Abstract
The major histocompatibility complex (MHC) plays a key role in disease resistance and is the most polymorphic gene region in vertebrates. Although habitat fragmentation is predicted to lead to a loss in MHC variation through drift, the impact of other evolutionary forces may counter this effect. Here we assess the impact of selection, drift, migration, and recombination on MHC class II and microsatellite variability in 14 island populations of the Aegean wall lizard Podarcis erhardii. Lizards were sampled from islands within the Cyclades (Greece) formed by rising sea levels as the last glacial maximum approximately 20,000 before present. Bathymetric data were used to determine the area and age of each island, allowing us to infer the corresponding magnitude and timing of genetic bottlenecks associated with island formation. Both MHC and microsatellite variation were positively associated with island area, supporting the hypothesis that drift governs neutral and adaptive variation in this system. However, MHC but not microsatellite variability declined significantly with island age. This discrepancy is likely due to the fact that microsatellites attain mutation‐drift equilibrium more rapidly than MHC. Although we detected signals of balancing selection, recombination and migration, the effects of these evolutionary processes appeared negligible relative to drift. This study demonstrates how land bridge islands can provide novel insights into the impact of historical fragmentation on genetic diversity as well as help disentangle the effects of different evolutionary forces on neutral and adaptive diversity.
Highlights
Understanding how fragmentation affects genetic variation and viability of wild populations is of primary importance to conservation biology
There was no significant correlation between the number of individuals sequenced within each island (NMHC) and either ARMHC or P, indicating that the sample sizes within islands were sufficient to give an unbiased estimate major histocompatibility complex (MHC) diversity
We hypothesized that drift was the predominant force and that neither selection, migration nor recombination was sufficient to overcome its cumulative effects. In keeping with this second hypothesis, we find that patterns of MHC variation appear to be largely governed by drift
Summary
Understanding how fragmentation affects genetic variation and viability of wild populations is of primary importance to conservation biology. The Aegean wall lizard Podarcis erhardii (Figure 1) is an ideal study organism for assessing the effects of island fragmentation on adaptive variation. With few exceptions, this species is found throughout Cyclades and does not readily disperse between islands (Hurston et al, 2009; Roca, Foufopoulos, Valakos, & Pafilis, 2009). This study capitalizes on this unique continental shelf island system to assess how drift, selection, migration, and recombination have impacted MHC variability in P. erhardii populations that have been subject to fragmentation events of different magnitude (island area) and duration (island age). Genomic DNA was extracted using phenol–chloroform, as described in Sambrook, Russell, and Maniatis (2001)
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