Reductive genome evolution, host-symbiont co-speciation and uterine transmission of endosymbiotic bacteria in bat flies

Bat flies of the family Nycteribiidae are known for their extreme morphological and physiological traits specialized for ectoparasitic blood-feeding lifestyle on bats, including lack of wings, reduced head and eyes, adenotrophic viviparity with a highly developed uterus and milk glands, as well as association with endosymbiotic bacteria. We investigated Japanese nycteribiid bat flies representing 4 genera, 8 species and 27 populations for their bacterial endosymbionts. From all the nycteribiid species examined, a distinct clade of gammaproteobacteria was consistently detected, which was allied to endosymbionts of other insects such as Riesia spp. of primate lice and Arsenophonus spp. of diverse insects. In adult insects, the endosymbiont was localized in specific bacteriocytes in the abdomen, suggesting an intimate host-symbiont association. In adult females, the endosymbiont was also found in the cavity of milk gland tubules, which suggests uterine vertical transmission of the endosymbiont to larvae through milk gland secretion. In adult females of Penicillidia jenynsii, we discovered a previously unknown type of symbiotic organ in the Nycteribiidae: a pair of large bacteriomes located inside the swellings on the fifth abdominal ventral plate. The endosymbiont genes consistently exhibited adenine/thymine biased nucleotide compositions and accelerated rates of molecular evolution. The endosymbiont genome was estimated to be highly reduced, ∼0.76 Mb in size. The endosymbiont phylogeny perfectly mirrored the host insect phylogeny, indicating strict vertical transmission and host-symbiont co-speciation in the evolutionary course of the Nycteribiidae. The designation Candidatus Aschnera chinzeii is proposed for the endosymbiont clade.