Genome Plasticity in Obligate Parasitic Phytoplasmas

Phytoplasmas are uncultivable, wall-less, insect-transmitted, and phytopathogenic prokaryotes belonging to the class Mollicutes. They are obligate, intracellular parasites with a very wide host range of plants and insects which have great economic impact. They possess the smallest genomes known among cellular organisms with a very low G + C content and irregular GC skew. Phytoplasma genomes vary significantly in size and DNA composition due to loss and acquisition of genomic DNA. Several vital genes and standard functional metabolic pathways are missing. Genome plasticity rendered by DNA perturbations may account for phytoplasma diversification, pathogenicity, and adaptation to diverse hosts and environmental milieus. Genome optimization in reductive evolution is characterized by instability and deletion of unnecessary genes. Several mobile genetic elements found in phytoplasma such as plasmids, insertion sequence-like elements, potential mobile units or sequence-variable mosaics, and phytoplasmal repeated extragenic palindromes play a vital role in adaptive evolution.