Recent theoretical and experimental models have revealed the role played by evolution during species spread, and in particular have questioned the influence of genetic drift at range edges. By investigating the spread of an aquatic invader in patchy habitats, we quantified genetic drift and explored its consequences for genetic diversity and fitness. We examined the interplay of gene flow and genetic drift in 36 populations of the red swamp crayfish, Procambarus clarkii, in a relatively recently invaded wetland area (30 years, Brière, northwest France). Despite the small spatial scale of our study (15 km2), populations were highly structured according to the strong barrier of land surfaces and revealed a clear pattern of colonization through watercourses. Isolated populations exhibited small effective sizes and low dispersal rates that depended on water connectivity, suggesting that genetic drift dominated in the evolution of allele frequencies in these populations. We also observed a significant decrease in the genetic diversity of isolated populations over only a 2‐year period, but failed to demonstrate an associated fitness cost using fluctuating asymmetry. This study documents the possible strong influence of genetic drift during the spread of a species, and such findings provide critical insights into the current context of profound rearrangements in species distributions due to global change.
Multispecies population genetics is an emerging field that provides insight relevant to conservation biology and community ecology. However, to date, this approach is limited to species with available genetic resources. The use of thousands of single‐nucleotide polymorphism (SNP) markers developed from recent genotyping‐by‐sequencing (GBS) technologies is a roadmap for the study of nonmodel species, but remains cost prohibitive when several, distantly related species are involved. We aimed to overcome this issue using a single HiSeq3000 run of restriction‐site associated DNA sequencing (RAD‐Seq) to retrieve SNP markers for 40 diverse species including plants, invertebrates, fish, and mammals. We developed a Python‐based pipeline to isolate c. 100–500 high‐quality SNP markers for each species that could be genotyped through classical PCR amplification methods. To assess the quality of these markers, we validated our approach on c. 160 of the characterized SNPs for each of 18 Neotropical fish species from the river Maroni (French Guiana, South America), using the MassARRAY iPLEX platform from Agena Bioscience (San Diego, CA, USA). A run of the pipeline applying stringent filtering parameters enabled the successful design of between 130 and 3,492 SNP markers for 30 of the 40 study species. Relaxing pipeline parameters allows for an increase in the number of detected SNPs. Across the 18 species from French Guiana, an average of 85% of markers were successfully amplified, polymorphic, and scored in ≥90% of individuals (c. 200 individuals per species). The great majority (>98%) of these markers were at Hardy–Weinberg equilibrium in each sampling site from the river Maroni. This SNP discovery was performed at the cost of c. $US110 for each of the 40 species. Genotyping was performed at the cost of c. $US6000 for each of the 18 fish species with an average of 200 individuals per species. This strategy was found cost‐and‐time efficient to develop hundreds of SNP markers for a large range of nonmodel species, which can be used to investigate ecological and evolutionary questions that do not require whole‐genome coverage.
Understanding the effect of human induced landscape fragmentation on gene flow and evolutionary potential of wild populations has become a major concern. Here, we investigated the effect of riverscape fragmentation on patterns of genetic diversity in the freshwater resident brook lamprey (Lampetra planeri) that has a low ability to pass obstacles to migration. We also tested the hypotheses of i) asymmetric gene flow following water current and ii) admixture with the closely related anadromous L. fluviatilis ecotype having a positive effect on L. planeri genetic diversity. We genotyped 2472 individuals, including 225 L. fluviatilis, sampled in 81 sites upstream and downstream from barriers to migration, in 29 West-European rivers. Linear modelling revealed a strong positive relationship between the distance to the source and genetic diversity, consistent with expected patterns of decreased gene flow into upstream populations. However, the presence of anthropogenic barriers had a moderate effect on spatial genetic structure. Accordingly, we found evidence for downstream-directed gene flow, supporting the hypothesis that barriers do not limit dispersal following water flow. Downstream L. planeri populations in sympatry with L. fluviatilis displayed consistently higher genetic diversity. We conclude that genetic drift and slight downstream gene flow mainly drive the genetic make up of upstream L. planeri populations whereas admixture between ecotypes maintains higher levels of genetic diversity in L. planeri populations sympatric with L. fluviatilis. We discuss the implications of these results for the design of conservation strategies of lamprey, and other freshwater organisms with several ecotypes, in fragmented dendritic river networks.
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