Abstract
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 (FST = 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.
Highlights
Limited gene flow between populations of organisms due to distance, barriers or low dispersal ability is known to result in genetically structured populations
From the ND4 gene sequences from 78 samples, we identified 11 haplotypes with a haplotype diversity Hd = 0.87, nuclear diversity π = 0.009 and 18 variable positions (Fig. 2B)
Considering the ever-increasing challenges for conserving diversity in the tropics, there is a critical need for studying the genetic diversity of populations and factors influencing the spatial distribution of diversity
Summary
Limited gene flow between populations of organisms due to distance, barriers or low dispersal ability is known to result in genetically structured populations. To explain speciation and genetic variation in space, the concept of isolation-bydistance (IBD) (Wright, 1943) and the role of environmental barriers, such as landscape features in limiting species distribution (Wallace, 1876), have been considered. While landscape features can be strong barriers, the dispersal pattern of a species results from the combined effects of inherent dispersal capacity and inhibiting factors, which in turn has significant implications on evolution and population genetics (Grosberg & Cunningham, 2001; Palumbi, 1994). The limitation imposed by barriers on dispersal could be especially critical for small vertebrates due to their low dispersal capacity (Schippers et al, 1996; Cushman et al, 2006; Wang, Savage & Bradley Shaffer, 2009)
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