Symbioses: Plant, Animal, and Microbe Interactions
Castillo, Dean M. , Pawlowska, Teresa .
Molecular evolution in bacterial endosymbionts of fungi.
Bacterial endosymbionts are rarely found in fungi. The notable exceptions include: (1) Burkholderia rhizoxinica and B. endofungorum associated with plant pathogenic Rhizopus microsporus, and (2) Candidatus Glomeribacter gigasporarum endosymbionts of arbuscular mycorrhizal fungi, Glomeromycota. Rhizopus endosymbionts provision the host with toxins important for plant pathogenesis and are indispensable to the host asexual reproduction. In contrast, the life history of Glomeribacter and its role in the biology of arbuscular mycorrhizal fungi are unclear. To better understand the association of Glomeribacter with Glomeromycota, we studied patterns of molecular evolution in these bacteria, and compared them with the patterns in other symbiotic and free-living beta-proteobacteria. The symbiotic bacteria included Burkholderia associates of Rhizopus, and Ca. Tremblaya princeps obligate endosymbionts of mealybugs (Pseudococcidae). The free-living relatives were B. cepacia and B. glathei. The nearly neutral theory predicts that in small asexual populations, mildly deleterious mutations will drift to fixation at a faster rate than in large and/or sexual populations. Consequently, obligate endosymbionts are expected to exhibit: (1) rates of molecular evolutions that are faster than in free-living relatives, and (2) reduced stability of ribosomal RNA molecules. We found that the Glomeribacter lineage evolved significantly slower than the Tremblaya lineage but significantly faster than Rhizopus endosymbionts and free-living Burkholderia species. Thermal stability of rRNA in Glomeribacter was higher than in Rhizopus endosymbionts and free-living Burkholderia, whose rRNA molecules were, in turn, more stable than those of Tremblaya. We also compared trajectories of rRNA evolution in all three endosymbiotic lineages and found that those of Glomeribacter and Tremblaya were significantly different. Taken together, our observations indicate that Glomeribacter follows a unique pattern of molecular evolution that is very different from closely related facultative endosymbionts of fungi and obligate endosymbionts of insects. Based on this pattern, we will speculate about the lifestyle of Glomeribacter.
Log in to add this item to your schedule
1 - Cornell University, Plant Pathology & Plant-Microbe Biology, 334 Plant Science Building, Ithaca, NY, 14853-5904, USA
Presentation Type: Oral Paper:Papers for Topics
Location: Wasatch B/Cliff Lodge - Level C
Date: Wednesday, July 29th, 2009
Time: 8:00 AM