CYTOLOGICAL VARIATION ANTI EVOLUTION WITHIN PELARGONIUM SECTION HOAREA (GERANIACEAE)

Chromosome numbers of 65 species of sect. Hoarea have been determined. These show three basic chromosome numbers, x = 11, 10 and 9. Only a f ew species are tetraploid. In five species both diploid and tetraploid cytotypes are reported. Several cases of deviations in chromosome num bers and cytological abnormalities were found, most of these being rel ated to the presence of B chromosomes that occur in eight species. Evi dence is presented to suggest that the basic chromosome numbers of x = 10 and x = 9 are derived from x = 11 by centric fusion. Although vari ation in basic chromosome number within Pelargonium has been the subje ct of detailed study, this is the first time that evidence has been fo und for a mechanism of change in basic number, that of centric fusion by Robertsonian translocation. For the species of sect. Hoar-ea with x = 9, where the evidence for Robertsonian translocation is greatest, t his process has probably taken place quite recently. In contrast to re sults from other sections of the genus Pelargonium, the three differen t basic numbers of sect. Hoarea do not contradict its delimitation as a natural taxon.