Academic journal article Genetics

A General Model to Explore Complex Dominance Patterns in Plant Sporophytic Self-Incompatibility Systems

Academic journal article Genetics

A General Model to Explore Complex Dominance Patterns in Plant Sporophytic Self-Incompatibility Systems

Article excerpt

ABSTRACT

We developed a general model of sporophytic self-incompatibility under negative frequency-dependent selection allowing complex patterns of dominance among alleles. We used this model deterministically to investigate the effects on equilibrium allelic frequencies of the number of dominance classes, the number of alleles per dominance class, the asymmetry in dominance expression between pollen and pistil, and whether selection acts on male fitness only or both on male and on female fitnesses. We show that the so-called "recessive effect" occurs under a wide variety of situations. We found emerging properties of finite population models with several alleles per dominance class such as that higher numbers of alleles are maintained in more dominant classes and that the number of dominance classes can evolve. We also investigated the occurrence of homozygous genotypes and found that substantial proportions of those can occur for the most recessive alleles. We used the model for two species with complex dominance patterns to test whether allelic frequencies in natural populations are in agreement with the distribution predicted by our model. We suggest that the model can be used to test explicitly for additional, allele-specific, selective forces.

(ProQuest-CSA LLC: ... denotes formulae omitted.)

THE population genetics of plant species with sporophytic self-incompatibility (SSI) are notoriously difficult to study both empirically and theoretically because of the complex dominance relationships occurring among alleles. WRIGHT (1939) developed a theory for gametophytic self-incompatibility (GSI), a genetic system to avoid self-fertilization involving recognition between a protein expressed in the haploid pollen and two codominantly expressed pistil proteins, and identified negative frequency-dependent selection as the major evolutionary force promoting allelic diversity. According to Wright's theory, selection under GSI is symmetric among alleles, and the only relevant feature of an allele is its current population frequency. In SSI, however, the incompatibility phenotypes of pollen and pistils are determined by the diploid genotypes of the paternal and maternal plants, respectively, and are governed by complex dominance interactions among alleles that introduce asymmetrical selection among alleles (BATEMAN 1952; SCHIERUP et al. 1997). A general understanding of the population genetics of SSI has also been difficult because different authors investigated different, often nonoverlapping model representations of SSI, sometimes with unstated assumptions, so that their outcomes have been difficult to compare (Table 1). Differences among models comprise: (1) frequencydependent selection acting on male fitness only (corresponding to Wright's assumption) vs. selection on both male and female fitnesses, named "fecundity selection" in VEKEMANS et al. (1998); (2) expression of dominance in both pollen and pistils vs. codominance in pistil and dominance in pollen; and (3) occurrence of at most one allele per dominance class along a hierarchical ladder of dominance vs. allowing several alleles per dominance class.

Previous analyses of deterministic models of SSI have shown the following:

1. Recessive alleles should reach higher equilibrium frequencies than more dominant alleles, the so-called "recessive effect" (BATEMAN 1952; SAMPSON 1974). The reason is that negative frequency-dependent selection tends to homogenize the frequencies of the phenotypic classes (the "isoplethy" hypothesis). Because of dominance, recessive alleles can be present in more phenotypic classes than are dominant alleles, and they thus reach higher total frequencies (COPE 1962).

2. Within a given class of dominance, individual allelic frequencies should be inversely related to the number of alleles in that class, the so-called "small number effect" (SAMPSON 1974). This arises because within a given dominance class, alleles are selectively equivalent and are thus expected to reach identical frequencies at equilibrium (as in completely symmetric models of balancing selection such as GSI). …

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