The evolution of self-fertility in apomictic plants

Abstract

Self-fertilization and apomixis have often been seen as alternative evolutionary strategies of flowering plants that are advantageous for colonization scenarios and in bottleneck situations. Both traits have multiple origins, but different genetic control mechanisms; possible connections between the two phenomena have long been overlooked. Most apomictic plants, however, need a fertilization of polar nuclei for normal seed development (pseudogamy). If self-pollen is used for this purpose, self-compatibility is a requirement for successful pollen tube growth. Apomictic lineages usually evolve from sexual self-incompatible outcrossing plants, but pseudogamous apomicts frequently show a breakdown of self-incompatibility. Two possible pathways may explain the evolution of SC: (1) Polyploidy not only may trigger gametophytic apomixis, but also may result in a partial breakdown of SI systems. (2) Alternatively, frequent pseudo self-compatibility (PSC) via aborted pollen may induce selfing of pseudogamous apomicts (mentor effects). Self-fertile pseudogamous genotypes will be selected for within mixed sexual-apomictic populations because of avoidance of interploidal crosses; in founder situations, SC provides reproductive assurance independent from pollinators and mating partners. SI pseudogamous genotypes will be selected against in mixed populations because of minority cytotype problems and high pollen discounting; in founder populations, SI reactions among clone mates will reduce seed set. Selection for SC genotypes will eliminate SI unless the apomict maintains a high genotypic diversity and thus a diversity of S-alleles within a population, or shifts to pollen-independent autonomous apomixis. The implications of a breakdown of SI in apomictic plants for evolutionary questions and for agricultural sciences are being discussed.

Fig. 1

Model for the evolution of self-compatibility in pseudogamous apomicts. The apomicts (stars) are assumed to arise within a SI diploid sexual population (circles). A SI apomict (black star) largely fails to establish within the sexual populations because of a minority cytotype disadvantage and lower fertility than sexuals. SI pollen donors suffer from high pollen discounting and will disappear from the population (Noirot et al. 1997). If pollinated by another apomictic genotype (gray star), seeds may establish in some spatial distance to the source population (1), but this SI plant must be still pollinated by another genotype (2). The SI apomict surrounds itself with SI offspring (3) and goes extinct after the first generation (4) because of mother–offspring and sibling incompatibility. Any self-fertile genotype will be selected for