Chromosome numbers of some southernmost species of Mimosa L. (Leguminosae)

Abstract

Chromosome numbers of 9 species of Mimosa which grow in the southern limit of the South American centre of diversification were determined, all of them are new reports except the one of M. oligophylla. M. vellosiella, M. oligophylla and M. flagellaris were diploids with 2n=2x=26; M. velloziana and M. adpressa were tetraploids with 2n=4x=52; M. sprengelii, M. ramulosa and M. rocae were octoploids with 2n=8x=104; and one accession of M. balansae was diploid while the other was tetraploid. All of them were multiples of the basic chromosome number x=13. The polyploid species were more frequent at higher latitudes, this increment was also accompanied with a higher ploidy level of the polyploids southernward. All the polyploids found belong to the sections with most advanced characters of the genus, which supports the phylogeny proposed for Mimosa by Barneby. Data suggest that the extension of the genus toward the temperate zones might have be done mainly by polyploid species, which may have had advantages over the diploids in colonising habitats restricted to the latter.