The research in my lab focuses on using DNA and protein sequences to build molecular phylogenies in order to investigate evolutionary relationships among organisms and to answer questions in evolutionary biology, taxonomy and biogeography.
Our research falls in 4 main areas:
1) Evolutionary relationships of the pulmonate land snails and slugs
2) Evolutionary relationships in the planktonic foraminifera
3) Snails as intermediate hosts for medically important parasites
1) Evolutionary Relationships in the Pulmonate Land Snails and Slugs (Pulmonata: Stylommatophora)
Land snails and slugs represent one of the largest invasions of the land, comprising some 30,000-35,000 species. They are major components of many ecosystems, and they include the vectors of many serious diseases. They have also become important models for studies on the mechanisms of evolution and are eminently suitable as subjects for biogeographic studies of early tectonic events.
Evolutionary Relationships and Taxonomy
Until recently, the origins and the deep-level evolutionary relationships of the major groupings of the land snails and slugs remained virtually unknown. Anatomical studies gave confusing and conflicting results, such that there appeared to be almost as many classifications as there were classifiers.
Working in the lab
Our recent molecular work has begun to unravel the evolutionary relationships within the land snails and slugs. We have sequenced part of the ribosomal RNA gene cluster for some 260 species of land snails and slugs in the Pulmonate suborder Stylommatophora. Although our molecular phylogeny largely agrees with the traditional taxonomy at the level of families, at deeper levels the molecular tree markedly disagrees with the traditional taxonomy. Remarkably, the Orthurethra (previously considered to be ancestral) appear to be relatively advanced, indicating that supposedly primitive features such as the orthurethran kidney are in fact derived. This finding is leading to a radical reinterpretation of early land snail evolution.
Adult Achatina achatina and baby
We are currently extending our molecular work on the Stylommatophora to include more taxa and to obtain new genes in order to provide independent confirmation of the relationships in the rDNA tree. In addition, we are expanding our phylogeny to include other non-stylommatophoran pulmonate taxa. We are also conducting in-depth phylogenetic studies to determine the relationships within the major groups of land snails and slugs, in particular the Orthurethra, Achatinoidea, Helicoidea and Limacoidea.
Population Genetics and Intraspecific Phylogeography
In addition to our work on land snail phylogeny, we are also examining population-level genetic variation within the land snails Cepaea hortensis, Achatina fulica and Helix aspersa.
The white lipped banded snail Cepaea hortensis is distributed across Europe, and is also found in Iceland, Greenland, as well as down the east coast of Canada and the USA. This unusual distribution pattern has led to much speculation concerning how C. hortensis got to North America and it has been suggested that the Vikings may have played a part in their introduction. We are using genetic data to examine variation in C. hortensis across continental Europe, its spread to Great Britain and Ireland and its subsequent migration to Iceland, Greenland and North America. Our genetic data so far reveal that a single genetic type exists in North America and Iceland. Interestingly, this genetic type is also present in Scandinavia, suggesting that the Vikings may indeed have played a part in the transfer of C. hortensis to Iceland and the New World.
The giant African land snail Achatina (=Lissachatina) fulica originated in East Africa but is now distributed across the tropics. It is an intermediate host for the nematode parasite Angiostrongylus cantonensis, which can cause eosinophilic meningitis in humans. We are using genetic data to track the migration of A. fulica out of Africa and the possible link between the spread of A. fulica and A. cantonensis.
The common garden snail Helix aspersa (=Canatreus aspersus) is a European species which has now been distributed across the globe. We are examining genetic variation in H. aspersa populations to track the movements of this land snail around the world.
Nantarat, N., Tongkerd P., Sutcharit, C., Wade, C. M., Naggs, F. & Panha, S., 2013. Phylogenetic relationships of the operculate land snail genus Cyclophorus Montfort, 1810 in Thailand. Molecular Phylogenetics and Evolution. 70: 99-111.
Wade, C. M., Mordan, P. B. & Naggs, F. 2006. Evolutionary relationships among the Pulmonate land snails and slugs (Pulmonata, Stylommatophora). Biological Journal of the Linnean Society 87: 593-610.
Wade, C. M. Mordan, P. B. & Clarke, B. C. 2001. A phylogeny of the land snails (Gastropoda: Pulmonata). Proceedings of the Royal Society (London): Biological Sciences 268: 413-422.
2) Evolutionary Relationships in the Planktonic Foraminifera
The planktonic foraminiferan Globigerinoides sacculifer
The planktonic foraminifera are an important group of unicellular protists that form part of the marine zooplankton and are globally distributed across the world's oceans. Their shells are readily fossilised in oceanic sediments and form a high-resolution archive of past climate. The planktonic foraminifera therefore play a vital role in the campaign to understand the processes of climate change.
Microfossils of the planktonic foraminiferan
Genetic studies of the planktonic foraminifera have provided a great deal of information about evolutionary relationships within the group. Interestingly, the evolutionary transition between a benthic and a planktonic way of life appears to have occurred several times with the planktonic spinose species (foraminiferans with spines) clustering separately from the planktonic non-spinose species. We are currently undertaking new molecular studies of the foraminifera utilising both the SSU rRNA gene and other nuclear genes in order to further investigate the evolutionary relationships within the group.
