Is population genetic structure of vascular plants shaped more by ecological or geographic factors? A study case on the Mediterranean endemic Centaurea filiformis (Asteraceae)

Abstract

All known populations of the Sardinian endemic Centaurea filiformis Viv. (Asteraceae) were studied in order to understand the impact of both geographic and ecological factors on the genetic structuring of this species. Fourteen populations and 234 individuals were sampled. The demographic structure of the populations and the reproductive ecology were estimated in 28 plots. Population genetic analyses were based on SSR markers. Genetic structure was investigated by spatial Bayesian methods. Average densities of 0.51 individuals m were detected, with a prevalence of adults. Ten species of pollinators were identified; C. filiformis ability to self-pollinate and myrmecochory were demonstrated experimentally. The populations displayed an average heterozygosity value of H = 0.576 and high genetic differentiation (overall F = 0.218). Bayesian analysis suggests that five is the most probable number of gene pools of origin. A strong correlation between geographic distances and genetic distances among populations was highlighted. The demographic population structure of C. filiformis is dominated by adults, suggesting that it is a stable-regressive or senile species, investing more in local persistence than colonisation ability. Despite the scattered distribution, the populations studied do not present evidence of genetic erosion. The analysis of genetic differentiation reveals very high differentiation levels among populations, thus indicating that effective barriers exist against gene flow. A general conclusion is that population distribution results in a clear genetic structure for the populations studied, and that geography and not ecology is shaping the present distribution of this species.

Islands as a crossroad of evolutionary lineages: A case study of Centaurea sect. Centaurea (Compositae) from Sardinia (Mediterranean Basin)

“…Hybridization is generally considered to be more probable in recently diversified groups, and lineage sorting of ancestral polymorphisms is more frequent in those that have diversified rapidly [85], but this is of no help in our case: Centaurea has diversified recently and has radiated rapidly [83]. In our case, we favor the hybridization hypothesis because intermediate morphological individuals have been described in the Sardinian species [30,40]. S1 Table).…”

“…However, bract appendages, which are the main character in the classification of sect. Centaurea, are quite different (see also [40]). Thus, we posit that the subordination of C. ferulacea to C. filiformis (e.g.…”

“…Intermediate individuals from the Monte Oseli population in the north present ptDNA from C. filiformis, whereas intermediate individuals from Cala Sisine in the south present C. ferulacea ptDNA. Hybridization is surely bidirectional because the hybrids are probably homoploid, and a "hybrid zone" was already defined on genetic grounds by [40]. Furthermore, individuals clearly identified as C. filiformis on morphological grounds (Cala Fuili, black circle in Fig 1) present ptDNA from the C. ferulacea line; this suggests that the coastal strip from Baunei to Cala Fuili may represent the ecological corridor followed by C. ferulacea in its expansion to the north.…”

“…aggregate from the mainland. Centaurea ferulacea, C. filiformis and C. horrida are longliving perennial diploids with 2n = 18 [29,[38][39][40]. Centaurea horrida is a pulvinular chamaephyte with spiny leaves and solitary capitula with shortly fimbriate bract appendages, growing exclusively on coastal cliffs, forming part of the dwarf vegetation of northwestern Sardinia (with a single remote isolated population on the islet of Tavolara, northeasten Sardinia).…”

The Endemic Vascular Flora of Sardinia: a Dynamic Checklist with an Overview of Biogeography and Conservation Status

“…Sardinia hosts around 25% of the 932 taxa native to Italy that were assigned to a risk category, mostly due to human disturbance [32,33]; of these taxa, 99 are endemic to the island. However, other studies accounted for further specific demographic, ecological and environmental causes of extinction risks, such as in the case of the endemic taxa belonging to the genera Anchusa [16,34], Astragalus [35] and Centaurea [36,37] or in the case of specific environments, such as wet habitats [38], pastures [39], coastal sand dunes [40,41] and small satellite islets [42,43]. Of the 21 vascular plants currently known in Sardinia and listed in the Annex II of the Habitats Directive (43/92/EEC), 17 are endemic [44].…”