2001 The Society for the Study of Evolution. All rights reserved.
Evolution, 55(5), 2001, pp. 869879
PARASITES AND THE EVOLUTION OF SELF-FERTILIZATION
ANEIL F. AGRAWAL1 AND CURTIS M. LIVELY
Department of Biology, Indiana University, Bloomington, Indiana 47405-3700
Abstract. Assuming all else is equal, an allele for selfing should spread when rare in an outcrossing population and
rapidly reach fixation. Such an allele will not spread, however, if self-fertilization results in inbreeding depression so
severe that the fitness of selfed offspring is less that half that of outcrossed offspring. Here we consider an ecological
force that may also counter the spread of a selfing allele: coevolution with parasites. Computer simulations were
conducted for four different genetic models governing the details of infection. Within each of these models, we varied
both the level of selfing in the parasite and the level of male-gamete discounting in the host (i.e., the reduction in
outcrossing fitness through male function due to the selfing allele). We then sought the equilibrium level of host
selfing under the different conditions. The results show that, over a wide range of conditions, parasites can select for
host reproductive strategies in which both selfed and outcrossed progeny are produced (mixed mating). In addition,
mixed mating, where it exits, tends to be biased toward selfing.
Key words. Breeding-system evolution, cross-fertilization, mixed mating, parasites, Red Queen hypothesis, self-
fertilization, sexual reproduction.
Received May 22, 2000. Accepted December 22, 2000.