Calyx cells and polyDNAvirus (PDV) production in the ovary of Cotesia Flavipes (Hymenoptera, Braconidae)

A. A. Zacaro 1,3 , H. Conte 2 & C. Cruz-Landim 1

1 Departamento de Biologia, Instituto de Biociências, UNESP, Rio Claro, SP, Caixa Postal 199, Brasil; 2 Departamento de Biologia Celular e Genética, UEM, Maringá, PR, CEP: 87020-900, Brasil; 3

The parasitoid braconid Cotesia flavipes is a gregarious wasp used as a biocontrol agent against the sugarcane borer Diatraea saccharalis (Lepidoptera, Pyralidae). Some species of Braconidae and Ichneumonidae endoparasitic wasps harbor viruses belonging to the family Polydnaviridae, which are produced in specialized ovarian cells (Calyx Cells, CC). The virus particles are released inside the lumen of the oviducts forming the major part of the calyx fluid. During the oviposition, egg and calyx fluid are injected into the host haemocoel. The genomic virus information is apparently integrated in the wasp's genome as proviruses; it is a mutualistic relationship playing an important role during the endoparasitoid life history. Basically, polydnavirus (PDV) avoids egg encapsulation by the immune system of the host. Morphologically, each PDV particle produced in the calyx cells of C. flavipes is formed by several electron-dense nucleocapsids embedded in a less electron-dense matrix and enclosed by a single plasmic membrane. Female and adult C. flavipes specimens were dissected in fixative solution (0.1M cacodylate buffer, 2.5% glutaraldehyde, 3% sucrose) and the fixed ovaries were routinely processed for transmission electron microscopy (TEM). The ultrathin sections were observed in a Phillips CM100 TEM. We recognize in the CC three morphological stages of PDV production, which seems to be different to previous reports made in other braconids (i.e. Apanteles galleriae). The intranuclear formation of the PDV envelope in C. flavipes is initiated after nucleocapsid production since no empty envelopes were ever observed. In comparison to the 2nd and 3rd stage, the CC in the 1st stage are smaller, have cytoplasmic inclusions and irregular nuclear morphology with no signs of the production of viral envelopes or nucleocapsids. The 2nd stage, however, is characterized by hypertrophied CC showing great amount of free ribosomes in the cytoplasm and nuclei containing free nucleocapsids and PDV particles. The nuclei of the CC in the 2nd stage have a peripheral halo adjacent to the nuclear envelope which is formed by free electron-dense nucleocapsids embedded in a less electron-dense euchromatic region and a central region characterized by the presence of complete PDV particles. CC in the 3rd stage are characterized by the decreased amount of the cytoplasm and by the presence of many complete PDV particles inside the nucleus, though single nucleocapsids are associated with electron-dense viroplasm between and adjacent to nuclear envelope invaginations.

Index Terms: endoparasitoid; virus particles; viroplasm; nucleocapsid.

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