The genomes of four phytoplasmas have been sequenced: "onion yellows", "aster yellows witches' broom" ("Candidatus Phytoplasma asteris"), "Ca. Phytoplasma australiense", and "Ca. Phytoplasma mali". Phytoplasmas have very small genomes, with extremely low GC content (sometimes as little as 23%, which is thought to be the lower threshold for a viable genome).Dikinson, M. Molecular Plant Pathology (2003) BIOS Scientific Publishers In fact, the Bermuda grass white-leaf phytoplasma ("Ca. P. cynodontis") has a genome size of only 530 kb, one of the smallest known genomes of all living organisms. The larger phytoplasma genomes are around 1350 kb in size. The small genome size of phytoplasma is attributable to reductive evolution from Bacillus/Clostridium ancestors. Phytoplasmas have lost ≥75% of their original genes, and can thus no longer survive outside of insects or plant phloem. Some phytoplasmas contain extrachromosomal DNA such as plasmids.
