The population structure and floral biology of Amborella trichopoda (Amborellaceae)

The shrubs and small trees of Amborella trichopoda are functionally unisexual and the populations are dioecious, male biased, and occur primarily in clumps. Floral size dimorphism reported for this species was confirmed by differences in floral biomass. At the level of the inflorescence, there were significantly greater numbers of male versus female flowers/inflorescence. No differences were observed between male and female plants in height, stem number, and diameter at the ground level. Male flowers bear 6 to 21 stamens and female flowers 3 to 6 spirally arranged carpels and staminodes that mimic the fertile androecia in male flowers. Flowering within a population was synchronous, and flowers of Amborella trichopoda are both insect- and wind-pollinated. A wide variety of insects ranging in size from ca. 1 mm to 7 cm in length pollinate the flowers, indicating a generalist pollination system. Beetles involved in pollination dwell in the forest litter but also spend hours on the leaves, flowers, and branches feeding on pollen. Pollen is the reward for insects as there is an absence of detectable floral volatiles and nectars, and anthers lack secretions or food bodies. A free-flowing stigma secretion was occasionally present, but it was not consumed by pollinators. Structural studies indicate that the stigma is of the dry-type, and the pollinators probably visit female flowers because of the mimetic role of the staminodes. The combination of wind and insect pollination exhibited in A. trichopoda is rare in basal angiosperms. Gall midges, parasitoid wasps, and thrips utilize floral tissue as a breeding site, impeding reproduction. Two species of gall-inducing midges (Cecidomyiidae) insert egg(s) into the gynoecia of developing flower buds, converting one or more ovaries into galls. Parasitoid wasps (Chalcidae) lay eggs in the galls that develop into larvae that prey upon the midge maggots. The Cecidomyiidae expanded with the angiosperms, but the earliest fossils of gall-inducing gall midges occur in the Miocene. Deceptive mechanisms involving numerous floral traits in small bisexual and unisexual flowers are common in the ANITA group and other basal angiosperms.