Spatial dynamics of cytoplasmic male sterility

Introduction Conflict often leads to nonequilibrium fluctuations as attackers spread, host defences increase, and counter-attack evolves. Spatial processes play a crucial role when attack types arrive from other populations and import of matching defence follows. We focus on cytoplasmic male sterility (CMS), an ideal system in which to study the spatial dynamics of conflict. Cytoplasmically inherited genes in the mitochondria sometimes cause male sterility by interfering with pollen development. Reallocation from pollen to ovules increases the number of seeds. This reallocation to seeds benefits the mitochondrial CMS genes, which are transmitted through seeds but not through pollen. Reproductive reallocation causes a conflict with nuclear genes, which are transmitted through both pollen and seeds. Consistent with this idea of conflict, nuclear restorers occur that counteract cytoplasmic effects and restore pollen fertility. A plant appears as a normal hermaphrodite when it has a male-sterile cytoplasm and matching nuclear restorers. Wild populations of CMS plants maintain distinct cytoplasmic genotypes (cytotypes). Each cytotype causes male sterility by an apparently different mechanism because each responds to a particular subset of nuclear restorer alleles. Nuclear restorer alleles are typically polymorphic at several loci, with each allele specialized for restoring pollen fertility when associated with a particular cytotype. The frequencies of cytotypes, restorer alleles, and male-sterile (female) plants often vary over space. We focus on the processes that influence spatial variation in gene frequencies. We start by summarizing the key observations and theories. With the theory as our guide, we analyse the best studies of natural populations currently available. Finally, we consider prospects for the future. The arrival of molecular tools brings great opportunity to measure the spatial distributions of genotypes and to test theories about the spatial dynamics of conflict. In the course of our chapter, we develop a new model of CMS dynamics. We argue that the relative allocation of resources to pollen and ovules by hermaphrodites interacts with the spatial dynamics of gene frequency fluctuations driven by conflict.