Publications

4374

Summary Holobiont phenotype results from a combination of host and symbiont genotypes as well as from prevailing environmental conditions that alter the relationships among symbiotic members. Corals exemplify this concept, where shifts in the algal symbiont community can lead to some corals becoming more or less thermally tolerant. Despite linkage between coral bleaching and disease, the roles of symbiotic bacteria in holobiont resistance and susceptibility to disease remains less well understood. This study thus characterizes the microbiome of disease‐resistant and ‐susceptible Acropora cervicornis coral genotypes (hereafter referred to simply as ‘genotypes’) before and after high temperature‐mediated bleaching. We found that the intracellular bacterial parasite ‘ Ca. Aquarickettsia rohweri’ was strikingly abundant in disease‐susceptible genotypes. Disease‐resistant genotypes, however, had notably more diverse and even communities, with correspondingly low abundances of ‘ Ca. Aquarickettsia’ . Bleaching caused a dramatic reduction of ‘ Ca. Aquarickettsia’ within disease‐susceptible corals and led to an increase in bacterial community dispersion, as well as the proliferation of opportunists. Our data support the hypothesis that ‘ Ca. Aquarickettsia’ species increase coral disease risk through two mechanisms: (i) the creation of host nutritional deficiencies leading to a compromised host‐symbiont state and (ii) the opening of niche space for potential pathogens during thermal stress.

Abstract Background ‘Candidatus Phytoplasma ulmi’ is the agent associated with elm yellows and has been categorised in the European Union as a quarantine pathogen. For central and northern European countries, information on the occurrence and distribution of the pathogen and its impact on elms is scarce, so a survey of native elm trees has been conducted in Germany. Results About 6500 samples from Ulmus minor, Ulmus laevis and Ulmus glabra, were collected nationwide. Phytoplasma detection was performed by applying a universal 16Sr DNA-based quantitative PCR (qPCR) assay and a novel ‘Ca. P. ulmi’ specific qPCR assay targeting the 16S–23S spacer region. Both assays revealed that 28% of the samples were infected by ‘Ca. P. ulmi’, but infection rates of the elm species and regional incidences differed. The phytoplasma presence in the trees was not correlated to disease-specific symptoms. The survey identified a regional disparity of infection which was high in east, south and central Germany, whereas only a few infected sites were found in the western and northern parts of the country. Monitoring the seasonal titre of ‘Ca. P. ulmi’ in an infected tree by qPCR revealed a high colonisation in all parts of the tree throughout the year. Conclusions ‘Ca. P. ulmi’ is widely present in elms in Germany. The rare occurrence of symptoms indicates either a high degree of tolerance in elm populations or a low virulence of pathogen strains enabling high infection rates in a long-living host.