Drosophila testacea can be found in temperate regions of Europe, extending to East Asia.

[1] This hybrid inviability (see Haldane's rule)) may be due to selfish X chromosomes and co-evolved suppressors.

[2] Alternately, differences in sex pheromone (e.g. vaccenyl acetate) reception could underlie female readiness and male willingness to copulate.

[3] The antimicrobial peptide gene Diptericin B has been pseudogenized in D. testacea and likely its sister species, D.

[4] This was due to a lack of Acetobacter bacteria in its environment, relaxing Natural selection on Diptericin B, which is specifically evolved to combat Acetobacter.