The effects of historical fragmentation on major histocompatibility complex class II β and microsatellite variation in the Aegean island reptile, Podarcis erhardii

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.

Long-Term Habitat Fragmentation Is Associated With Reduced MHC IIB Diversity and Increased Infections in Amphibian Hosts

“…The ecological and evolutionary impacts of long-term fragmentation on land-bridge island fauna has been studied in a number of systems (Karr, 1982;Aguilar et al, 2004;Hurston et al, 2009;Estrada-Villegas et al, 2010;Duryea et al, 2015;Belasen et al, 2016;Santonastaso et al, 2017). In a previous study examining immunogenetic diversity in island vertebrates, MHC diversity remained high while overall diversity dropped (Aguilar et al, 2004).…”

Evolutionary genetics of immunological supertypes reveals two faces of the Red Queen

“…A puzzling observation about MHC gene evolution is that some studies report that MHC diversity is primarily affected by drift 51 – 53 , whereas others show that the high polymorphism is maintained by balancing selection 54 , 55 . This contradiction is reconciled when realising that genetic drift (as well as selection) acts on the alleles, whereas balancing selection acts on the immunological function of the alleles, defined by their ST.…”

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

“…This reflects the limited overlap of haplotypes between populations. In the Cycladic Archipelago in the Aegean Sea, Aegean wall lizard’s populations show little or no contemporary gene flow between different island populations ( Santonastaso et al, 2017 ). This is a group of continental islands comparable to the Andaman Islands as they were also formed due to rise in sea levels in the LGM.…”