Spatial genetic structure of Aquilegia taxa endemic to the island of Sardinia

Abstract

Background and aims: The Mediterranean Basin is one of the most important regions for the Earth's plant biodiversity; however, the scarcity of studies on fine scale patterns of genetic variation in this region is striking. Here, an assessment is made of the spatial genetic structure of all known locations of the three Sardinian endemic species of Aquilegia in order to determine the relative roles of gene flow and genetic drift as underlying evolutionary forces canalizing the divergence of Sardinian Aquilegia taxa, and to see if the spatial genetic structure found fits the current taxonomic differentiation of these taxa.

Methods: DNA from 89 individuals from all known locations of Aquilegia across Sardinia was analysed by means of amplified fragment length polymorphism (AFLP) markers. Both principal co-ordinates analysis (PCoA) and Bayesian clustering analyses were used to determine the spatial genetic structure irrespective of any taxonomic affiliation. Historical effects of gene flow and genetic drift were assessed by checking for the existence of isolation-by-distance patterns.

Key results: STRUCTURE and PCoA analyses revealed a pattern of genetic variation geographically structured into four spatial genetic groups. No migration-drift equilibrium was detected for Aquilegia in Sardinia, when analysed either as a whole or in individual groups. The scenario approached a Case III pattern sensu Hutchinson and Templeton, which is associated with extreme isolation conditions where genetic drift has historically played a dominant role over gene flow.

Conclusions: The pattern of genetic variation of Sardinian taxa of Aquilegia indicates that genetic drift has been historically more influential than gene flow on population structure of Sardinian species of Aquilegia. Limited seed dispersal and divergent selection imposed by habitat conditions have been probably the main causes reinforcing post-Pleistocene geographical isolation of Aquilegia populations. The spatial genetic structure found here is not fully compatible with current taxonomic affiliations of Sardinian Aquilegia taxa. This is probably a consequence of the uncoupling between morphological and genetic patterns of differentiation frequently found in recently radiated taxa.

Fig. 1.

Study area and results of the genetic structure analyses. The 19 known locations of Aquilegia in Sardinia are distributed over the Central and Eastern mountain systems. Four locations at Monte Corrasi (North): Arco Corrasi (8), Punta Corrasi (11), Ahottadoglios (14) and Palumbrosa (15). Four Eastern locations: Gorropu Canyon (19), Pischina Urtaddala (18), Flumineddu (7) and Codula Orbisi (1). Five Central locations: Badde Enis (2), Talana (3), Rio Correboi (4), Rio Olai (5) and Monte Spada (6). Three locations at SE-Gennargentu Massif: Baccu Seardu (12), Su Furciddu (17) and Calavrighe Grossu (9). Finally, three locations in the Tacchi region: Monte Arquerì (10), Funtana Dorada (13) and Funtana Sa Ceraxia (16). Filled symbols represent populations already described as A. barbaricina (circles), A. nugorensis (squares) and A. nuragica (diamond). Open symbols (triangles) represent Aquilegia locations not previously described. Spatial genetic structure is inferred by a model-based clustering method implemented in STRUCTURE. At each location, pie charts indicate the mean proportion of membership of individuals at each location for the K = 4 genetic groups. The right-hand bar diagram indicates membership of each individual for the 4 genetic groups. Spatial genetic groups are indicated to the right of the figure.

Fig. 2.

Principal co-ordinates analysis of Sardinian Aquilegia individuals based on Euclidean distances of AFLP phenotypes. Axis 1 (34·7 %) and axis 2 (23·2 %) segregated populations into four groups: Monte Corrasi (upper-left cloud), Gorropu Canyon (lower cloud), Pischina Urtaddala (right small cloud) and a huge group with Central, Eastern, Gennargentu Massif and Tacchi populations (large upper-right cloud). Each symbol represents a different location.

Fig. 3.

Pairwise F_ST values among populations plotted against the straight-line distance separating them with its regression line for (A) the whole set of Sardinian populations and for spatial genetic groups where this comparison was possible. (B) Corrasi, (C) CCD and (D) GTF. The slope was significantly different from 0 at CCD and marginally significant at Corrasi. Locations 18 (Pischina) and 7 (Flumineddu) were excluded from this representation due to their genotypic (Pischina) or geographic (Flumineddu) segregation from the rest of the individuals of their respective groups (CCD and GTF, respectively).