Spot the difference

"The problem is," says Professor Roger Thorpe, "one green pit viper looks like another green pit viper, even to a professional zoologist. But venoms can differ widely between different species." In sub-tropical Asia, where a snakebite can be disabling or even fatal, understanding the biodiversity of pit vipers is clearly important. "Anyone responsible for snake-bite therapy may wish to know which snakes live where so that they can have an appropriate antidote to hand, and similarly those working on snake toxins need to know the origin of their samples."

At Bangor, Professor Thorpe and Anita Malhotra are unravelling the complex family relationships of the large group of Asian pit vipers, Trimeresurus. "We’ve got together a phenomenal collection of over 600 tissue and venom samples," says Professor Thorpe. "That enables us to tackle questions that people haven’t been able to look at in the past because they’ve been working from a smaller database."

He and his collaborators have collected almost all the samples themselves during trips to Asia. They catch an animal in the wild, anaesthetise it, take a blood sample and a photograph, record morphological information, then let the animal go again (or their local collaborators keep it as a reference specimen).

This first-hand information can be crucial. "We know exactly where the specimens come from," explains Professor Thorpe. "This is often critically important, because snakes of the same or closely related species can have different venoms if they are living in different places." Professor Thorpe, Dr Malhotra and colleagues made this rather surprising discovery in 1995, and suggested an evolutionary explanation. "Sometimes the venom is very actively a subject of natural selection. This is driven by diet; the venom is being selected to subdue a particular prey in a given area."

A further complication is that existing morphological keys are often inadequate. Many were generated from museum specimens, which may have lost their colours and patterns when preserved in alcohol. "People frequently mis-identify these species from museum keys," says Dr Malhotra. "So we’re working on live animals in the wild, to try and pick up as much morphological information as we can."

Over the last decade, molecular techniques to analyse the DNA of these animals have also played a key role in clarifying the taxonomy. "It’s not till you get into the molecular side that you really start to understand the extent of the biodiversity, what snake lives where, and what antivenom should be available in that area," says Professor Thorpe.

Back in the lab, the team analyses nuclear and mitochondrial DNA to build phylogenies, or gene trees, which reveal how closely related different species actually are. Some of the results are surprising. "We think there are about 45 species of Trimeresurus, but you can suddenly find out that what you thought was one species is actually made up of five," explains Professor Thorpe. "Even the genus Trimeresurus may be an artificial one," adds Dr Malhotra. "At the end of our work we may end up with maybe half a dozen different genera."

Unfortunately, someone bitten by a pit viper doesn’t have time to send snake DNA off to a lab to find out what has bitten them. "We’re looking at the snake’s genes and then going back to the animals again, to produce more accurate diagnostic morphological characters which can be used to identify the snake out in the field," says Dr Malhotra.

Having gained a clearer view of the underlying relationships of Asian pit vipers, she is now starting to look more closely at venom evolution in these snakes and the forces driving it. Her work with Professor Thorpe to uncover the complex relationships between the species and their venoms will benefit other areas of research: it will frame the work of toxinologists and immunologists, but also ultimately benefit people who share a habitat with the deadly pit vipers.

See also

• Screen test: Article describing research into traditional Nigerian snakebite remedies
• Going ballistic: Article describing the use of DNA immunisation to create cheap effective antivenoms

External links

• Professor Roger Thorpe at the University of Wales, Bangor: Research details
• Dr Anita Malhotra at the University of Wales, Bangor: Research details

Further reading

Professor Roger Thorpe
Daltry J, Wüster W, Thorpe R S (1996). Diet and snake venom evolution. Nature. 379: 537–540.

Thorpe R S, Black H, Malhotra A (1996). Matrix correspondence tests on the DNA phylogeny of the Tenerife Lacertid elucidates both historical causes and morphological adaptation. Systematic Biology. 45: 335–343.

Thorpe R S (1996). The use of DNA divergence to help determine the correlates of evolution of morphological characters. Evolution 50: 524–53.1

Thorpe R S, Malhotra A (1996). Molecular and morphological evolution within small islands. Philosophical Transactions of the Royal Society, B Lond. 351: 815–822

Thorpe R S, Wüster W, Malhotra A (1997). Venomous Snakes: Ecology, Evolution and Snakebite. Oxford University Press. 276pp.

Malhotra A, Thorpe R S (2000). A phylogeny of the Trimeresurus group of pit-vipers: New evidence from a mitochondrial gene tree. Molecular Phylogenetics and Evolution 16: 199–211.

Malhotra A, Thorpe R S (2000). The dynamics of natural selection and vicariance in the Dominican anole: patterns of within-island molecular and morphological divergence. Evolution. 54(1): 245–58.

Pook C E, Wuster W, Thorpe R S (2000) Historical biogeography of the Western Rattlesnake (Serpentes: viperidae: Crotalus viridis), inferred from mitochondrial DNA sequence information. Molecular Phylogenetics and Evolution. 15(2): 269–82.

Tarkhnishvili D N, Thorpe R S, Arntzen J W (2000) Pre-pleistocene refugia and differentiation between populations of the caucasian salamander (Mertensiella caucasica). Molecular Phylogenetics and Evolution. 14(3): 414–22.

Giannasi N, Thorpe R S, Malhotra A (2000). A phylogenetic analysis of body size evolution in the Anolis roquet group (Sauria: Iguanidae): character displacement or size assortment? Molecular Ecology. 9(2): 193–202.

Guebitz T, Thorpe R S, Malhotra A (2000). Phylogeography and natural selection in the Tenerife gecko Tarentola delalandii: testing historical and adaptive hypotheses. Mol Ecol 9: 1213–1221.

Richard M, Thorpe R S (2000). Highly polymorphic microsatellites in the lacertid Gallotia galloti from the western Canary Islands. Mol Ecol. In press.

Stenson A, Malhotra A, Thorpe R S (2000). Highly polymorphic microsatellite loci from the Dominican anole (Anolis oculatus) and their amplification in other bimaculatus series anoles. Mol Ecol. In press.

Giannasi N, Thorpe R S, Malhotra A (2001). The use of amplified fragment length polymorphisms (AFLP) in determining species trees at fine taxonomic levels: Analysis of a medically important snake, Trimeresurus albolabris. Mol Ecol. In press.

Giannasi, N C, Malhotra A, Thorpe R S (2001). Nuclear and mitochondrial DNA phylogenies of the Trimeresurus complex: Implications for the gene versus species tree debate. Molecular Phylogenetics and Evolution. In press.

Surget-Groba Y, Heulin B, Guillaume C P, Thorpe R S, et al (2001). Intraspecific phylogeography of Lacerta vivipara and the evolution of viviparity. Molecular Phylogenetics and Evolution. In press.

Dr Anita Malhotra
Relevant publications listed above and also includes:

Malhotra A, Thorpe R S (1997). Size and shape variation in a Lesser Antillean anole, Anolis oculatus (Sauria: Iguanidae) in relation to habitat. Biol. J. Linn. Soc. 60: 53–72.

Malhotra A, Thorpe R S (1997). Microgeographic variation in scalation of Anolis oculatus (Dominica, West Indies): a multivariate analysis. Herpetologica 53: 49–62.