Effects of chimerism in bacterial social performance

Background/Question/Methods

The total productivity or performance of heterogeneous groups can be determined by the presence or absence of interactions between their constituents. Although relevant to understanding the dynamics of natural systems, studies testing the effects of community composition on total productivity tend to focus only on pairwise combinations of species or strains. We used several strains recently isolated from nature of the soil bacterium Myxococcus xanthus to test the effect of different degrees of chimerism on the total group performance in three social behaviors displayed by this species. We created cultures with 1 to 5 different strains in all possible strain combinations. To analyze swarming ability and predation, we spotted chimeric cultures of M. xanthus on the center of agar plates with nutrients or without nutriet but containing prey lawns. We then measured distance covered by the expanding colony after 4 days. To analyze developmental ability, we spotted the same chimeric cultures on nutrient-free agar plates and harvested the fruiting bodies resulting from the starvation conditions 5 days later. We then counted the number of viable spores produced.

Results/Conclusions

We found that predation and developmental proficiency by M. xanthus are negatively affected by the number of strains in the chimeric group, but we found no correlation between group performance and average genetic distance of the individuals within the group. This suggests that a decrease in performance of chimeric groups can be a stronger force leading to population differentiation and kin discrimination than has been previously considered. Our results thus have important implications for our understanding of the evolution and maintenance of the diversity and structure of microbial soil populations.