The eggshell of the almond wasp Eurytoma amygdali (Hymenoptera, Eurytomidae) - 1. Morphogenesis and fine structure of the eggshell layers

Abstract

The almond wasp Eurytoma amvgdali (Hymenoptera: Eurytomidac) feeds and oviposits exclusively in almonds and therefore is characterized as an insect of economic importance. Its meroistic polytrophic ovaries include follicles with a tri-partitc configuration. The mature follicles exhibit two filaments occupying the two poles of the egg. One is the micropylar filament while the other might serve for respiration since it is likely that its flattened end layers remain outside the almond fruit. The eggshell is formed by aposition and the follicle cells, which surround the follicle until the end of oogenesis, may be responsible for protein synthesis and secretion which finally lead to the assembly of the eggshell. The eggshell comprises the thin vitelline membrane, possibly a ‘wax’ layer of waterproofing function, a transluscent layer which appears amorphous even at the end of choriogenesis, a granular layer, including large and small electron-dense granules, and finally a columnar layer very similar to layers found in other insect species of the same or different orders. Peroxidase is histochemicalLY found for the first time in an eggshell of the Hymenoptera order: the tranluscent layer in particular is positively stained (electron-dense). Two possible roles of this peroxidatic activity are discussed, first, in comparison to other fruit-infesting insects, we assume that elastic chorion is produced through the function of peroxidase induced bonds (resilin-type bonds), very important for avoiding premature breaking, while being oviposited through a narrow ovipositor. Second, referring to other studies, this layer can play a bactericidal role for additional embryonic itprotection.