No abstract

We surveyed female-biased sex ratio traits (SR traits) in a moth, Ostrinia scapulalis (Lepidoptera: Crambidae) from six geographic locations in Japan. A small proportion (8.3%) of females showed the SR trait. About half of them were associated with a Wolbachia infection (SRw+), whereas the remaining ones were not (SRw-). No significant differences in frequencies were detected among locations and years for either SRw+ or SRw- females. In SRw+ matrilines, the SR trait was inherited. Curing the Wolbachia infection by tetracycline at the larval stages resulted in all-male broods in the next generation. These characteristics were identical to the Wolbachia-induced SR trait observed previously in the related species Ostrinia furnacalis. The Wolbachia strain in O. scapulalis was indistinguishable from that in O. furnacalis in terms of the wsp and ftsZ gene sequences, suggesting either that infection originated from a common ancestor of the two hosts or that Wolbachia was transmitted horizontally between the host species. In SRw- matrilines, on the other hand, the SR trait was not due to bacterial infection but some other maternally inherited element, since antibiotic treatment had no effect and bacterial PCR did not give the characteristic product. In these matrilines, the SR trait occasionally failed to show, suggesting the presence of nuclear factor(s) suppressing the SR trait.

Sex ratio distortion toward females (SR trait), induced by a Wolbachia bacterium, has been reported in two species of the Ostrinia furnacalis group, viz., O. furnacalis , and O. scapulalis (Lepidoptera: Crambidae). In addition, an SR trait caused by abacterial, unidentified agent(s) is known in O. scapulalis . Here we examined the SR trait in four other species of the furnacalis group, viz., O. orientalis , O. zaguliaevi, and O. zealis from Japan, and O. nubilalis from central and eastern Europe. Wolbachia infection was detected in three O. orientalis females and in one O. zaguliaevi female, and the infection was always associated with the SR trait. In terms of wsp and ftsZ gene sequences, Wolbachia strains in O. orientalis and O. zaguliaevi were indistinguishable from each other, or from those in O. furnacalis and O. scapulalis . These findings suggest that Wolbachia strains in the four Ostrinia species are probably identical. In addition, one O. zealis female, which was negative in diagnostic PCRs for Wolbachia and general bacteria, produced an all-female brood. This trait was very similar to the SR trait caused by abacterial agent(s) in O. scapulalis .