Collecting Benthic Foraminifera in Norfolk
Hidden Diversity and the Implications for Reconstructing Past Climate
One of the most interesting outcomes of genetic studies of the planktonic foraminifera concerns the extent of differentiation within individual morphospecies (species defined on the basis of morphological differences). Individual mophospecies of planktonic foraminifera show an exceptionally high level of genetic diversity in their SSU rRNA genes, and many include more than one genetically distinct entity. Indeed, some of these genetic types may warrant classification as separate 'cryptic' species. This finding is of interest because of the role of foraminiferal microfossils in reconstructing past climates. For climate reconstruction it has been assumed that each 'morphospecies' is a single entity with a specific ecological (and thus climatic) preference. If the distinct genetic types within a morphospecies are in fact adapted to different habitats, and exhibit different ecological and climate preferences, then the assumption that each morphospecies is characteristic of a particular climate would be wrong. If so, there could be significant errors in the current models of climate reconstruction. Recent work suggests that different genetic types are indeed associated with different environments. If it does become possible to distinguish these newly recognised genetic types in the fossil record, the role of the foraminifera as indicators of past climate could be greatly enhanced.
Picking Foraminifera in the Lab
Biogeography and Gene Flow
Genetic studies of the planktonic foraminifera have also begun to illuminate the processes of diversification and speciation in the oceans. Despite the high degree of genetic diversity observed in their SSU rRNA genes, we have found identical sequence types (genotypes) in individuals collected at opposite ends of the globe in several morphospecies. Perhaps most remarkable is our discovery of identical rRNA genotypes in individuals collected from the Arctic and Antarctic subpolar regions within each of the cool-water morphospecies Globigerina bulloides, Turboratalita quinqueloba and Neogloboquadrina. This is surprising, as these morphospecies are only found in the high latitudes and are absent from the tropical regions, which are considered a formidable barrier to gene flow. Similarly, we have found identical rRNA genotypes within the warm-water morphospecies Orbulina universa and Globigerinoides sacculifer in individuals collected from the Caribbean and Coral Seas, and also in individuals from the Eastern Pacific and Mediterranean in O. universa. These findings are important because they suggest that gene flow can occur on a global scale within the planktonic foraminifera, with genetic intermixing between populations as far apart as the Arctic and Antarctic, or the Pacific and Atlantic. Nevertheless, our genetic data also reveals the existence of clear barriers to gene flow in the oceans. For example, genotypes of the planktonic foraminiferan Neogloboquadrina pachyderma do not transit the shallow Bering Sea with one genetic type found in the Arctic Ocean and North Atlantic, and a different genotype in the North Pacific.
The RRS Charles Darwin
Seears, H. A., Darling, K. F. & Wade, C. M. 2012 Ecological partitioning and diversity in tropical planktonic foraminifera. BMC Evolutionary Biology 12: 54.
Darling, K. F., Kucera, M. & Wade, C. M. 2007. Global molecular phylogeography reveals persistent Arctic circumpolar isolation in a marine planktonic protist. Proceedings of the National Academy of Sciences (USA) 104: 5002-5007.
Darling, K. F., Kucera, M., Pudsey, C. J., & Wade, C. M. 2004. Molecular evidence links cryptic diversification in polar planktonic protists to Quaternary climate dynamics. Proceedings of the National Academy of Sciences (USA) 101: 7657-7662.
Darling, K. F., Wade, C. M., Stewart, I. A., Kroon, D., Dingle, R. & Leigh Brown, A. J. 2000. Molecular evidence for genetic mixing of Arctic and Antarctic subpolar populations of planktonic foraminifers. Nature 405:43-47.
3) Snails as intermediate hosts for medically important parasites
Several nematodes use snails and slugs as intermediate hosts, including nematodes of both medical and veterinary importance. Nematodes of the genus Angiostrongylus are of particular interest. They include Angiostrongylus cantonensis, which can cause eosinophilic meningitis in humans, and the French Heartworm Angiostrongylus vasorum, a veterinary parasite that infects dogs. We have developed a molecular technique for the identification of 3rd juvenile stage A. cantonensis and A. vasorum in snail and slug intermediate hosts. Using this technique we are undertaking a global survey of the incidence of A. cantonenis in snails and slugs and a UK-wide survey of the incidence of A. vasorum in snail and slug hosts. In addition, we are determining the incidence of adult nematodes within UK land snails and slugs.
Isolating nematodes from snail hosts
Aquatic snails also act as intermediate hosts for trematodes of the genus Schistosoma that lead to schistosomiasis (bilharzia) in humans. We are examining the link between Biomphalaria and Schistosomiasis in Lake Victoria.
Standley, C. J., Wade, C.M. & Stothard, J. R. 2011. A fresh insight into transmission of Schistosomiasis: a misleading tale of Biomphalaria in Lake Victoria. PLoS ONE 6 (10) e26563.
Fontanilla, I. K. C. & Wade, C. M. 2008. The small subunit (SSU) ribosomal (r) RNA gene as a genetic marker for identifying infective 3rd juvenile stage Angiostrongylus cantonensis. Acta Tropica 105: 181-186.
We are using bioinformatics tools to look at how differences in evolutionary rate between organisms can mislead phylogeny estimation (long branch attraction). We are also using bioinformatics techniques to examine questions in developmental biology and in particular we are interested in the evolutionary history of the nodal gene that has an important role in left/right asymmetry.
Evans, T, Wade, C. M., Chapman F. A., Johnson, A. D. & Loose, M. 2014. Acquisition of germ plasm accelerates vertebrate evolution. Science 344(6180): 200-203