THE STABILITY OF THE SYMBIOSIS BETWEEN DIOECIOUS FIGS AND THEIR POLLINATORS: A STUDY OF FICUS CARICA L. AND BLASTOPHAGA PSENES L.

Each Ficus species depends on a specific mutualistic wasp for pollination. The wasp breeds on the fig, each larva destroying a female flower. It is, however, not known why the wasps have not evolved the ability to use all female flowers. In "dioecious" figs, the wasp can only breed in the female flowers of the "male" trees, so that pollination of a female tree is always lethal. The wasps should therefore be selected to avoid female trees. Field data is presented showing that the fruiting phenology of the dioecious fig Ficus carica is such that this selection does not occur: syconia are not receptive at the same time on "male" and female trees. Most wasps are forced to emerge from the syconia of "male" trees at a time when they will not be able to reproduce, whether they avoid female trees or not. This aspect of the life cycle of the wasp, although noticed, has been obscured in most previous studies. It is shown that the fruiting phenology of Ficus carica, which stabilizes the symbiosis, is the result of short-term selective pressures on the male function of the trees. Such selective pressures suggest a possible pathway from monoecy to dioecy in Ficus under seasonal climates. © 1987 The Society for the Study of Evolution.