Pelagibacter Subgroups Ib, II, IIIa, IIIb, IV and V Proto-mitochondria Neorickettsia Wolbachia Anaplasma Ehrlichia Midichloria Orientia Rickettsia The phylogenetic relationship between these two groups has yet to reach consensus in the scientific literature.

[7][8] However, later studies suggested that this relationship is false and was due to a phylogenetic artefact, which artificially groups independent AT-rich and fast-evolving lineages (Rickettsiales and Pelagibacterales have both properties) together.

In their classification, the relation between the two orders is retained in the subclass, the Rickettsidae, which include the Rickettsiales, the Pelagibacteriales, and the extinct protomitochondrion (mitochondria themselves are not bacteria, but organelles).

[11] Reduction in genome size, % GC and coding density and genes are generally attributed to genetic drift and Muller's ratchet.

Similarly, Muller's ratchet is activated through the lack of recombination and horizontal gene transfer (the eukaryotic host cell is a natural barrier).