Ibero-Balearic plant disjunctions: genomic support for rapid and recurrent long-distance colonizations of the endangered Diplotaxis ibicensis (Brassicaceae) despite no dispersal syndromes.

Abstract

Diplotaxis ibicensis is a western Mediterranean plant endemic to the Balearic Islands and a small area of the eastern Iberian Peninsula. This study investigates the biogeographical relationships among 16 populations across its entire distribution using two complementary approaches: (i) an ITS-based phylogenetic analysis to reconstruct Diplotaxis species relationships and divergence times, and (ii) phylogenomic and population genomic analyses of D. ibicensis via genotyping-by-sequencing (GBS) to assess divergence among populations. The ITS phylogeny supports D. ibicensis as a monophyletic group closely related to the western Mediterranean species D. siettiana, D. brevisiliqua and D. ilorcitana. A time-calibrated tree estimates the most recent common ancestor of these four species in the Pliocene-Pleistocene (3.68-0.77 Ma), well after the Messinian Salinity Crisis (5.96-5.3 Ma). GBS analyses identified six well-supported clades of D. ibicensis individuals, four of them restricted to single islands and the mainland, while two clades contained populations from different islands. These results suggest that rapid long-distance dispersal (LDD) events occurred across permanent marine barriers, followed by secondary colonizations among distant islands, but strong isolation in others. The highest genetic diversity (π) occurs on the island of Ibiza, while the greatest genetic differentiation (F_ST) is found between mainland and Cabrera island populations. Recurrent LDD events contrast with the absence of specialized LDD traits in pollen, fruits and seeds. A review of studies of Ibero-Balearic species reveals similar patterns of LDD across the so-called Ibiza Channel. Conservation strategies are proposed based on patterns of genetic diversity and differentiation.