Self-Incompatibility Systems in the Flowering Plants

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SELF-INCOMPATIBILITY SYSTEMS IN THE FLOWERING PLANTS D. CHARLESWORTH* . . . full of experiments that may be repeated and discussed by intelligent gardeners and of ideas that may soonerfructify in their minds than in those of any other class of naturalists.—Gardeners Chronicle, review of Darwin's Origin of Species, December 1859 Introduction It is very easy to overlook the fact that plants have interesting and complex sex lives, even though most of us appreciate the beauty of flowers, which are the reproductive organs of flowering plants. For most of human history, a practical knowledge of the facts of plant reproduction has been widespread among people who had an interest in seed or fruit production, that is, farmers and gardeners, and even among country children who noticed that some holly trees do not produce berries (they are male plants) or paid attention to the different forms of primrose flowers. Despite the great interest of breeding systems and of the recognition mechanism involved in the phenomenon of self-sterility, and the potential importance of these facts in plant breeding, surprisingly little attention has been given to the study of self-incompatibility in plants, especially at the molecular level. Botanical science has tended to ignore them, and botany textbooks are full of anatomical and taxonomic facts, while modern botany has concentrated on biochemistry—both passing over functional studies above the cellular level. Although the fundamental principles of sexual reproduction in plants, and the role of pollen in fertilising ovules via the stigmata of flowers, were adequately demonstrated experimentally by Camerarius in 1694, and modes of pol- *Department of Biology, University of Chicago, 915 East 57th Street, Chicago, Illinois 60637.© 1987 by The University of Chicago. All rights reserved. 0031-5982/87/3002-0528101.00 Perspectives in Biology and Medicine, 30, 2 ¦ Winter 1987 \ 263 lination by insects and by the agency of wind were reviewed in detail by Sprengel in 1793, it was not until Darwin thought about plants from an evolutionary perspective that the function and mechanisms of plant sex began to become clear. As in so many branches of biology, an evolutionary viewpoint brings out more of the fascinating complexity of the real world than a purely mechanistic approach. Darwin was the first to understand that self-fertilisation is not common and that many species of flowering plants have mechanisms that prevent self-fertilisation, such as separation of the sex functions in time, so that even though pollen is capable of functioning in the pistil (female parts) of the same individual (i.e., the plants are self-compatible), few of the seeds produced under natural conditions will be products of self-fertilisation, unless the plant has many flowers, and they are unsynchronised. In 1862, Darwin published his interpretation of many of the features of orchid flowers as being devices to reduce or prevent self-fertilisation [I]. Darwin also was the first to understand the significance of such outbreeding mechanisms in terms of avoidance of inbreeding depression , which he studied in numerous plant species [2]. Darwin also contributed to understanding of the ways in which plants avoid selffertilisation [3] as he discovered many cases of self-incompatibility, including the self-incompatibility system in plants with different flower forms, such as primroses and Oxalis [4]. Today, the study of self-incompatibility in plants is on the verge of a new major advance that will certainly come through molecular studies that have very recently been started of the genes that control incompatibility types. The available data are so intriguing and puzzling that there seems little doubt that further study will be very rewarding. But the understanding of plant breeding systems also has value in showing how much more complex and interesting beings plants are than one might suspect. Because plants cannot move, one does not suspect their ability to discriminate between their own and other sources of pollen and their elaborate devices that ensure that they will father progeny of other plants, produce seed, and avoid production of poor-quality seed. When one knows something of these facts, one sees plants with new respect, and the beauty of flowers in gardens or in the countryside is enhanced by appreciation of the significance of what one...