Phylogenetic and cytogenetic relationships among species of Oxalis section Articulatae (Oxalidaceae)

Oxalis section Articulatae Knuth (Oxalidaceae) comprises a group of five species with a restricted distribution in the southern part of South America. To assess the current circumscription of section Articulatae and the evolutionary mechanisms involved in its diversification, we used three plastid DNA sequences and the nuclear ribosomal transcribed spacer (ITS) to build a molecular phylogeny. A cytogenetic analysis of the group including chromosome number, heterochromatin distribution and determination of nuclear DNA content was also performed. Cytologically, all species presented symmetrical karyotypes except for O. linarantha with a bimodal karyotype although it maintained the same haploid number as the other species (n = 7). Nuclear DNA contents in the Articulatae species are among the lowest for the genus. The O. linarantha 2C value represents the minimum value known for Oxalis, extending the genome size range from 0.58 to 41.10 pg, representing approximately a 70-fold difference, and one of the highest ranges known for an angiosperm genus. Molecular, chromosomal and morphological data suggest that section Articulatae should be redefined to include only those species with rhizomatous stems while the tunicate-bulbous species O. linarantha should be placed elsewhere. Different ploidy levels were found, ranging from 2x to 7x. Diploids and polyploids showed a geographic structure; diploids are widely distributed, while polyploids are found in the uplands of eastern Uruguay and southern Brazil. Haplotype sharing and chromosome data suggest that at least some of section Articulatae polyploids may be the result of interspecific hybridization and appear to have originated very recently.