Two Snake Genomes Equals a Good Reading Day

It’s a good week for snake genomics, because PNAS has published both the Burmese python genome (Castoe et al. 2013) and the king cobra genome (Vonk et al. 2013). The related papers come from separate research teams (the python people mostly from Colorado and the cobra cabal the Netherlands), albeit with significant overlap between them. The world of snake molecular biology is a small one, after all.

The python group planted its snake genome flag in the ground more than two years ago with a paper describing their first draft assembly, and I have been eagerly awaiting the  results of their full-blown analysis. It was well worth the wait. As the python is known for its feast-or-famine metabolism (the small intestine can grow up to three times in size after gulping a meal half the python’s body mass), the researchers provided a very elegant analysis of the differential expression of genes in digestive organs before and after a meal and show, basically, that different genes are turned on or off either pre- or post-feeding. Very cool.

Figure 3B from Castoe et al. (2013), showing genes that have undergone positive selection on the vertebrate lineage leading to snakes.

Figure 3B from Castoe et al. (2013), showing genes that have undergone positive selection on the vertebrate lineage leading to snakes.

The authors of the python paper also investigated how snakes evolved their iconic and constrained morphology – because, after all, snakes lack legs and are equipped with a feeding apparatus equivalent to a human being swallowing a 16lb Thanksgiving turkey whole, and the molecular bases of these adaptations are unresolved. They analyzed a large number of genes shared across vertebrates – called orthologs – and found that during the evolution of the vertebrate lineage leading to snakes, many genes associated with skull and spinal development, metabolism and other functions experienced a faster rate of evolution than in other lineages. Also very cool.

Next up: king cobra. Why do we need both a python AND a cobra genome? The answer lies in the difference between these two snakes. One of them – and I hope you guessed cobra – is venomous. So it is no surprise that much of the justification for sequencing the king cobra genome included a need to understand the evolutionary origins and maintenance of the genes that control venom production. The cobra genome contains a multitude of protein families that underwent a significant expansion during cobra evolution that resulted in what we see today – a highly potent mixture of toxins designed to ensure certain death to a chosen prey item.

The two genomes differ vastly in their qualities of assembly. Through a mixture of various sequencing methods, the python team was able to get an N50 value of 207kb, meaning 50% of the assembled chunks of contiguous sequence were at least 207,000 base pairs of DNA in length. That assures that the research team would be able to recover the majority of genes – exons and introns and all. The cobra genome by contrast has an N50 value of only 3,982 base pairs, meaning that some of the genes may be fragmentary and the length of many introns will remain unresolved. However, I think that using N50 as a the gold standard of genome assembly “quality” is misleading. Sequencing strategies that significantly raise N50 values cost more money. In this day and age of modern biology, where small labs or groups of researchers conjure up whatever resources they can for an in-house genome sequencing project, the most affordable strategy for however you wish to address your biological questions will probably suffice. Both of these snake genome papers make the cut in that regard, and they are a significant contribution to the field of reptilian genomics.


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