Genetic differentiation, local adaptation and phenotypic plasticity in fragmented populations of a rare forest herb

Due to habitat loss and fragmentation, numerous forest species are subject to severe population decline. Investigating variation in genetic diversity, phenotypic plasticity and local adaptation should be a prerequisite for implementing conservation actions. This study aimed to explore these aspects in ten fragmented populations of Physospermum cornubiense in view of translocation measures across its Italian range.

For each population we collected environmental data on landscape (habitat size, quality and fragmentation) and local conditions (slope, presence of alien species, incidence of the herbivorous insect Metcalfa pruinosa and soil parameters). We measured vegetative and reproductive traits in the field and analysed the genetic population structure using ISSR markers (STRUCTURE and AMOVA). We then estimated the neutral (F_ST) and quantitative (P_ST) genetic differentiation of populations.

The populations exhibited moderate phenotypic variation. Population size (range: 16-655 individuals), number of flowering adults (range: 3-420 individuals) and inflorescence size (range: 5.0-8.4 cm) were positively related to Mg soil content. Populations' gene diversity was moderate (Nei-H = 0.071-0.1316); STRUCTURE analysis identified five different clusters and three main geographic groups: upper, lower, and Apennine/Western Po plain. Fragmentation did not have an influence on the local adaptation of populations, which for all measured traits showed P_ST < F_ST, indicating convergent selection.

The variation of phenotypic traits across sites was attributed to plastic response rather than local adaptation. Plant translocation from suitable source populations to endangered ones should particularly take into account provenance according to identified genetic clusters and specific soil factors.