Identification of an Early Male-Killing Agent in the Oriental Tea Tortrix, Homona magnanima

Abstract

Arthropods are frequently infected with inherited symbionts, which sometimes confer fitness benefits on female hosts or manipulate host reproduction. Early male killing, in which infected males die during embryogenesis, is induced by some bacteria, such as Wolbachia and Spiroplasma. A female-biased sex ratio has been found in Homona magnanima, collected from a tea plantation in Japan. Here, we examined the male-killing trait in H. magnanima and identified the agent that induces early male killing. The sex ratio distortion (SR) strain produced only females and no males, and its egg hatch rate was significantly lower than that of the normal (N) strain. The N strain was infected with only Wolbachia, whereas the SR strain was infected with both Wolbachia and Spiroplasma. Antibiotic treatment with 0.10% tetracycline restored the 1:1 sex ratio in the SR strain. Females treated with 0.05% tetracycline were positive for Spiroplasma but not for Wolbachia and showed a female-biased sex ratio, whereas Wolbachia-positive females did not revert to male killing. When inoculated with a homogenate of the SR strain female, females infected with only Spiroplasma produced female-biased offspring. Sequence analysis of the 16S rRNA gene revealed that Spiroplasma sp. of H. magnanima belonged to the ixodetis clade. These results indicate that Spiroplasma was responsible for male killing in H. magnanima. Late male killing is induced in H. magnanima by an RNA-like virus, and therefore this is the first case in which different male-killing agents expressed at different times in the life cycle have been found within one host species.

Keywords: Spiroplasma; Wolbachia; bacteria; sex ratio distortion; symbiosis.