Population structure in the Andaman keelback, Xenochrophis tytleri: geographical distance and oceanic barriers to dispersal influence genetic divergence on the Andaman archipelago

Limited gene flow between populations due to geographic distance, presence of barriers or inherent low dispersal ability leads to the formation of genetically structured populations. Strong population structure indicates lowered levels or absence of gene flow which might lead to inbreeding and loss of genetic capacity to recuperate from anthropogenic stress and natural calamities. Terrestrial reptiles are generally known to have low dispersal abilities and few studies have explored drivers of their population structure on continental islands, where both anthropogenic stress and natural calamities are relatively common. We investigated the population structure and drivers of diversification of the Andaman keelback (Xenochrophis tytleri), an endemic, terrestrial and freshwater snake species in the Andaman archipelago, a continental group of islands in the Bay of Bengal. Data was collected from 86 individuals from seven islands and 78 individuals were sequenced for the gene Nuclear Dehydrogenase subunit 4 to identify the number of populations and distribution of genetic diversity across populations. We found 11 haplotypes on seven islands and observed high genetic differentiation between seven populations defined island-wise (F-ST = 0.82). We further tested the number of populations by incorporating spatial data into Bayesian Clustering Analysis (GENELAND) and identified six populations of the Andaman keelback. We tested for the influence of Isolation-by-distance on these populations. While the overall trend showed a positive correlation between geographic and genetic distance, a correlogram revealed that the positive correlation disappears beyond -20-40 km. We also tested for the presence of geographical barriers to gene flow using Monmonier's algorithm (SPADS), which identified five barriers to dispersal confirming that there are oceanic barriers to dispersal for some island populations of the Andaman keelback. As the Andaman Islands are arranged almost in a straight line from North to South, our data are insufficient to tease apart the roles of geographical distance and barriers to gene flow. We conclude that salt waters between near islands are weak barriers and as the geographical distance between islands increases, so does the strength of the barrier

Phylogeny and conservation status of the Indian egg-eater snake, Elachistodon westermanni Reinhardt, 1863 (Serpentes, Colubridae): Supplementary material

The Indian egg-eater (<i>Elachistodon westermanni</i>) is a monotypic species of the Genus <i>Elachistodon</i> distributed across the Indian sub-continent. In Africa, there are 13 species of egg-eating snakes of the Genus <i>Dasypeltis</i>. These two genera, <i>Elachistodon </i>and <i>Dasypeltis</i> were thought to be closely related due to similar diet specialization, and shared biogeographic history between the Indian sub-continent and the continent of Africa. In our study, we amplified three mitochondrial genes and one nuclear gene from <i>E. westermanni</i> and reconstructed molecular phylogeny utilizing published sequences to understand the evolutionary relationships between the African, and the Indian egg-eating snakes. We used morphological characters to reinforce our inferences on phylogenetic relationships. We show that the Indian egg-eater is sister to a set of cat snakes of the Genus <i>Boiga</i>,<i> </i>and it does not share recent ancestry with the African egg-eating snakes. Morphological character states point at similarities between <i>Elachistodon </i>and<i> Dasypeltis</i> only in characters associated with their feeding behaviour. <i>Elachistodon</i> <i>westermanni</i> was similar to the <i>Boiga </i>spp<i>.</i> in several other morphological characters, and we provisionally assign <i>E. westermanni</i> under the genus <i>Boiga</i>. Compilation of records of <i>E. westermanni</i> across the Indian sub-continent over the years revealed a positive "Lazarus" effect. We conclude that, the egg-eating behaviour and the associated morphological characters in the snake genera <i>Dasypeltis</i> and <i>Elachistodon </i>are a result of convergent evolution. Based on the conservation status of <i>E. westermanni</i>, it could serve as a flagship species to conserve important wildlife habitats that are being lost rapidly in India