Systematics and Biogeography of South American Tripsacum
Distribution patterns of the New World Andropogoneae genus Tripsacum (Gramineae) were studied in an effort to trace its origin and evolution. The genus is widely distributed from about 42(DEGREES)N to 24(DEGREES)S latitude in the Americas and islands of the Caribbean. The largest concentration of species occurs in Meso-America, with ten species mostly confined to south-central Mexico and adjacent Guatemala. Three species are restricted to South America. One species occurs in western Mexico and southern Arizona, and one species extends across the range of the genus. When the worldwide distribution of the Andropogoneae is plotted on a map of the continents as they are assumed to have been positioned during the Cretaceous period, a pattern of decreasing generic and specific densities that radiates from present day India as a center becomes evident. Within the Andropogoneae polyploidy shows a similar pattern, with diploids more common in the Indian subcontinent and polyploids increasing outward, suggesting a migration of the Andropogoneae out of this assumed center of origin. Since Tripsacum is absent from the Old World, the progenitor of this genus probably migrated to the New World (central Mexico) across Africa some 80 million years ago.
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