The avoidance of interference between the presentation of pollen and stigmas in angiosperms I. Dichogamy

Abstract

Dichogamy is the separation of the presentation of pollen and stigmas in time within a plant. It is a common but neglected feature of outcrossing angiosperms. Dichogamy has been almost universally interpreted as an outcrossing mechanism, but many dichogamous species are also self-incompatible (and sometimes also herkogamous and/or with unisexual flowers). In outcrossing species, there is almost invariably a clash between selection to place pollen and stigmas in similar positions for effective pollination and selection to keep the androecia and gynoecia apart to avoid interference between them. We suggest that the separation of pollen and stigmas acts in general to reduce this self-interference and it often also reduces self-fertilisation. Mechanisms preventing self-fertilisation primarily increase maternal fitness, whereas mechanisms avoiding self-interference primarily promote paternal fitness. Five independent ways of subdividing dichogamy are recognised: protandry or protogyny; intrafloral or interfloral dichogamy; complete or incomplete dichogamy; various intervals between the successive presentations of pollen and stigmas; asynchronous, hemisynchronous, or synchronous dichogamy (the latter of several subtypes). In a sample of British species, protandry is almost twice as common as protogyny in biotically pollinated species but protogyny is six times as common as protandry in abiotically pollinated species. To obtain testable hypotheses of the selective forces responsible for dichogamy, four selective forces that influence whether pollen or stigmas are presented first are examined. (1) Effectiveness in avoiding self-fertilisation; (2) Selection for prolonged pollen presentation; (3) Optimal positions for dispatching and receiving pollen; (4) Interference between stamens and carpels, involved in seven different contexts: (a) The relative ease of moving androecia and gynoecia; (b) Vertical wind-pollinated inflorescences; (c) Vertical animal-pollinated inflorescences; (d) Refuge, trap, and brood blossoms; (e) One type of sporophyll facilitates the presentation of the other; (f) Stamen signals or rewards; (g) Post-presentation changes in flowers. The most important factor overall may be the relative ease of moving stamens and carpels after they have functioned. For many species there may be a combination of selective causes of the direction of dichogamy. Besides pollen-stigma interference, other types of interference (in which two activities obstruct each other because they have the same optima) and conflicts (in which two activities have divergent optima) occur in plants.