Are leaf traits and genetic differentiation patterns divergent in endemic vs widespread congeneric plant species? Insights from the Mediterranean forest species pair Aegonychon calabrum-A. purpurocaeruleum (Boraginaceae)

Abstract Understanding trait and genetic variability within endemic plant species is key to elucidate their plasticity, resource-use attitudes and adaptive responses in variable habitat conditions. However, no studies have focused on both variability dimensions and their relationship in narrow Mediterranean endemics of the forest understorey. We investigated this topic in the species pair Aegonychon calabrum , a rare herb in southern Italy, and its related widespread congener A. purpurocaeruleum (Boraginaceae). After assessing intraspecific variation in Leaf Area (LA), Specific Leaf Area (SLA), Leaf Dry Matter Content (LDMC), and population genetic structure by AFLP fingerprinting, we used P ST –F ST comparisons to infer the proportion of genotypic and phenotypic differentiation driven by directional vs. stabilizing selection. We also examined whether individuals exhibit similar trait values due to their genetic proximity. Lower LA suggested lower resource-acquisitive ability in A. calabrum , but variation in traits and CSR scores was comparable to its widespread congener. Population heterozygosity was slightly lower than in A. purpurocaeruleum, not supporting genetic erosion in the endemic species. In both species, phenotypic differentiation was higher than genetic differentiation suggesting that directional or divergent selection is potentially acting on the examined leaf traits and thus on their resource use attitudes. Trait values positively correlate with genetic proximity in a clade formed 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 generally fitting the "refuge" model, populations of endemic understory herbs of Mediterranean forests can be subject to non-stabilizing selection.