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Self-incompatibility systems: barriers to self-fertilization in flowering plants
ANNE C. REA and JUNE B. NASRALLAH*
Department of Plant Biology, Cornell University, Ithaca, NY, USA
ABSTRACT Flowering plants (angiosperms) are the mostprevalent and evolutionarily advanced group of plants. Success of these plants is owed to several unique evolutionary adaptations that aid in reproduction: the flower, the closed carpel, double fertilization, and the ultimate products of fertilization, seeds enclosed in the fruit. Angiosperms exhibit a vast array of reproductive strategies, including both asexual and sexual, the latter of which includesboth self-fertilization and cross-fertilization. Asexual reproduction and self-fertilization are important reproductive strategies in a variety of situations, such as when mates are scarce or when the environment remains relatively stable. However, reproductive strategies promoting cross-fertilization are critical to angiosperm success, since they contribute to the creation of genetically diversepopulations, which increase the probability that at least one individual in a population will survive given changing environmental conditions. The evolution of several physical and genetic barriers to self-fertilization or fertilization among closely related individuals is thus widespread in angiosperms. A major genetic barrier to self-fertilization is self-incompatibility (SI), which allowsfemale reproductive cells to discriminate between “self” and “non-self” pollen, and specifically reject self pollen. Evidence for the importance of SI in angiosperm evolution lies in the highly diverse set of mechanisms used by various angiosperm families for recognition of self pollen tube development and preventing self-fertilization.
KEY WORDS: plant reproductive barrier, pollination,pollen-pistil interaction, S locus
As for all organisms, reproductive success is critical for survival in plants. All plants have the capacity to reproduce sexually, but given their predominantly sessile lifestyle, it is no wonder that plants have maintained the ability to reproduce without the need for a mate throughout their evolutionary history. Most plant species, but very few animalspecies, have the capacity to reproduce asexually. Even the most evolutionarily advanced and successful group of plants, the angiosperms (flowering plants), are capable not only of reproducing asexually, but can reproduce sexually via self-fertilization, whereby the egg and sperm from the same plant fuse to produce viable offspring. Hermaphroditism, the condition in which an individual has both maleand female reproductive structures, is a pre-requisite for self-fertilization. This condition and the ability to self-fertilize are relatively rare in animals but very common in plants. While self-fertilization has its advantages under favorable and relatively stable conditions, it can be problematic in more variable
and unpredictable environments. Since self-fertilization ultimately resultsin the production of genetically identical offspring, it can culminate in a population of low genetic diversity, making the perpetuation of populations difficult in the variable environments that plants encounter in nature and often cannot escape. It is therefore not surprising that throughout their history, plants have evolved many different mechanisms for preventing self-fertilization, or atleast promoting cross-fertilization, which allow them to avoid the potentially deleterious consequences of inbreeding. In this review, we outline the unique features of fertilization in flowering plants. We focus on the events that precede egg-sperm fusion, namely the intricate interactions that occur between pollen
Abbreviations used in this paper: ARC1, Armadillo repeat-containing protein 1;...