Are leaf traits and genetic differentiation patterns divergentin endemic vs widespread congeneric plant species? Insightsfrom the Mediterranean forest species pair Aegonychoncalabrum‑A. purpurocaeruleum (Boraginaceae)

Understanding trait and genetic variability within endemic plant species is key to elucidatetheir plasticity, resource-use attitudes and adaptive responses in variable habitat conditions.However, no studies have focused on both variability dimensions and their relationshipin narrow Mediterranean endemics of the forest understorey. We investigated this topicin the species pair Aegonychon calabrum, a rare herb in southern Italy, and its relatedwidespread congener A. purpurocaeruleum (Boraginaceae). After assessing intraspecificvariation in Leaf Area (LA), Specific Leaf Area (SLA), Leaf Dry Matter Content (LDMC),and population genetic structure by AFLP fingerprinting, we used PST–FST comparisonsto infer the proportion of genotypic and phenotypic differentiation driven by directionalvs. stabilizing selection. We also examined whether individuals exhibit similar trait valuesdue to their genetic proximity. Lower LA suggested lower resource-acquisitive ability inA. calabrum, but variation in traits and CSR scores was comparable to its widespreadcongener. Population heterozygosity was slightly lower than in A. purpurocaeruleum,not supporting genetic erosion in the endemic species. In both species, phenotypicdifferentiation was higher than genetic differentiation suggesting that directional ordivergent selection is potentially acting on the examined leaf traits and thus on theirresource use attitudes. Trait values positively correlate with genetic proximity in a cladeformed by two of the three A. calabrum populations, a pattern consistent with population-level niche conservatism. The case study of A. calabrum supports that, while generallyfitting the "refuge" model, populations of endemic understory herbs of Mediterraneanforests can be subject to non-stabilizing selection.