CYTOTAXONOMIC STUDIES IN THEMEDA-TRIANDRA FORSSK .4. A POPULATION CYTOGENETIC STUDY OF A CONTACT ZONE BETWEEN TETRAPLOID AND HEXAPLOID POPULATIONS

A population cytogenetic study was undertaken across a contact zone be tween Themeda triandra populations in the Transvaal Bushveld to the no rth of the Magaliesberg, and Highveld populations to the south. Fiftee n collection points were chosen and three plants collected at each poi nt. The northern-most collection point contained only tetraploids and those on the Highveld only hexaploids. In the vicinity of the Magalies berg Mountain range, contact between the tetraploids and hexaploids pr oduced pentaploid and decaploid hybrids. Meiotic pairing association a nalyses showed that both the tetraploids and hexaploids formed apomict ic clones, of which some were widely distributed and others were more limited. This indicates that facultative apomixis occurs among both th e tetra- and hexaploids and gives rise to genotypic and genomic segreg ants thus producing variation and forming new apomictic clones. The po ssible evolution of Themeda triandra in southern Africa is discussed.