Evolutionary patterns in the Dilatata group (<i>Paspalum</i>, Poaceae): A polyploid/agamic complex
Paspalum dilatatum Poir. and its related species are warm-season grasses native to temperate South America. The members of the Dilatata group include polyploid sexual and apomictic components, some of which have reached worldwide distributions. The common biotype of P. dilatatum is a complex apomictic pentaploid hybrid, and efforts to identify its putative ancestors have led to the accumulation of a significant amount of cytogenetic information about the relationships among biotypes within the Dilatata group. In general, past work in this complex has suffered from the lack of representative collections, and the low power of the techniques. In this study, I developed microsatellite markers, analyzed their transferability within the Dilatata group, and applied them to representative samples to analyze the evolutionary relationships within the group at different levels. The markers developed here show great power to detect recent hybridization and analyze genetic structure. The genetic structure of the sexual biotypes was described for a collection of P. dilatatum ssp. flavescens . This biotype is highly autogamous, and its genetic variability does not show significant geographical structure probably due to continuous disturbance of the roadside environment it inhabits. The relationships among the sexual and the apomictic components are analyzed, and the origin of the apomictic biotypes is discussed. Genetic recombination was not detected in the apomictic hexaploids and tetraploids. Among the pentaploids, a single clone and its somatic variants were found on all the continents and in almost all the collection sites in its native area. All the other apomicts in the group, including the recombinant pentaploids are hypothesized to be derived from the pentaploid form. The probable mechanisms involve either the production of unreduced female gametes or eu-triploid pollen grains by the pentaploids. This is probably the most extensive study ever attempted in this group, and it will undoubtedly change the direction of all future research in these species. The new recombinant forms will have to be analyzed and their productive potential assessed, while existing collections should be re-structured to reflect the unexpected distribution of the genetic variability they contain.