Asymmetrical gene flow between coastal and inland dunes in a threatened digger wasp

Abstract

Connectivity is a species- and landscape-specific measure that is key to species conservation in fragmented landscapes. However, information on connectivity is often lacking, especially for insects that are known to be severely declining. Patterns of gene flow constitute an indirect measure of functional landscape connectivity. We studied the population genetic structure of the rare digger wasp Bembix rostrata in coastal and inland regions in and near Belgium. The species is restricted to sandy pioneer vegetations for nesting and is well known for its philopatry as it does not easily colonize vacant habitat. It has markedly declined in the last century, especially in the inland region where open sand habitat has decreased in area and became highly fragmented. To assess within and between region connectivity, we used mating system independent population genetic methods suitable for haplodiploid species. Despite B. rostrata being a poor colonizer, gene flow between existing populations appeared not highly restricted. We found more pronounced genetic structure in the small and isolated inland populations as compared to the well-connected coastal region, including a pattern of asymmetrical gene flow from coast to inland. Most likely, demography within different landscape contexts is the main factor explaining the discrepancy in connectivity and asymmetrical gene flow between the different regions. This illustrates the importance of large populations for the species′ metapopulation persistence. Therefore, to improve the conservation status B. rostrata through increased connectivity, large sites with sand dunes need to be preserved or created, especially in landscapes with small and isolated populations.

Strong gene flow across an urbanized coastal landscape in a dune specialist digger wasp

“…We studied functional connectivity based on landscape genetics methods of the digger wasp Bembix rostrata in a human-altered coastal region. Overall, the populations of this dune-specialist insect are genetically well-connected in this region (Batsleer et al 2022a). Despite this overall genetic admixture, we could detect a consistent signal of urban features facilitating gene flow, in addition to the prominent isolation-by-distance (IBD).…”

“…Bembix rostrata is often considered to be a philopatric species, as they do not easily colonize vacant habitat and exhibit site fidelity throughout the nesting season (Nielsen 1945;Larsson 1986;Bogusch et al 2021;Batsleer et al 2022b). However, a population genetics study across Belgium shows that this does not preclude gene flow between existing populations, including dispersal to distant, more isolated populations (Batsleer et al 2022a).…”

“…DNA extraction and PCR amplification was performed as described in (Batsleer et al 2022a) and summarized hereafter. DNA is extracted from the wing tips with a protocol based on Chelex (Biorad).…”

“…33 polymorphic microsatellites are PCR-amplified for each sample and electropherograms are scored using Geneious Prime (Biomatters). Microsatellites that were excluded after assumption testing (Hardy-Weinberg, null alleles and linkage disequilibrium) in the population genetics study in (Batsleer et al 2022a) were also excluded here, resulting in 21 microsatellites for the subsequent genetic analysis. A total of 539 samples were successfully amplified.…”

“…Although a population genetics study shows that the subregion is genetically well connected (Batsleer et al 2022a), the influence of the surrounding landscape on genetic relatedness between individuals remains unexplored. We use a resistance raster approach to quantify resistance values of the distinctive landscape types in our study area.…”