Horizontal transfer of bacteriophage WO. (a) Phage WO can transfer between two different Wolbachia strains that coinfect the same host cell [22,23,25,31]. The phage becomes lytic (i) and lyses its Wolbachia host cell (ii). An active phage particle then attaches to a phage-free Wolbachia that coinfects the same host cell (iii) and injects its DNA (iv), at which point the DNA integrates into the chromosome (v). (b) Phage WO might also, hypothetically, be transmitted paternally by sperm from an infected male to the egg of a female carrying a phage-free Wolbachia. Once the sperm fertilizes the egg (i) the transported phage is released (ii) and can infect Wolbachia as in steps (iii-v) above.