Deep genetic structure at a small spatial scale

Species with small geographic ranges do not tend to have a high genetic structure, but some land snail species seem to be an exception.Xerocrassa montserratensis, an endangered land snail endemic to Catalonia (northeastern Iberian Peninsula), is an excellent model to study the processes affecting the phylogeography of specialized species of conservation concern. This species is restricted to xerophilous stony slopes and occurs within a small and fragmented area of ca. 500 km2. We sequenced the COI barcode region of 152 individuals from eight sites covering the entire range of the species. We found four genetic groups mostly coincident with their geographic distribution: a central ancestral group containing shared haplotypes among five localities and three groups restricted to a single locality each. Two of these derived groups were geographically and genetically isolated, while the third and most differentiated group was not geographically isolated. Geomorphologic and paleoclimatic processes during the Pleistocene can explain the divergence found between populations of this low dispersal species with historical fragmentation and secondary contacts.

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Fig: Distribution range, habitat and sampling information of Xerocrassa montserratensis. (A) Distribution map of X. montserratensis with shell photographs of the three morphologically described subspecies. The arrows point to their locus typicus. (B) Location and codes of the eight sampling sites (Table). (C) Photography of a bare stony slope, the preferred habitat of X. montserratensis. (D) Distribution of bare stony slopes (red patches) in Sant Llorenç del Munt i l’Obac Natural Park including the three locations sampled in this park. The map in (A) was performed with ArcGIS 10x (ESRI,). The maps in (B,D), are in UTM coordinates for zone 31 T, and have been plotted with Surfer20 (Golden Software).

Nonetheless, recent passive large dispersal through streams was also detected in the central group. Overall, our study uncovered four evolutionary units, partially matching morphologically described subspecies, which should be considered in future conservation actions.

Català, C., Bros, V., Castelltort, X. et al. Deep genetic structure at a small spatial scale in the endangered land snail Xerocrassa montserratensis. Sci Rep 11, 8855 (2021). https://doi.org/10.1038/s41598-021-87741-7

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