On the Evolutionary Costs of Self-Incompatibility: Incomplete Reproductive Compensation Due to Pollen Limitation

Abstract

Pollen limitation affects plants with diverse reproductive systems and ecologies. In self-incompatible (SI) species, pollen limitation may preclude full reproductive compensation for prezygotic rejection of pollen. We present a model designed to explore the effects of incomplete reproductive compensation on evolutionary changes at a modifier locus that regulates the level of SI expression. Our results indicate that incomplete reproductive compensation greatly increases the evolutionary costs of SI, particularly in populations with low S-allele diversity. The evolutionary fate of modifiers of SI expression depends on the rate at which they are transmitted to future generations as well as the effects of SI on offspring number and quality. Partial SI expression can represent a stable condition rather than an evolutionarily transient state between full expression and full suppression. This unanticipated result provides the first theoretical support for the evolutionary stability of such mixed mating systems, the existence of which has recently been documented.

Sporophytic self-incompatibility in Senecio squalidus (Asteraceae): S allele dominance interactions and modifiers of cross-compatibility and selfing rates

“…In general, it seems that S-modifier loci are maintained as part of SI systems when they contribute to the regulation of outcrossing or compensate for conditions where the reproductive costs of full SI are high (Vallejo-Marín and Uyenoyama, 2004;Harder et al, 2007;Busch and Schoen, 2008).…”

“…Avoidance of inbreeding depression in the presence of a considerable accumulated genetic load is considered to be a major force for the maintenance of SI (Vallejo-Marín and Uyenoyama, 2004;Porcher and Lande, 2005). However, the relationship is not a simple one because genetic load is also itself responsive to selection and coevolves with mating system (Latta, 1994;Glemín et al, 2001).…”

“…Although direct suppression of S locus genes is sometimes the causal factor of SI breakdown, empirical evidence suggests that unlinked S-modifier loci that generate quantitative variation in SI expression are more often the rule (McClure et al, 2000). A theoretical understanding is emerging of the range of conditions under which mixed mating systems that combine SI and intermediate selfing rates are evolutionary stable (Vallejo-Marín and Uyenoyama, 2004;Porcher and Lande, 2005;Harder et al, 2007;Johnston et al, 2009). These conditions are the result of interplay between several factors including inbreeding depression and reproductive assurance.…”

The Paradox of Clonality and the Evolution of Self-Incompatibility

“…On the other hand, if pollen or pollinator availability is low, pollen grains that are rejected by the self-incompatibility mechanism may not be substituted, and thus some ovules would go unfertilized. The evolutionary maintenance of self-incompatibility depends on the relative benefit of producing higher quality offspring and the relative costs incurred by potential reduction in offspring number 8 Plant clonality can affect the relative benefits and costs of self-incompatibility through its effects on the persistence and spatial distribution of genotypes. For instance, Vallejo-Marín and O'Brien 1 suggest that clonality provides reproductive assurance in colonizing taxa by allowing genotypes to persist and propagate even in the absence of conditions conducive to seed production.…”

“…1). 8,12,13 It is important to note that both the maintenance and the breakdown effects mentioned www.landesbioscience.com Plant Signaling & Behavior The Paradox of Clonality and Self-Incompatibility above are expected to occur in the same ecological conditions, namely when the availability of pollen, pollinators, or compatible mates limit seed set (collectively known as pollen limitation).…”