There was a high population structuring according to the sampling location. Regarding significant population-wise comparisons, the extent of differentiation ranged from moderate (Tea-Cabe) to strong (Lerez-Cabe). Lerez, Albarellos, and
Cabe were the poorest localities in terms of genetic diversity. An explanation for this might be that they are at the edge of the species' range. Gene flow may be more restricted between two groups of localities, as revealed by STRUCTURE: POP1 (Lerez-Albarellos) and POP2 (Tea-Cabe). This hypothesis requires further investigation using highly variable co-dominant markers.
Albarellos may have a high conservation value, as (1) it is the only studied locality not significantly differentiated from the others (Table 3) and (2) its geographic condition is intermediate between other sites inhabited by M. splendens. The reservoir Albarellos (336 ha, built in 1971) is the only sampled locality not included as SCI in the Natura-2000 network. Such an intermediate site may be important for a species with less dispersal and higher structuring than O. curtisii. Low dispersal in M. splendens is due to its ecological requirements rather than its actual dispersal ability, as anisopterans are usually good fliers. These facts point to the importance of preserving appropriate habitats and connectivity between them for M. splendens, a species less flexible in ecological demands, and clearly linked to remnants of autochthonous forests (Cordero Rivera 2006).
Seven out of the nine known Northwest Iberian populations of M. splendens were found in natural rivers. The remaining two sites were located in man-made hydroelectric reservoirs, where aquatic/riverine vegetation is lacking. Despite the negative effect of reservoirs on some insect groups due to habitat destruction (Turner 2007) and fragmentation (Watanabe and Omura 2007), it will be interesting to determine whether Albarellos resulted from a recent colonization from other river basins (e.g., Tea) or descended from individuals previously present in the river.
There was a significant population structuring according to the sampling location. Regarding population-wise comparisons, the extent of differentiation ranged from moderate (Lerez-Deza) to strong (Tea-Cabe). AMOVA results (Table 5) suggested the presence of phylogeographic structure in our data. Clustering of O. curtisii populations, according to the branching pattern showed by the UPGMA tree (Fig. 2b), yielded a nearly significant value (FCT = 0.101, P = 0.06). This putative structure is congruent with the isolation-by-distance pattern inferred through Mantel's test. Accordingly, O. curtisii appears more widely distributed across the region studied than M. splendens. The former shows farther dispersal than the latter (Azpilicueta Amorin et al. 2007).
Arnoia was the only sampled locality not classified as SCI. We recommend this locality to be assigned a certain level of protection. This is based on the fact that Arnoia (n = 10) showed higher genetic variability than Cabe (n = 18). Both populations are geographically and phylogenetically close (Figs. 1 and 2). However, gene flow between them is expected to be low (Table 3). It was somewhat surprising that the highest level of genetic diversity was found in Lerez (Table 1). This is because
Lerez is probably the studied location suffering the most harmful human activity, e.g., intensive forestry with eucalypt plantations all over the basin, leisure facilities, and small dams.
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