Floral biology and reproduction in Poa (Poeae: Gramineae)

Flowers in the cosmopolitan genus Poa L. are predominantly hermaphrodite but many departures from this sex form occur in the New World. Dioecism is primarily a South American breeding system with about three times as many dioecious species as in the rest of the world. Gynomonoecism is a Central and South American trait heavily represented in Andean Peru and Bolivia. This zone of gynomonoecism separates dioecism in North and South America. Gynodioecism, a convenient evolutionary position on the pathway to dioecism, is relatively infrequent and in North America is of indeterminate form in several taxa. Apomixis has long been recognised in European Pea; in western North America, apospory has invaded dioecious species and generated populations of pistillate plants. In Peru and Bolivia, several taxa are composed exclusively of plants with pistillate flowers, but these have arisen from gynomonoecious progenitors. Poa is of Eurasian origin and migrated to North America and thence to South America. Sex-form kinds and frequencies are in stark contrast in the two parts of the continent, but are explicable in evolutionary terms. The selection pressures generating the deviations from hermaphroditism and their timing are unknown.