Initially discovered in D. neotestacea, mushroom-feeding flies are commonly infected with the trypanosomatid parasite Jaenimonas drosophilae.

[7] The major innate immunity pathways of Drosophila are found in D. neotestacea, however the antimicrobial peptide Diptericin B has been lost.

These evolutionary patterns in mushroom-breeding Drosophila and other fruit flies suggests that the immune system's effectors (like antimicrobial peptides) are directly shaped by host ecology.

The S. poulsonii strain of D. neotestacea has spread westward across North America due to the selective pressure imposed by the sterilizing nematode parasite Howardula aoronymphium.

[4] While S. poulsonii can be found in other Drosophila species, the D. neotestacea strain is unique in defending its host against nematode infestation.

[10] 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.

[11][12] These toxins cut a conserved structure in ribosomal RNA, ultimately changing the nucleotide sequence at a specific site.

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