FEMALE-BIASED SEX-RATIO IN THE ASIAN CORN-BORER, OSTRINIA-FURNACALIS - EVIDENCE FOR THE OCCURRENCE OF FEMINIZING BACTERIA IN AN INSECT

The maternally inherited, female-biased sex ratio in many arthropods h as been attributed to infection with micro-organisms. Male killing, th elytoky and feminization are recognized as the mechanisms of the bacte ria-induced sex ratio distortion in arthropods. A sex ratio distortion towards the female has been found in the Japanese population of the A sian corn borer Ostrinia furnacalis (Lepidoptera). In the present stud y, inheritance of the sex ratio distortion in the Asian corn borer and its underlying mechanism were investigated. Two of the 13 field-colle cted females produced all-female offspring. The female-biased sex rati o was maternally inherited for three generations. There was no differe nce in egg hatchability between thelygenic and normal crosses. Treatme nt with tetracycline for two generations resulted in production of onl y male offspring. Considering the sex determination system in lepidopt eran insects, it was concluded that chromosomal males are feminized by a cytoplasmic agent(s), most probably parasitic bacteria. This is the first report of the occurrence of feminizing bacteria in insects. The sex ratio distorter was considered to occur at low frequency in the A sian corn borer population.