XVI International Botanical Congess
Cytological investigation of species of Pelargonium has revealed variation not only in basic chromosome number [x=4, (7), 8, 9,10, 11, 17, 19, 21] but also in chromosome size, and these characters have proved to be of value for the sectional classification of the genus. For species within a section, basic chromosome number and size are fairly uniform. Rarely, e.g. in section Hoarea, there is variation in number (x= 9, 10, 11) and in this section there is some evidence for reduction in chromosome number by Robertsonian fusion. A molecular phylogeny based on mtDNA and cpDNA sequence data supports the view that x = 11 is basal in the genus but that Pelargonium has two lineages, one with small and the other with larger chromosomes, and that reduction in chromosome number has been derived independently in each lineage.