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BackgroundSpiroplasma citri comprises a bacterial complex that cause diseases in citrus, horseradish, carrot, sesame, and also infects a wide array of ornamental and weed species. S. citri is transmitted in a persistent propagative manner by the beet leafhopper, Neoaliturus tenellus in North America and Circulifer haematoceps in the Mediterranean region. Leafhopper transmission and the pathogen's wide host range serve as drivers of genetic diversity. This diversity was examined in silico by comparing the genome sequences of seven S. citri strains from the United States (BR12, CC-2, C5, C189, LB 319, BLH-13, and BLH-MB) collected from different hosts and times with other publicly available spiroplasmas.ResultsPhylogenetic analysis using 16S rRNA sequences from 39 spiroplasmas obtained from NCBI database showed that S. citri strains, along with S. kunkelii and S. phoeniceum, two other plant pathogenic spiroplasmas, formed a monophyletic group. To refine genetic relationships among S. citri strains, phylogenetic analyses with 863 core orthologous sequences were performed. Strains that clustered together were: CC-2 and C5; C189 and R8-A2; BR12, BLH-MB, BLH-13 and LB 319. Strain GII3-3X remained in a separate branch. Sequence rearrangements were observed among S. citri strains, predominantly in the center of the chromosome. One to nine plasmids were identified in the seven S. citri strains analyzed in this study. Plasmids were most abundant in strains isolated from the beet leafhopper, followed by strains from carrot, Chinese cabbage, horseradish, and citrus, respectively. All these S. citri strains contained one plasmid with high similarity to plasmid pSci6 from S. citri strain GII3-3X which is known to confer insect transmissibility. Additionally, 17 to 25 prophage-like elements were identified in these genomes, which may promote rearrangements and contribute to repetitive regions.ConclusionsThe genome of seven S. citri strains were found to contain a single circularized chromosome, ranging from 1.58 Mbp to 1.74 Mbp and 1597-2232 protein-coding genes. These strains possessed a plasmid similar to pSci6 from the GII3-3X strain associated with leafhopper transmission. Prophage sequences found in the S. citri genomes may contribute to the extension of its host range. These findings increase our understanding of S. citri genetic diversity.

Potato psyllid (Bactericera cockerelli) is an important pest of potato, tomato and pepper. Not only could a toxin secreted by nymphs results in serious phytotoxemia in some host plants, but also over the past few years B. cockerelli was shown to transmit "Candidatus Liberibacter solanacearum", the putative bacterial pathogen of potato zebra chip (ZC) disease, to potato and tomato. ZC has caused devastating losses to potato production in the western U.S., Mexico, and elsewhere. New knowledge of the genetic diversity of the B. cockerelli is needed to develop improved strategies to manage pest populations. Mitochondrial genome (mitogenome) sequencing provides important knowledge about insect evolution and diversity in and among populations. This report provides the first complete B. cockerelli mitogenome sequence as determined by next generation sequencing technology (Illumina MiSeq). The circular B. cockerelli mitogenome had a size of 15,220 bp with 13 protein-coding gene (PCGs), 2 ribosomal RNA genes (rRNAs), 22 transfer RNA genes (tRNAs), and a non-coding region of 975 bp. The overall gene order of the B. cockerelli mitogenome is identical to three other published Psylloidea mitogenomes: one species from the Triozidae, Paratrioza sinica; and two species from the Psyllidae, Cacopsylla coccinea and Pachypsylla venusta. This suggests all of these species share a common ancestral mitogenome. However, sequence analyses revealed differences between and among the insect families, in particular a unique region that can be folded into three stem-loop secondary structures present only within the B. cockerelli mitogenome. A phylogenetic tree based on the 13 PCGs matched an existing taxonomy scheme that was based on morphological characteristics. The available complete mitogenome sequence makes it accessible to all genes for future population diversity evaluation of B. cockerelli.

Abstract Background Huanglongbing (HLB) is a highly destructive disease of citrus production worldwide. 'Candidatus Liberibacter asiaticus', an unculturable alpha proteobacterium, is a putative pathogen of HLB. Information about the biology and strain diversity of 'Ca. L. asiaticus' is currently limited, inhibiting the scope of HLB research and control. Results A genomic region (CLIBASIA_05640 to CLIBASIA_05650) of 'Ca. L. asiaticus' showing hyper-sequence variation or locus mosaicism was identified and investigated using 262 bacterial strains (188 from China and 74 from Florida). Based on the characteristic electrophoretic profiles of PCR amplicons generated by a specific primer set, eight electrophoretic types (E-types) were identified, six E-types (A, B, C, D, E, and F) in China and four E-types (A, C, G, and H) in Florida. The 'Ca. L. asiaticus' strains from China consisted predominately of E-type A (71.3%) and E-type B (19.7%). In contrast, the 'Ca. L. asiaticus' strains from Florida was predominated by E-type G (82.4%). Diversity of 'Ca. L. asiaticus' in China was also evidenced. Strains from the high altitude Yunnan Province consisted of five E-types with E-type B being the majority (62.8%), whereas strains from the low altitude coastal Guangdong Province consisted of only two E-types with E-type A as the majority (97.0%). Sequence analyses revealed that variation of DNA amplicons was due to insertion/deletion events at CLIBASIA_05650 and the downstream intergenic region. Conclusions This study demonstrated the genomic mosaicism of 'Ca. L. asiaticus' resulted from active DNA insertion/deletion activities. Analyses of strain variation depicted the significant inter- and intra-continent diversity of 'Ca. L. asiaticus'.

Clonal replicates of Prunus dulcis cv ‘Butte,’ P. webbii and their interspecific hybrid P 63-61 were inoculated with Xylella fastidiosa strain M23 and evaluated for almond leaf scorch disease and subsequent winter curing of infections during three growing seasons. Initial inoculations established greater than 90% infection in each of the accessions, based on PCR diagnoses from petiole tissues sampled near the inoculation site. Classic leaf scorch symptoms were evident in each population during the first growing season in a controlled greenhouse environment. Trees were removed from the greenhouse during the winters to accumulate chill hours and to provide the possibility of winter curing X. fastidiosa infections. Both PCR diagnostics and in vitro cultivation were used during the second and third growing seasons to determine the persistence of X. fastidiosa in clones among the three populations. Tree survival and the degree of winter cured infections differed among the three populations, with P. webbii and P 63-61 demonstrating enhanced levels of survivorship over ‘Butte.’ After two cycles of ambient winter temperatures and subsequent growth, ‘Butte’ averaged 21.2% winter cured trees with 73.1% mean survival. Tree survival and winter cured infections were nearly 100% for both P. webbii and P 63-61, demonstrating the utility of P. webbii in almond breeding efforts aimed at reducing tree vulnerability to X. fastidiosa infections.

A study was conducted to analyze extrachromosomal DNAs associated with the Italian clover phyllody phytoplasma strains collected in Cavazzo carnico and with the phylogenetically related Vaccinnium witches' broom and walnut witches' broom phytoplasmas. Results indicated that size variation was not associated with plant host specificity. Findings also showed the presence of highly conserved ORFs with substantially invariant putative translation products.

Clamped homogeneous electric field (CHEF) electrophoresis analysis of the genomic structures of Listeria monocytogenes reveals two individual genomic divisions associated with flagellar (H) antigen type and containing serovar strains. ApaI enzyme digestion of division I and II serovar isolates induces the formation of cluster IA, and clusters IIA and IIB, respectively. These results reveal a correlation between pheno- and genotypic characteristics of L. monocytogenes.