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Mosaicism of horizontally transferred potential mobile units in the genome of 'Candidatus Phytoplasma solani' strain SA-1

Backgrounds Small and repeat-rich genomes are characteristic for the members of the genus ‘Candidatus Phytoplasma’, endocellular phytopathogens transmitted and hosted by phloem-feeding insects. While phytoplasma economical importance is increasing, experimental studies are still lagging due to the inability to grow a pure phytoplasma culture. For most phytoplasma species, one of the hallmarks of their behaviour is the adaptability to different plant and insect hosts, especially in ‘Ca. P. solani’ that has a broad host range as is transmitted by polyphagous insects. Objectives The aim of this work was to learn about the background of its plasticity and adaptability by sequencing the genome of ‘Ca. P. solani’ strain SA-1. Methods Whole plant samples of infected periwinkle were used for generation of libraries sequenced on the Illumina MiSeq platform. De novo assembly generated the draft genome of 19 contigs with a total size of 821, 322 bp. Repetitiveness and abundance of potential mobile units (PMUs) was confirmed and found responsible for the incomplete genome assembly. Moreover, a mosaicism of PMUs and PMU-like regions was found, corroborated both by the analyses of PMU organization and gene content as well as the molecular phylogeny of PMU-related genes (dnaB, dnaG, hflB, tmk, fliA, tra5). Conclusions This finding suggests a horizontal transfer of PMUs from related ('Ca. P. australiense') and more distant ('Ca. P. asteris' and 'Ca. P. mali') phytoplasmas. We hypothesize that SA-1 strain gained a mosaic of PMUs, a trait not previously described for phytoplasma genomes, allowing high genome plasticity and successful adaptation to various plant and insect hosts.

genome, plasticity, phytoplasma, mobile genetic elements

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