Isozyme polymorphism and organization of the agamic complex of the Maximae (Panicum maximum Jacq., P. infestum Anders, and P. trichocladum K. Schum.) in Tanzania.

The tribe Maximae (Panicum maximum Jacq., P. infestum Anders., P. trichocladum K. Schum.) includes two sympatric pools with different modes of reproduction and ploidy levels: an apomictic and tetraploid pool on the one hand, and a smaller, sexual and diploid pool on the other. From an analysis of isozyme polymorphism five main results were evident. First, overall polymorphism is considerable showing that apomixis does not lead to a reduction in diversity. Second, the isozyme polymorphism of the two pools is similar, and this may be explained by reciprocal gene flow (low, but continuous) between these two pools. Third, maximum local polymorphism is due to the simultaneous presence of P. maximum sexuals and P. infestum apomicts. A continuum exists between the two species. Fourth, a high proportion of rare alleles, arising from introgression from P. infestum, characterized the isozyme polymorphism. These rare alleles, strongly counter-selected at the diploid level, are maintained by apomixis; the frequency of triplex or quadruplex genotypes was nevertheless low. Fifth, the heterozygosity level within apomicts is not higher than that of sexuals, showing that the apomixis-polyploidy combination does not lead to a higher frequency of very heterozygous individuals.