Aphidius ervi Haliday (Hymenoptera, Braconidae) is an endophagous parasitoid of several macrosiphine aphid species. Host physiology and metabolism are considerably redirected by the parasitoid in order to support growth and development of its pre-imaginal stages. Parasitized pea aphids, Acyrthosiphon pisum (Harris) (Hemiptera, Aphididae), are castrated by the parasitoid venom, injected at the oviposition along with the egg. The two most abundant proteins of A. ervi venom (21 kD and 36 kD), when injected into nonparasitized hosts, target germarial cells of the ovarioles and induce cellular alterations similar to those observed in naturally parasitized hosts. Teratocytes, cells deriving from the embryonic membrane of A. ervi, show an intense histolytic activity, especially against formed host embryos, further contributing to the castration process. These cells of embryonic origin have an impressive metabolic capacity, as suggested by the massive exchange of material with the host environment and by the synthesis of two abundant parasitism specific proteins (15kD and 45 kD), which appear to be of nutritional importance for the developing A. ervi larva. This biosynthetic role of teratocytes is significantly supported by a parasitoid-mediated increase of the amino acid supply by aphid bacterial endosymbionts. Bateriocytes (host cells harbouring bacterial symbionts of the genus Buchnera) of parasitized aphids strongly interact with teratocytes, and, unlike other host tissues, do not show any degeneration. Experimental evidence indicates that amino acid biosynthesis of endosymbionts is considerably enhanced, resulting in a much higher production of essential aromatic amino acids. A physiological model integrating the host regulation factors, both of maternal and embryonic origin, and describing the nutritional exploitation strategy of the host is proposed.

Index terms: pea aphid; Braconidae; Buchnera; teratocytes; venom; castration

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