Clusters of diverse genes existing as multiple, sequence-variable mosaics in a phytoplasma genome.

Phytoplasmas are cell wall-less prokaryotes living as obligate parasites and pathogens of plants and insects, making them attractive subjects for studies to gain a greater understanding of transkingdom parasitism and pathogenicity. During a study of two phytoplasma genomes, we obtained evidence for previously unreported clustering of genes, pseudogenes, mobile genetic elements, intergenic repeat units, and repetitive extragenic palindromes that occur in multiple, homologous clusters in some phytoplasma genomes. The clusters represent previously unrecognized mosaics, possibly assembled through multiple events of targeted mobile element attack, duplication, recombination, and rearrangement. Multiple clusters could conceivably afford potential for genome reduction through homologous recombination. Differences in the sizes and multiplicity of such clusters possibly account for some of the previously reported but unexplained variations in genome size among closely related phytoplasma strains.