Understanding the Causes and Genetic Effects of Thelytoky in the Aphelinidae: A Key to Improving Biological Control

Helytoky is a type of parthenogenesis whereby females produce only female offspring without the involvement of males, even where males are occasionally produced. In the last few years, strong circumstantial evidence has implied that thelytoky can be caused by micro-organisms called Wolbachia in at least some species of wasps. The thelytoky can be "cured" by treatment with antibiotics. Further Wolbachia-like organisms can be found in microscopic examinations and genetically identified through their DNA. The aphelinid wasps, and especially species in the genus Aphytis, are among the most important of all classical biological control agents. Aphytis species are critical in the biological control of scale insect pests in commercial orchards and ornamental plantings. About 30% of Aphytis species are thelytikous, of which we were able to study three in detail. In all three, thelytoky was curable by treatment with antibiotics and Wolbachia were identified morphologically and through their DNA. In contrast, Wolbachia were not detectable in biparental species of Aphytis. Studies of Wolbachia gene sequences obtained from Aphytis showed that they were most closely related to those from a very distantly related wasp, Muscidifurax uniraptor, strongly implying that the Wolbachia can be horizontally transferred. As revealed by electron microscopy, the Wolbachia show a strong association with the nurse and follicle cells of the female wasps.