Spiroplasma poulsonii is a maternally transmitted symbiont, meaning it is primarily inherited through the female germ line.

In an interview with the Global Health Institute, Dr. Toshiyuki Harumoto said: "To our knowledge, Spaid is the first bacterial effector protein identified to date that affects host cellular machinery in a sex-specific manner...”[6] The S. poulsonii strain of Drosophila neotestacea can defend its host from attack by nematodes and parasitic wasps.

[7] This defence is important enough that S. poulsonii has spread westward across North America due to the selective pressure imposed by the sterilizing nematode parasite Howardula aoronymphium.

[8] The mechanism through which S. poulsonii protects flies from nematodes and parasitic wasps relies on the presence of toxins called ribosome-inactivating proteins (RIPs), similar to sarcin or ricin.

Spiroplasma poulsonii likely avoids damaging its host fly by carrying parasite-specific complements of RIP toxins encoded on bacterial plasmids.