Low genetic diversity and high gene flow in the Aquatic Warbler (Acrocephalus paludicola), a threatened marshland songbird with a fragmented breeding range

Abstract

Breeding habitat loss and depletion of genetic diversity can have critically negative impacts on species, and especially so for habitat specialists. The Aquatic Warbler Acrocephalus paludicola is a threatened European songbird that breeds in fens, which have been lost and fragmented over centuries. We used microsatellite loci to perform a spatial and longitudinal comparison, comparing samples collected recently (contemporary) with those obtained about two decades earlier (historical) from c. 25% of the breeding range (Biebrza and Polesie) of the Aquatic Warbler. With these data we explored changes in genetic diversity, expansions and reductions of population size, population structure, and gene flow. Allelic richness, expected heterozygosity, the number of effective and private alleles, and effective population size (NE ≈ 200) were low and comparable across time and space. We detected a genetic bottleneck in contemporary Biebrza, and a less certain bottleneck in both historical and contemporary Polesie. Across space, Biebrza and Polesie were not genetically differentiated in either period. All the historical samples clustered together, but in the contemporary samples a small part of Polesie clustered separately (FST = 0.011). Across time, Biebrza and Polesie showed low but significant differentiation (FST = 0.026–0.064), and historical and contemporary groups clustered apart. A principal coordinate analysis on genetic distance detected three groups, which spanned sampling locations. In the temporal comparison, the contemporary group and contemporary Biebrza had elevated mean within‐group pairwise relatedness. We did not recover signals for sex‐biased dispersal, asymmetric gene flow or isolation‐by‐distance using the molecular data. Our results suggest that the studied populations show: (1) impoverished genetic diversity, (2) a change in allele frequencies over the two decades studied and (3) high gene flow between distant breeding sites, implying high resilience to habitat fragmentation that should facilitate the success of recolonising restored habitat patches by Aquatic Warblers.

Inbreeding depression depends on the body size and environmental conditions in a threatened songbird, the aquatic warblerAcrocephalus paludicola

“…The aquatic warbler represents a non-model species for which it is difficult to establish within-pair relatedness – because there is no pair bond, and complete father-to-offspring assignment requires high effort to capture males over a large area. Return rates of first-year birds are challenging to study because of their low natal site fidelity, resulting most probably from high juvenile mortality during the first winter migration and potentially also high post-natal dispersal of young (Kubacka et al . 2023).…”

“…Genotypes were called and filtered with the ref_map.pl programme, which utilises the gstacks and the populations modules. In gstacks , the var_alpha (alpha threshold for discovering SNPs) was set to 0.01, and all the samples were assumed to belong to one population, because of the extensive gene flow known in the aquatic warbler (Kubacka et al . 2023).…”

“…A recent study demonstrated that genetic diversity in the aquatic warbler is low and comparable to that of other threatened bird species (Evans & Sheldon 2008, Vitorino et al . 2019, Kubacka et al . 2023).…”