Population genetics of nodule bacteria: simulation of cyclic processes in bacterial-plant systems.

摘要

A model allowing the analysis of the role of reproduction in legume nodules and transfer of the symbiotic sym genes in the evolution of legume facultative symbionts, nodule bacteria (rhizobia), was elaborated. The model describes the formation of new symbiont populations as a result of the sym gene transfer from initial symbionts to avirulent soil bacteria and reproduction of the formed virulent recombinants in nodules. The microevolution of a closed bacterial-plant system was analysed. The systemwas assumed to have several constant parameters: the numbers of initial symbionts and local avirulent bacteria, the frequency of the formation of new symbionts as a result of the sym gene transfer, the total number of nodules formed by initial and new symbionts, and the number of bacteria released into the soil after the degradation of a nodule. According to the model, the micro-evolution of the system consists of a number of cycles, each involving: (1) transfer of the sym genes from nodule strains to local bacteria, which produces new virulent symbionts; (2) competition of virulent strains (initial and new symbionts) for nodulation; (3) in planta reproduction and release of virulent clones into the soil; and (4) competition of virulent and avirulent local strains for ex planta saprophytic living. A recurrent equation, which allows the estimation of the numbers of new symbionts at all evolutionary cycles, was obtained. The microevolution rate of the system depended on the number of nodules formed by bacteria on plant roots and, to the least extent, on the numbers of initial symbionts.