Evolutionary divergence of the insect disease-encoding Serratia plasmid pADAP Supplementary Materials
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The larvae of the New Zealand grass-grub (Costelytra givenii), cause significant damage to New Zealand’s pastures. Two diseases of grass-grub larvae, instigated by strains of Serratia entomophila and S. proteamaculans, have been identified and are being used as commercial biocontrol agents. The main virulence determinants of these Serratia strains, an Anti-feeding-prophage and an ABC-toxin-complex, are encoded on a 153-kb conjugative plasmid pADAP.
Recently 75 Serratia strains with atypical disease phenology were sequenced with the goal of defining evolutionary points of divergent between plasmid variants and define potential co-evolution between plasmid and host.
Phylogenetic analysis of the conserved plasmid “backbone”, residing between a conserved integrase (Int2) and the end of a conjugative pili cluster (PilL), revealed clustering of all the S. entomophila plasmids. Within the predicted backbone region, several intergenic replication and conjugation regions contained DNA inserts, one of which, positioned between TraG and TraC, demarcates chronic disease related plasmids from the non-chronic plasmids. These inserted regions were used as a marker to track the evolutionary divergence between the plasmids variants.
A key finding was that all ABC-toxin-complex variants are co-located with a fimbria cluster, eluding to a role of fimbriae for the proper functionality of the ABC-toxin-complex.