Host castration by Aphidius ervi venom proteins

The braconid Aphidius ervi Haliday (Hymenoptera, Braconidae) is an endophag ous parasitoid of the pea aphid, Acyrthosiphon pisum (Harris) (Homoptera, A phididae). Parasitized host aphids show different degrees of castration, a response that is total when parasitoid oviposition takes place in first ins tar hosts. Deleterious effects on the host reproductive system are already evident by 24 h following parasitization, before egg hatching. The effect o f parasitoid venom on A. pisum ovaries has been studied by performing micro injections in non-parasitized host aphids and observing the cellular altera tions of the apical germaria of ovarioles. Venom injection reproduced the s ame alterations observed in parasitized aphids, while injections of saline solution did not induce any detectable change. By 24 h, the germarial cells of both venom-treated aphids and parasitized aphids showed the absence of the nucleolus and of electron-dense material around the nucleus, frequently referred to as "nuage material". By 48 h more evident signs of degeneratio n were observed, suggesting the possible occurrence of apoptosis. The bioac tive component of the venom was both heat- and protease-sensitive. The acti vity was found in purified fractions that were highly enriched in two prote ins with an approximate molecular mass of 21 kD and 36 kD, respectively. Th ese macromolecules are the most abundant components of A. ervi venom and, u nlike many venom proteins of studied parasitic Hymenoptera, are not glycosy lated and appear to be subunits of an oligomeric protein. The adaptive sign ificance of host castration is discussed. (C) 2000 Elsevier Science Ltd. Al l rights reserved.