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Project Progress

We continued our studies of invertebrate-associated bacteria using Wolbachia pipientis endosymbionts that infect up to %70 of all arthropod species. The Wolbachia symbiosis is an appropriate model for investigating how acquisitions of endosymbionts shape highly speciose groups of eukaryotes. Among metazoan hosts, the innate immune system in particular serves as the first line of communication to foreign endosymbionts and represents an ancient weapon necessary for million of species that lack a sophisticated adaptive immune response. We showed this year that adults of one infected insect species have low infection whereas in the closely related species adults have ~100-fold higher titers to the same strain. This unambiguous difference is due mostly to differences in infection density of similarly infected tissues and an expanded tissue tropism. We also showed stability of the high infection titers for 50+ generations rather than attenuation after a few generations and finally we demonstrated reversal of the infection titers from high to low densities upon transinfection back to the resident species. Based on these four lines of evidence, we conclude that an unambiguous genetic difference between the host species controls a ~100-fold change in endosymbiont densities. These results will provide information on the genetic basis of variation in immunocompetence to intracellular, inherited bacteria and may reveal new genes or mechanisms involved in the cross-talk between eukaryotes and their bacterial endosymbionts.

Our studies of long-term bacterial mutualists of insects examine the genomic consequences of lifestyle transitions from a free-living to a host-dependent existence. This year, we discovered that homopolymeric tracts persist in these endosymbiont genomes and have important functional effects. In a comparative genomic analysis, we demonstrated that genomes of insect mutualists contain surprisingly high frequencies of homopolymeric regions, especially polyA tracts. Such tracts are considered deleterious in bacteria because they are prone to transcriptional slippage. Using experimental approaches, we showed that RNA polymerase has low fidelity along polyA tracts in the ant mutualist, Blochmannia, and generates a pool of heterogeneous transcripts. This slippage typically disrupts reading frames of genes, but occasionally restores the reading frame of so-called 'pseudogenes’ that contain frameshifts. Transcriptional slippage could be exploited to manipulate gene expression in small genomes that lack most gene regulation functions. This study has been accepted (pending minor revision) in PNAS.