SELFISH DNA AND BREEDING SYSTEM IN FLOWERING PLANTS

In many species, some individuals carry one or more B chromosomes: ext ra, or supernumerary chromosomes not part of the normal complement. In most well-studied cases, B's lower the fitness of their carrier and p ersist in populations only because of accumulation mechanisms analogou s to meiotic drive. It has been suggested that such genomic parasites are expected to persist only in outcrossed sexual species, in which un infected lines of descent can be continuously reinfected; in inbred or asexual species, all selection is between lines of descent, and the g enomic parasites are either lost or must evolve into commensals or mut ualists. Here we present a simple population genetic model of the effe ct of outcrossing rate on the frequency of B chromosomes, and find tha t outcrossing facilitates the spread of parasitic B's, but inhibits th e spread of mutualists. Data compiled from the literature on breeding system and B chromosomes of British plants indicate that B's are much more likely to be reported from obligately outcrossed species than inb red species. These results support the ideas that most B chromosomes a re parasitic, and that breeding systems play a central role in the bio logy of selfish genes.