Evolutionary lineages of Ni-hyperaccumulation and taxonomy in Euro-Mediterranean Alysseae (Brassicaceae): insights from nrDNA sequence data

Ni-hyperaccumulation is a rare form of physiological specialization shared by a small number of angiosperms growing on ultramafic soils. The remarkable proportion of hyperaccumulators in the family Brassicaceae offer a broad range of opportunities to investigate the expression, regulation and evolution of the genetic traits underlying such a specialization. However, the evolutionary patterns of metal accumulation in Brassicaceae are still incompletely known. We used non coding nrITS sequences and a phylogenetic approach to assess relationships among Ni-hyperaccumulators in tribe Alysseae at the genus, species, and below-species level, in order to test monophyly vs. polyphyly of this trait within such an important group. Molecular data show that the ability for Ni-hyperaccumulation in Alysseae has a double origin, appeared in the clades of Bornmuellera-Leptoplax and Alyssum sect. Odontarrhena. Lack of affinity between Leptoplax emarginata and Peltaria, a member of Thlaspideae into which L. emarginata is placed by some authors, implies that Ni-accumulation did not originate in Thlaspideae, unlike commonly believed. In Bornmuellera-Leptoplax this ability represents an early synapomorphy appeared from an ancestor shared with the calcicolous, sister clade of Mediterranean Ptilotrichum. In A. sect. Odontarrhena it has multiple origins even within the three Europaean clades retrieved by DNA sequences. Lack of geographic cohesion suggests that accumulation ability has been lost or gained over the different serpentine areas of south Europe through independent events of adaptation and selection associated with changes in the expression of functional genes. Genetic continuity and phenotypic plasticity within and between populations of the A. murale s.l. complex call for a reduction of the number of Ni-hyperaccumulator taxa to be formally recognized.