Dynamic structures in phytoplasma genomes: sequence-variable mosaics (SVMs) of clustered genes

Emergence of the phytoplasma clade from an Acholeplasma-like ancestor gave rise to an intriguing group of cell wall-less prokaryotes through a remarkable and continuing evolutionary process. In a ceaseless progression, phytoplasmas have evolved reduced genomes, lost biochemical pathways for synthesis of nutrients supplied by hosts, and gained capabilities for transkingdom parasitism and pathogenicity in plants and insects. While continued genome degradation has made phytoplasmas increasingly host dependent, their small, AT-rich genomes have evolved conspicuous flexibility that enables rapid responses to host signals, successful evasion of host surveillance, and adaptation to shifting environments encountered during obligate, transkingdom parasitism. Recent work revealed that multiple, sequence-variable mosaics (SVMs) of clustered genes and repetitive extragenic palindromes are characteristic features of genome architecture in phylogenetically diverse phytoplasma species. SVMs are apparently of ancient origin, while current forms result from dramatic and more recent events. The dynamic nature of SVMs could account for their composite structure and potential for rapid changes significant in phytoplasma-host interactions.