Publications

4505

Abstract Many insects depend on obligate mutualistic bacteria to provide essential nutrients lacking from their diet. Most aphids, whose diet consists of phloem, rely on the bacterial endosymbiont Buchnera aphidicola to supply essential amino acids and B vitamins. However, in some aphid species, provision of these nutrients is partitioned between Buchnera and a younger bacterial partner, whose identity varies across aphid lineages. Little is known about the origin and the evolutionary stability of these di-symbiotic systems. It is also unclear whether the novel symbionts merely compensate for losses in Buchnera or carry new nutritional functions. Using whole-genome endosymbiont sequences of nine Cinara aphids that harbour an Erwinia-related symbiont to complement Buchnera, we show that the Erwinia association arose from a single event of symbiont lifestyle shift, from a free-living to an obligate intracellular one. This event resulted in drastic genome reduction, long-term genome stasis, and co-divergence with aphids. Fluorescence in situ hybridisation reveals that Erwinia inhabits its own bacteriocytes near Buchnera’s. Altogether these results depict a scenario for the establishment of Erwinia as an obligate symbiont that mirrors Buchnera’s. Additionally, we found that the Erwinia vitamin-biosynthetic genes not only compensate for Buchnera’s deficiencies, but also provide a new nutritional function; whose genes have been horizontally acquired from a Sodalis-related bacterium. A subset of these genes have been subsequently transferred to a new Hamiltonella co-obligate symbiont in one specific Cinara lineage. These results show that the establishment and dynamics of multi-partner endosymbioses can be mediated by lateral gene transfers between co-ocurring symbionts.

‘Candidatus Liberibacter solanacearum’ (Lso), transmitted by the potato psyllid (Bactericera cockerelli), is the putative causal agent of potato zebra chip disease. The bacterial pathogen infects a wide range of solanaceous plants (both wild and cultivated species), among which are peppers, potatoes, and tomatoes. Currently there are two commonly detected, genetically distinct haplotypes of Lso (A and B) identified from potatoes in the United States. To determine whether there are interactions between Lso haplotypes and different solanaceous hosts, experiments were conducted in the greenhouse in which pepper, potato, and tomato plants were infested with psyllids carrying Lso A, B, or an A and B mix (AB) or with psyllids free of Lso. Host plants were grown in pots in cages on the greenhouse benches and infested with six psyllids per plant. In addition, eight pepper cultivars were similarly infested for deeper understanding of host–haplotype interactions. Approximately 7 weeks after infestation, adult psyllids in each cage were counted to determine the impact of Lso haplotype–host interactions on psyllid survival and plants were sampled and tested molecularly for Lso. Individual psyllids carrying haplotypes B or AB and those free of Lso copiously reproduced on all three hosts, and leaf tissue from each plant tested positive for the respective Lso except those infested with Lso-negative psyllids. However, psyllids carrying Lso A did not survive on peppers but survived and abundantly reproduced on potatoes and tomatoes. In addition, samples from peppers infested with psyllids carrying Lso A tested negative for Lso. However, peppers infested with individual psyllids carrying Lso AB tested positive for Lso A, indicating that the presence of B may be required for infection by Lso A and psyllid survival on peppers. The different pepper cultivars infested with psyllids carrying Lso A showed similar results to the haplotype–host interaction tests, suggesting that cultivar may not be a factor in Lso A–pepper host interactions. Results from these studies suggest that Lso A may affect host selection by psyllids either for nutrition or laying of eggs. Mechanisms involved in preventing psyllid reproduction on peppers, once identified, will have significant implications for potential psyllid management.

Summary The marine sponge Halichondria panicea inhabits coastal areas around the globe and is a widely studied sponge species in terms of its biology, yet the ecological functions of its dominant bacterial symbiont ‘ Candidatus Halichondribacter symbioticus’ remain unknown. Here, we present the draft genome of ‘ Ca . H. symbioticus’ HS1 (2.8 Mbp, ca. 87.6% genome coverage) recovered from the sponge metagenome of H . panicea in order to study functions and symbiotic interactions at the genome level. Functional genome comparison of HS1 against closely related free‐living seawater bacteria revealed a reduction of genes associated with carbohydrate transport and transcription regulation, pointing towards a limited carbohydrate metabolism, and static transcriptional dynamics reminiscent of other bacterial symbionts. In addition, HS1 was enriched in sponge symbiont specific gene families related to host–symbiont interactions and defence. Similarity in the functional gene repertoire between HS1 and a phylogenetically more distant symbiont in the marine sponge Aplysina aerophoba , based on COG category distribution, suggest a convergent evolution of symbiont specific traits and general metabolic features. This warrants further investigation into convergent genomic evolution of symbionts across different sponge species and habitats.

Summary Citrus huanglongbing (HLB) is the most devastating citrus disease worldwide. ‘ Candidatus Liberibacter asiaticus’ (Las) is the most prevalent HLB causal agent that is yet to be cultured. Here, we analysed the flagellar genes of Las and Rhizobiaceae and observed two characteristics unique to the flagellar proteins of Las: (i) a shorter primary structure of the rod capping protein FlgJ than other Rhizobiaceae bacteria and (ii) Las contains only one flagellin‐encoding gene flaA (CLIBASIA_02090), whereas other Rhizobiaceae species carry at least three flagellin‐encoding genes. Only flgJ Atu but not flgJ Las restored the swimming motility of Agrobacterium tumefaciens flgJ mutant. Pull‐down assays demonstrated that FlgJ Las interacts with FlgB but not with FliE. Ectopic expression of flaA Las in A. tumefaciens mutants restored the swimming motility of ∆ flaA mutant and ∆ flaAD mutant, but not that of the null mutant ∆ flaABCD . No flagellum was observed for Las in citrus and dodder. The expression of flagellar genes was higher in psyllids than in planta . In addition, western blotting using flagellin‐specific antibody indicates that Las expresses flagellin protein in psyllids, but not in planta . The flagellar features of Las in planta suggest that Las movement in the phloem is not mediated by flagella. We also characterized the movement of Las after psyllid transmission into young flush. Our data support a model that Las remains inside young flush after psyllid transmission and before the flush matures. The delayed movement of Las out of young flush after psyllid transmission provides opportunities for targeted treatment of young flush for HLB control.