RESUMO
BACKGROUND: The lesser grain borer Rhyzopertha dominica (Coleoptera: Bostrichidae) is a stored-product pest beetle. Early histological studies dating back to 1930s have reported that R. dominica and other bostrichid species possess a pair of oval symbiotic organs, called the bacteriomes, in which the cytoplasm is densely populated by pleomorphic symbiotic bacteria of peculiar rosette-like shape. However, the microbiological nature of the symbiont has remained elusive. RESULTS: Here we investigated the bacterial symbiont of R. dominica using modern molecular, histological, and microscopic techniques. Whole-mount fluorescence in situ hybridization specifically targeting symbiotic bacteria consistently detected paired bacteriomes, in which the cytoplasm was full of pleomorphic bacterial cells, in the abdomen of adults, pupae and larvae, confirming previous histological descriptions. Molecular phylogenetic analysis identified the symbiont as a member of the Bacteroidetes, in which the symbiont constituted a distinct bacterial lineage allied to a variety of insect-associated endosymbiont clades, including Uzinura of diaspidid scales, Walczuchella of giant scales, Brownia of root mealybugs, Sulcia of diverse hemipterans, and Blattabacterium of roaches. The symbiont gene exhibited markedly AT-biased nucleotide composition and significantly accelerated molecular evolution, suggesting degenerative evolution of the symbiont genome. The symbiotic bacteria were detected in oocytes and embryos, confirming continuous host-symbiont association and vertical symbiont transmission in the host life cycle. CONCLUSIONS: We demonstrate that the symbiont of R. dominica constitutes a novel bacterial lineage in the Bacteroidetes. We propose that reductive evolution of the symbiont genome may be relevant to the amorphous morphology of the bacterial cells via disruption of genes involved in cell wall synthesis and cell division. Genomic and functional aspects of the host-symbiont relationship deserve future studies.
RESUMO
The soft-electron beam (low-energy electrons) and gamma-radiation sensitivities of phosphine-resistant (PHR) and -susceptible (PHS) strains of adults lesser grain borer Rhyzopertha dominica (F.) were studied, with particular reference to DNA damage assessed using single-cell electrophoresis (comet assay). Results showed that mortality in adult R. dominica varied significantly between both PHR and PHS strains. Adults of the PHR strain were found to be more tolerant toward soft-electron and gamma radiation than adults of the PHS strain. Studies on the longevity of strains showed that mean survival time and dose rate were highly correlated with both strains and treatments. Results also showed that adults of the PHR strain lived longer than adults of PHS strain for both treatments. Radiation sensitivity indices, however, decreased as radiation dose increased in both strains. Analysis of DNA damage, after 40- and 160-Gy gamma radiation, was carried out using cells obtained from both strains. Gamma-irradiated adults of both strains showed typical DNA fragmentation, compared with cells from nonirradiated adults, which showed more intact DNA. Investigations using the comet assay showed that tail length, moment, olive-tail moment, percentage of tail DNA, and percentage of DNA damage were all greater in the PHS strain compared with the PHR strain and the control insects. Results also showed that DNA damage remained at a constant level for up to 24 h after irradiation. The results have been discussed in relation to the observed strain differences in radiation sensitivity and resistance to phosphine.