A Survey of the Reproductive Biology of the Myrtoideae (Myrtaceae)

The Myrtoideae usually present small, epigynous, 4-5-merous, polystemonous flowers that last one day. Bee-pollination in which pollen is the sole reward is the dominant pollination system. Nectar has been best documented in Syzygium but probably also occurs in other bee-pollinated genera. The most common bee visitors are Apidae: Meliponinae and Bombinae. Bird- and mammal-pollination occur in Old World Syzygium with nectar as the primary reward. Bird-pollination with petals as the reward occurs in New World Acca and Myrrhinium. General floral morphology is very uniform, while inflorescence types and flowering strategies are very diverse. Stigmas are dry and ovules are anatropous, hemicampylotropous or anacampylotropous and have an outer 2-6-layered integument and an inner 2-layered integument or a single integument. The ovary usually contains more ovules than will form seeds. Flowering strategies vary from mass-flowering types, in which the flowering episode typically takes only a few days, to steady-state types of up to 90 days duration. Flowering at dry/wet season transition is common in seasonal climates, and fire-induced flowering is found occasionally. Outbreeding is probably widespread, although both self-compatible and self-incompatible species exist. The self-incompatible species have self pollen tubes penetrating the micropyles, so preferential outcrossing may be maintained by a late-acting mechanism. Cryptic dioecy, in which female flowers have "mimic" sterile anthers occurs in several genera. Apomixis occurs in Syzygium and this has been reported to be linked to the polyembryony found in this genus. Myrtoid fruits are fleshy berries or drupes, dispersed by birds, bats, and other small mammals. Fruit size, color, texture, and number of seeds are all very variable. Seed coats may be absent to bony, but have a smooth surface. The endosperm is mainly digested by the developing embryo. Early embryology is relatively uniform but final embryo morphology varies widely across the genera. Germination times vary from 10 days to over 2 years and seed viability periods from 15 days to 1 year.