Avoidance of encapsulation in the absence of VLP by a braconid parasitoid of Drosophila larvae: an ultrastructural study

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Abstract
The hymenopteran Asobara tabida Nees (Braconidae: Alysiinae) parasitizes the larval stages of Drosophila species. In D. melanogaster, 90% of A. tabida eggs avoid encapsulation and are found embedded within the tissues (digestive tube, fat body, tracheae, etc.) of their larval host. Electron microscopy was used to investigate if the embedment of the parasitic egg within the tissues of its host may prevent encapsulation. Study revealed that the outer layer of the A. tabida egg consists of a fibrous exochorion that exhibits adhesive properties within the tissues of D. melanogaster. Three hours after oviposition, any basement membrane surrounding the host tissues that is in contact with the parasitic egg is stuck to the exochorion. Attachment rapidly increases. Nine hours post oviposition, the A. tabida egg is almost totally surrounded by a sheath of host tissue, with only limited areas of the surface remaining uncovered and exposed to attack by the host hemocytes. It is therefore concluded that the adhesion of A. tabida eggs to the host tissues is likely to contribute to their protection from encapsulation during embryogenesis. Also, electron microscopy reveals that no virus-like particles occur in A. tabida, unlike any other braconid parasitoid studied. The absence of virus-like particles is discussed with regard to the importance of host regulation of the particles in other braconid species.

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