Reconstructing Atheris viper evolution in Africa

Posted by Michele Menegon and Stuart Marsden

A paper published recently in Molecular Phylogenetics & Evolution by Michele Menegon, PhD student in the group, Simon Loader and Sylvain Ursenbacher (University of Basel), William Branch (Nelson Mandela Metropolitan University), Tim Davenport (Wildlife Conservation Society), and Stu, reconstructs the evolutionary history of Atheris vipers across Africa.

 

The team analysed mitochondrial DNA to uncover phylogenetic relationships among Africa's wonderful Atheris viper species, including Matilda’s Horned Viper (Atheris matildae), a species recently discovered by Michele, Tim Davenport and Kim Howell (Menegon et al. 2011).

 

The recently discovered Matilda's Horned Viper Atheris matildae (Photo: Michele Menegon)

The study used molecular clock estimates to age taxa, and clades within ten of the genus’ 15  species. This was done to investigate whether the timing of the divergence within Atheris corresponds to specific geographic and climatic events. In other words, where did the forest-associated species we find today in the mountains of the EAM hotspot come from, and how does their current pattern of distribution and endemism across mountains fit with geological changes such as the formation of the Rift Valley, and climatic changes over the past 20 million years?

Africa's dry lowlands form barriers to dispersal in forest-associated reptiles and amphibians (Photo: Michele Menegon)

Analyses indicate that the early diversification of the genus probably took place in lowland Central/West Africa around 15-18 mya, potentially coinciding with the final phase of the existence of the ‘pan-African forest’ (a period when moist forest stretched right across the continent). The descendants from this first colonization from Central Africa to Eastern Africa are species currently confined to high mountains of Mozambique (A. mabuensis) and Kenya (A. desaixi). The presence of these species on distant mountain tops, with no known sister species from intervening mountains, may indicate that there have been many extinctions of species since the initial colonisation and divergence.

Atheris rungweensis (Photo: Michele Menegon)

More recently, between seven and nine million years, we highlighted divergences of extant species from the fragmented montane forests of the Albertine Rift, Southern Highlands and Eastern Arc Mountains. This phylogenetic pattern suggests a more recent colonization into these areas, delayed by around 7 million years from the first appearance of the genus in East Africa (A. nitschei and A. rungweensis in the Albertine Rift and A. ceratophora, A. matildae and A. barbouri in the Eastern Arc). 

Atheris nitschei (Photo: Michele Menegon)

A possible reason for this late colonization is the physical and climatic barrier of the Great Rift Valley. Such narrowing of dispersal routes might have promoted a ‘C-shaped’ dispersal route with a northern and a southern front – where species’ ‘relatedness’ tracks the species along these routes. Interestingly, despite the late colonization of the mountain ranges east of the Albertine Rift, a high proportion of current Atheris diversity occurs there, highlighting the likely role of mountains in promoting speciation and species accumulation.

The exquisite Uzungwe viper Atheris barbouri, considered one of the rarest snakes in the world (Photo: Silvia Ceppi)

 
Of the 15 recognised species, only one (A. squamigera) has a particularly wide distribution ranging from Nigeria in the west, across the Congolian forest belt to western Kenya in the east and Angola in the south (Phelps 2010). All other Atheris species have small known ranges. For example, A. desaixi is found in just two localities in the Kenyan highlands, A. mabuensis in two mountain blocks in Mozambique, A. acuminata is known for a single locality in Uganda and the newly described A. matildae in a single forest fragment in the Southern Highlands of Tanzania. Of course, these restricted-range vipers, like other herps of the Eastern Arc such as Nectophrynoides toads, will face intense battles to avoid extinction in the future. Michele’s PhD will further examine the protection offered to these herps across the region. 

Maintaining intact Afromontane forests holds the key to conserving restricted-range Atheris vipers (Photo:Michele Menegon)

References

Menegon M, Davenport TRB, Howell KM. (2011). Description of a new and critically endangered species of Atheris (Serpentes: Viperidae) from the Southern Highlands of Tanzania, with an overview of the country's tree viper fauna. Zootaxa 3120: 43-54. (Atheris matildae sp. nov.)

Menegon, M., Loader, S.P., Marsden, S.J. Branch, W., Davenport, T. & Ursenbacher, S. (2014). The genus Atheris (Serpentes: Viperidae) in East Africa: phylogeny and the role of rifting and paleoclimate in shaping the current pattern of species diversity. Molecular Phylogenetics & Evolution 79: 12-22.

Phelps, T. (2010). Old World Vipers, A Natural History of the Azemiopinae and Viperinae. Edition Chimaira, Frankfurt am Main, Germany, 558 pp.