An unexpected occurrence of <i>Alyssum rossetii (Brassicaceae</i>) in the Pyrenees, a new species for the Spanish flora

Alyssum rossetii is recorded for the first time in Spain from a single locality in the Pyrenees. After the type locality in the Valpelline valley (Pennine Alps, Italy), it is only the second known locality of this species. Morphology, diploid chromosome number (2n = 16), relative genome size, and ITS sequences confirm that the Pyrenean plants are conspecific with the Alpine ones and should be classified as A. rossetii. Phylogenetic analyses placed A. rossetii within the clade of A. sect. Gamosepalum comprising species from Turkey and demonstrated that A. rossetii is only distantly related to other Alyssum species occurring in the Pyrenees and adjacent regions: A. alyssoides, A. cacuminum, A. collinum, A. fastigiatum, A. granatense, A. montanum, and A. simplex. An identification key to all Alyssum species known from the Pyrenees is presented.