Studying Caribbean lizards when you are based in Northern Europe is maybe not the most obvious thing to do. But I couldn’t resist the charm of Anolis and embarked on a postdoc project with the aim of unlocking some of their mysteries. Since I have a background in comparative genomics, I was particularly excited about one odd feature of the green anole genome: unlike other vertebrates, it is remarkably cluttered with transposable elements.
Transposable elements (or TEs for short) are popularly referred to as jumping genes because they can copy and paste themselves within a genome. Traditionally TEs have been considered to be a ‘junk’ part of the genome, selfishly proliferating in an arms race with the host genome that is trying to keep TEs in check. As a defense, the host genome is usually restricting TEs from entering functionally important regions. But in the green anole even the Hox gene clusters, developmental control regions of the genome that are usually kept neat and tidy, got invaded by these TEs.
Even junk can become valuable in a different context. Indeed, there is circumstantial evidence that TEs can contribute to diversification and adaptation. For example, genomic incompatibilities arising from TE insertions have therefore been suggested to promote reproductive isolation. In other words, proliferation of TEs should be positively associated with speciation. Furthermore, some evolutionary innovations, like the mammalian placenta, appear to involve co-option of TEs for gene regulation.
Does the odd feature of the green anole genome indicate that something interesting is going on with TEs also in the evolutionary history of Anolis lizards? My study published in the Proceedings of the Royal Society of London B is a first attempt to take a closer look.
To this end, I compared the DNA sequences of Hox gene clusters of 30 lizard and snake species, including 20 Anolis species. I reconstructed the history of TE invasions of Anolis lizards and linked this to patterns of diversification across the phylogeny. The results revealed that there was a burst of TE activity in the lineage leading to extant Anolis. It did not stop there – TEs have continued to accumulate during speciation events, such that extant Anolis whose evolutionary history is characterized by many speciation events also have accumulated more TEs than lineages with relatively fewer speciation events. This finding supports the hypothesis that proliferation of TEs contributes to reproductive isolation, but what is cause and what is consequence remains to be seen.
Could TE activity also have contributed to the morphological differences that characterize Anolis ecomorphs? Well, I did not find evidence for this as yet, but this hypothesis is much more difficult to test since we need to learn more about developmental genetics to know where in the genome we should look. Nevertheless, I think this study shows that we can begin to unravel the genomics of adaptive radiation of these wonderful lizards!