Reproductive biology of two tropical dioecious fig species: Implications for the maintenance and evolution of dioecy in Ficus.

摘要

Fig trees (Ficus spp., Moraceae) are pollinated by species-specific mutualist wasps (Hymenoptera: Chalcidoidea: Agaonidae). In dioecious figs, male trees produce pollen and pollinator wasps and provide food and shelter for wasp larvae, while female trees produce seeds. The reproductive phenology of two dioecious fig species, Ficus exasperata and F. hispida, was observed over a two year period in two sites, a strongly seasonal site and a relatively aseasonal site in south India. Branches on male and female trees of these species were mapped and the fates of the figs on them followed through weekly censuses, to quantify the phenology of dioecious figs in the tropics, and to test the validity of an hypothesis relating the evolution of dioecy in figs to their reproductive phenology in seasonal environments. The phenology of F. exasperata was seasonal, and corresponded closely to that predicted by the hypothesis for dioecious figs in seasonal environments. The aseasonal phenology of F. hispida was different from that predicted. Several other benefits of sexual specialization were found in both fig species, such as greater ovule production, more foundresses, longer duration of and increased variation in developing phase in figs from female than from male trees, and greater crop overlap in female trees than in male trees. These advantages of differential resource allocation to sexual function, combined with phenological data from F. hispida, indicated that a second route to dioecy existed in Ficus. Another major issue concerned the stability of the dioecious fig/wasp mutualism. In the F. hispida system, in which receptive periods on male and female trees overlapped frequently, individual wasp pollinators should be strongly selected to avoid female figs, in which they cannot reproduce. However, in an experiment conducted to determine whether wasps could distinguish between sexes when presented with a choice of receptive figs of both sexes, it was found that wasps could not distinguish between fig sexes. The results suggested that other factors may be involved in limiting the ability of wasps to choose between sexes, serving to stabilize the dioecious fig/wasp mutualism.