodiles and turtles."
Alföldi, J. et al. Nature http://dx.doi.org/10.1038/nature10390 (2011).
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Sweetlove, L. Lizard genome unveiled. Nature (2011). https://doi.org/10.1038/news.2011.512