Are Ophiolitic Substrates Drivers for Reticulate Evolution in <i>Armeria</i> (Plumbaginaceae)?

Ultramafic substrates can play a role in fostering ecological adaptation and microevolutionary dynamics. The <i>Armeria denticulata</i> complex includes two flowering plant species (<i>A. denticulata</i> and <i>A. saviana</i>): the former is a strict serpentinophyte endemic to Tuscany and western Liguria, while the latter grows on limestone/jasper in a small area of southern Tuscany. Intriguingly, northern Apennine populations of <i>A. arenaria</i> subsp. <i>praecox</i>, a subspecies otherwise endemic to the western Alps, grow on ophiolites. Finally, the central-southern Italian endemic <i>A. gracilis</i> is instead linked to limestone. We aimed at understanding whether substrate specificity and/or hybridization promoted speciation in the <i>A. denticulata</i> complex, despite similar ecological conditions failing to cause speciation in the nearby <i>A. arenaria</i>. We used Genome skimming and Illumina sequencing to assemble plastomes (152 kb) and data from the nuclear genome (ribosomal DNA subunits and 36 single-copy markers; 27 kb in total) used to infer phylogenies and evaluate different reticulate evolution scenarios by calculating gene tree probabilities under the Coalescent model. The phylogenomic analyses were complemented by morphometric data using a matrix of 134 individuals × 27 characters. Morphometric data were analyzed both by fitting a Gaussian Mixture Model to compute population-wise Jensen-Shannon Distances and a Neighbor-Net network, and by inferring a standard linear discriminant analysis. Both morphometric and phylogenomic results suggest <i>A. saviana</i> is a species of homoploid hybrid origin, involving <i>A. denticulata</i> s.str. (ovule donor) and <i>A. gracilis</i> (pollen donor). A single population of <i>A. denticulata</i> from inner Tuscany (Monte Ferrato) could have originated from an introgression/hybridization event between <i>A. denticulata</i> s.str. (pollen donor) and <i>A. arenaria</i> subsp. <i>praecox</i> (ovule donor). Accordingly, our results suggest that substrate specificity and hybridization/introgression prompted microevolutionary processes in the <i>Armeria denticulata</i> complex.