Evolutionary patterns in the Dilatata group (Paspalum, Poaceae)

Paspalum dilatatum Poir. and its related species are warm-season grasses native to the grasslands of temperate South America. The group comprises several sexual tetraploid forms and apomictic tetraploids, pentaploids, hexaploids, and heptaploids. Interest in several of these biotypes as forage grasses has led to the accumulation of abundant cytogenetic information, evolutionary hypotheses, and thorough field studies which make the group a very promising model for analysis of evolutionary processes in apomictic complexes. Microsatellite markers were used here to analyze the relationships among the apomictic biotypes and evolutionary pathways. Most apomictic biotypes were shown to be monoclonal and sexual recombination is probably very rare. Suggested mechanisms for the formation of apomicts involve either unreduced female gametes or euploid pollen grains from the pentaploid biotype. Even-ploid apomictics, including those cytologically capable of facultative apomixis, are monoclonal and seem to play a very minor role in the evolution of the complex. The relationships hypothesized among the apomicts are congruent with a single origin of apomixis in the group which in turn would be coded by a non-recombining genome.