New chromosome counts in Neotropical Machaerium Pers. species (Fabaceae) and their taxonomic significance

Machaerium comprises about 130 species and is widespread in the Neotropics, ranging from Mexico to Argentina. Because of the existing taxonomic complexes, circumscribing many of its species is far from easy. Cytological studies on Machaerium have only been carried out for four species and are still meager. This paper brings forward chromosome numbers for 18 species, 16 of which represent new counts. Chromosomes were very small in size. Meiotic behavior was observed only for M. opacum (n=10) and proved to be normal. The other species showed 2n=20, except for the tetraploids M. nyctitans, M. hirtum and M. sericiflorum, where 2n=40. Polyploidy possibly evolved independently in Machaerium since these species belong to different sections. Both M. hirtum and M. sericiflorum (tetraploids) were regarded as taxa distinct from M. aculeatum (diploid), although they used to be considered as synonyms. The importance of polyploidy for the evolution of species in this genus and its frequency within the different sections will be assessed as more records become available. It might also be possible to confirm whether its basic number is actually x=10, as results tend to indicate.