Genetics

ABSTRACT “ Candidatus Phycosocius bacilliformis” is an alphaproteobacterial ectosymbiont of the hydrocarbon-producing green alga Botryococcus braunii . We sequenced the whole genome of “ Ca. P. bacilliformis” BOTRYCO-2, isolated from a two-membered culture with B. braunii . The genome contains approximately 3.3 Mb, with an average G+C content of 56.91% and 3,125 predicted protein-coding genes.

Phytoplasmas of the apple proliferation group cause diseases in various fruit trees, mainly apples, pears and apricots. Immunodominant membrane protein, or ‘Imp’, shows high inter‐ and intraspecies gene variability, and is thus frequently used for population studies in these organisms. In this investigation, a survey was carried out of ‘ Candidatus Phytoplasma pyri’ imp gene diversity in the Czech Republic. In total, 12 imp genotypes that could be clustered into nine groups were identified. Using all publicly available data, 17 distinct imp genotypes (labelled A–K) were defined. Interestingly, genotypes C, H and I were more related to ‘ Ca . Phytoplasma mali’ than to other ‘ Ca . Phytoplasma pyri’ imp genotypes, indicating that some strains may represent a transitional type between the two species. As Imp probably plays an important role in host–pathogen interactions, description of its diversity in ‘ Ca . Phytoplasma pyri’ may help in understanding the epidemiology, symptomatology, and other aspects of pear decline disease. A simple RFLP PCR system is presented for rapid and cost‐effective discrimination of ‘ Ca . Phytoplasma pyri’ imp genotypes.

Peach orchards in the northeast of Spain were severely affected in 2012 by a previously unreported disease in this area. The symptoms included early reddening, leaf curling, decline, abnormal fruits, and in some cases death of the peach trees. All the infected peach samples were positive for ‘ Candidatus Phytoplasma pyri’, but none were infected by the ‘ Ca . Phytoplasma prunorum’. In this work, potential vectors able to transmit ‘ Ca . Phytoplasma pyri’ from pear to peach and between peach trees were studied and their infective potential was analysed at different times of the year. Transmission trials of the phytoplasma with potential vectors to an artificial feeding medium for insects and to healthy peach trees were conducted. Additionally, isolated phytoplasmas were genetically characterized to determine which isolates were able to infect peach trees. Results showed that the only insect species captured inside peach plots that was a carrier of the ‘ Ca . Phytoplasma pyri’ phytoplasma was Cacopsylla pyri . Other insect species captured and known to be phytoplasma transmitters were present in very low numbers, and were not infected with ‘ Ca . Phytoplasma pyri’ phytoplasma. A total of 1928 individuals of C. pyri were captured in the peach orchards, of which around 49% were phytoplasma carriers. All the peach trees exposed to C. pyri in 2014, and 65% in 2015, were infected by ‘ Ca . Phytoplasma pyri’ 1 year after exposure, showing that this species is able to transmit the phytoplasma to peach. Molecular characterization showed that some genotypes are preferentially determined in peach.