Viruses and the Advantage of Sex in Anthoxanthum odoratum : A Review(<SPECIAL ISSUE>INTERNATIONAL SYMPOSIUM : MAINTENANCE MECHANISM AND DIVERSITY OF PLANT SPECIES POPULATIONS)

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The nature of the selective forces responsible for the maintenance of sexual reproduction in populations remains controversial. Theoreticians have proposed a variety of mechanisms to explain how sex is adaptive, including varying environments, sib competition, mutational damage, and pathogens. In a well-studied population of the short-lived, perennial grass Anthoxanthum odoratum in a mown North Carolina field, significant fitness advantages for sexual vs. asexual offspring have been shown. Evidence from two recent experiments implicates barley yellow dwarf luteovirus (BYDV), a single-stranded RNA virus, as a cause of the short-term advantage for sex in this population. When planted in sites close to parents, asexual offspring were twice as frequently infected as sexual offspring. In other sites, infection was more frequent among common than rare genotypes. Viruses possess a unique combination of features which make them plausible candidates to favor sexual reproduction in plants. Viruses are pervasive, have subtle but significant fitness effects, have the potential for rapid mutation and evolution, and are spread by vectors whose behaviors are host frequency dependent. The ecological and evolutionary consequences of viruses in plant populations deserve further study.