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.

Tlic reproductive capacities of the females (male-steriles or mS) of l h y m u~ uuigarii L. ha\(. Iwcti i t ivestigated, in terms of size and weight of plants, seed production, germination ability and pollination. The data, collected during nine years under controlled conditions and in natul-al situations are discussed in comparison with the results known in the liter-aturr. It is suggestcd that the large diversity of characteristics promoting sexual polymorphistn could justifv the high and variable rates of tnS met in natural conditions in T. vulgaris.

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.

The present study shows that correlations exist between chemotype polymorphism, sexual polymorphism and the environment. These correlations fall into three categories: 1. a fairly tight correlation exists between soil type and chemotype structure of the thyme populations growing on it; with a soil change, a predictible chemotypic change occurs. 2. There is a clear correlation between a stable environment and low genetic diversity; conversely, in disturbed areas, high genetic diversity exists as indicated by higher outcrossing rates and high levels of chemotype diversity. 3. Thyme uses a strategy of low dispersal which allows, through individual selection, the precise specialisation of each population to its environment.

Six distinct chemotypes of the mediterranean 7'lzymus vulgaris L. are characterized by differences in the major monoterpenes produced. The nature of the monoterpenes is shown to be controlled by a series of loci. Each locus probably has a dominant and a recessive allele. Epistatic relationships between these loci are interpreted by assuming that terpenes are end products of branches from the same biosynthetic pathway. The most dominant types, which correspond to the monoterpenes produced at the beginning of the chain, show the most important potential genetic diversity. There is good evidence that regulatory genes are involved in this system.