From pollen to seed: strategies and barriers in orchid reproduction

Orchidaceae is one of the most diverse plant families among flowering plants. Orchid diversity is reflected in their varied morphophysiological adaptations. Orchids are notable for their variety of floral mechanisms, their pollen grains commonly released as units called pollinia, and their “dust-seeds” with undifferentiated embryos lacking endosperm. In addition, orchids usually show specialized pollination systems, as well as a wide variety of pre- and postzygotic barriers (morphological, temporal, geographical, and molecular), which preclude self-pollination, geitonogamy, and interspecific cross-pollination. This review addresses the specializations exhibited in the reproduction of orchids, from pollen production to seed formation, considering the role of the different pre- and postzygotic barriers in fruit and seed set. Furthermore, the relationship between reproductive systems and the final development of fruits and seeds in this vast plant family is discussed. The development of specialized reproductive mechanisms, through the evolutionary innovation of pollinia and hierarchical pre- and postzygotic reproductive barriers, strongly promotes outcrossing and ensures genetic compatibility. However, while these mechanisms enhance reproductive success, they simultaneously increase orchid vulnerability to environmental threats.