The Biology And Evolution Of The Mollicutes/Mycoplasma-Related Endobacteria Of Arbuscular Mycorrhizal Fungi

Abstract

Arbuscular mycorrhizal fungi (AMF) are obligate biotrophs that form symbiotic associations with the roots of terrestrial plants. All major lineages of AMF harbour cytoplasmic endobacteria related to the Mollicutes class, referred to as the Mollicutes/mycoplasma-related endobacteria (MRE). There is virtually nothing known about the biology of MRE, nor the nature of their association of with AMF. The research outlined in this thesis is the first biological study of MRE. The first study is the official classification of MRE, based on our phylogenetic analysis and in vitro cultivation attempts. Our study revealed that MRE consist of many types, with MRE from each host species displaying varying degrees of genetic diversity. We used the genetically uniform MRE population of the AMF Rhizophagus clarus, as the first representative for official classification of MRE; Candidatus Glomeriplasma moenium gen. nov., sp. nov. was proposed for this MRE population. The second study is a metagenomic study on three MRE populations. We discovered that MRE genomes are highly plastic, with evidence of numerous chromosomal rearrangements. Their minimal genome indicates complete metabolic dependence on their hosts. The MRE genome also contained many genes acquired from the AMF through horizontal gene transfer, including SUMO proteases, likely used to alter SUMOylation levels in their hosts, which has been shown to increase endobacterial fitness in other systems. The extent of MRE genome erosion, along with the large number of horizontally acquired fungal genes, suggests a high degree of coevolution between the partners, and highlights the significance of MRE in AMF biology. The third study is an investigation of the molecular evolution of MRE. Muller's ratchet is an evolutionary theory that predicts that asexual finite populations (e.g. endobacterial populations) will eventually lead to population extinction due to the continuous fixation of deleterious mutations combined with the random loss of the most fit individuals. We found that MRE escapes Muller's ratchet through the maintenance of recombination machinery that creates genetic diversity in their population. MRE are unique in this light, as endobacteria are generally unable to prevent the erosion of their recombination machinery.