Peripatric speciation in an endemic Macaronesian plant after recent divergence from a widespread relative

<div><p>The Macaronesian <i>Scrophularia lowei</i> is hypothesized to have arisen from the widespread <i>S</i>. <i>arguta</i> on the basis of several phylogenetic studies of the genus, but sampling has been limited. Although these two annual species are morphologically distinct, the origin of <i>S</i>. <i>lowei</i> is unclear because genetic studies focused on this Macaronesian species are lacking. We studied 5 <i>S</i>. <i>lowei</i> and 25 <i>S</i>. <i>arguta</i> populations to determine the relationship of both species and to infer the geographical origin of <i>S</i>. <i>lowei</i>. The timing of <i>S</i>. <i>lowei</i> divergence and differentiation was inferred by dating analysis of the ITS region. A phylogenetic analysis of two nuclear (ITS and ETS) and two chloroplast (<i>psbJ–petA</i> and <i>psbA–trnH</i>) DNA regions was performed to study the relationship between the two species, and genetic differentiation was analysed by AMOVA. Haplotype network construction and Bayesian phylogeographic analysis were conducted using chloroplast DNA regions and a spatial clustering analysis was carried out on a combined dataset of all studied regions. Our results indicate that both species constitute a well-supported clade that diverged in the Miocene and differentiated in the Late Miocene-Pleistocene. Although <i>S</i>. <i>lowei</i> constitutes a well-supported clade according to nDNA, cpDNA revealed a close relationship between <i>S</i>. <i>lowei</i> and western Canarian <i>S</i>. <i>arguta</i>, a finding supported by the spatial clustering analysis. Both species have strong population structure, with most genetic variability explained by inter-population differences. Our study therefore supports a recent peripatric speciation of <i>S</i>. <i>lowei</i>—a taxon that differs morphologically and genetically at the nDNA level from its closest relative, <i>S</i>. <i>arguta</i>, but not according to cpDNA, from the closest Macaronesian populations of that species. In addition, a recent dispersal of <i>S</i>. <i>arguta</i> to Madeira from Canary Islands or Selvagens Islands and a rapid morphological differentiation after the colonization to generate <i>S</i>. <i>lowei</i> is the most likely hypothesis to explain the origin of the last taxon.</p>