Cytological studies in four species of Mesosetum (Arthropogoninae) reveal the lowest chromosome number among the Neotropical Poaceae

Until recently, the basic chromosome number x = 8 was suggested for the Mesosetum genus, which was considered rare in the subtribe Arthropogoninae (Poaceae, Paspaleae). However, the number of accessions and species analysed is not large and the relation of chromosome numbers in this genus with others of the subtribe is uncertain. In this study, cytological and reproductive information in other species of Mesosetum was provided enabling a broad critical analysis of chromosomes evolution in the clade. Chromosome counting and meiotic behaviour profile were determined for six accessions of Mesosetum belonging to four different species. Mature embryo sac structure was determined to predict sexual or apomictic mode of reproduction. All accessions showed 2n = 8 chromosomes, number recorded for first time in Neotropical Poaceae and Paniceae s. l.. Chromosomes showed similar length and presence of one satellite pair in two perennial species. Meiosis was regular, with the formation of four bivalents whilst the embryo sacs were of Polygonum type. Although the basic chromosome number x = 10 appears to be plesiomorphic in Paspaleae tribe, current data indicate x = 4 as a possible additional number. These findings indicate that polyploidy could be involved in Mesosetum evolution and speciation.