A novel mechanism of balancing selection, a type of natural selection, in root pathogen Plasmodiophora brassicae, is presented. Recent studies have demonstrated that P. brassicae retains entire genomes in its population over time. The proposed mechanism for this retention is suppression of host resistance during infection by a virulent genotype (pathotype). This suppression of resistance provides an opportunity for infection by avirulent pathotypes, which would otherwise not be able to infect and reproduce in the resistant host cultivar. In this novel mechanism, which we propose to name as beta retention, a virulent genotype (alpha genotype) opens the door for infection by avirulent genotype (beta genotypes). Under other conditions, such as a different resistant cultivar or host, the alpha genotype could function as a beta genotype and vice versa. This novel mechanism of balancing selection is distinct from existing mechanisms in that entire genomes are retained in the population; other mechanisms maintain single genes or gene clusters. Retention of whole genomes has not previously been reported, probably because it occurs only in microbial plant pathogens and the beta genotypes could only be identified using whole-genome sequences. Beta retention is proposed as a separate mechanism of balancing selection.
Gossen et al. (Wed,) studied this question.