Expanding the southern range margin at the cost of massive asymmetric introgression: Armeria pungens (Plumbaginaceae)

Abstract

Armeria (Plumbaginaceae) is a primarily Mediterranean angiosperm genus with low pre- and postzygotic reproductive isolation, in which a number of taxa have been proposed to be of hybrid origin. The southernmost population of a coastal sand-dune Iberian-Corsica-Sardinian species (A. pungens) was reported to be introgressed by a sympatric congener (A. macrophylla) based on nrDNA ITS and plastid DNA Sanger sequences, morphometric data, genome size variation and ecological niche. A genotyping-by-sequencing (GBS) genomic study reveals that such introgression is markedly asymmetric towards A. pungens both for the nuclear and plastid genomes. For the latter, a plastid capture has occurred in the study site, which can be considered a hybrid zone. By contrast, genetic clustering analyses hardly reveal any introgression in its congener A. macrophylla. The introgression scenario is consistent with Currat et al.¿s (2008) model in which a species invading an occupied territory becomes massively introgressed with local genes. This is due to demographic imbalance between the two species, with reduced population densities of the invader at the colonization front. This scenario is facilitated by factors such as low interspecific reproductive isolation and remoteness of the source of the invasion, which are met in the Armeria hybrid zone. Although the recent origin of the introgressed population -and the hybrid zone- is so far based on circumstantial evidence, our results may provide clues for understanding how complex genetic admixture patterns are shaped in their early stages.