ABSTRACT
Previous studies have shown that the larva of Chelonus inanitus plays an essential role in inducing the precocious onset of metamorphosis in polydnavirus/venom-containing Spodoptera littoralis, and that this effect might be due to proteins released by the parasitoid larva into the host. Here we show that a 212kD protein is the predominant protein released in vitro by late first instar parasitoids. This protein does not accumulate anywhere in the parasitoid body. Polyclonal antibodies specific for the 212kD protein were produced and immunoblot analyses revealed that this protein begins to be released by parasitoid larvae when the precocious onset of metamorphosis is induced in the host. Amino acid sequences of the N-terminus and of internal fragments of the 212kD protein were determined and glycoprotein detection showed that this protein is glycosylated. The availability of specific antibodies and the amino acid sequence information should now allow us to clone and express the 212kD protein and to directly study its functional significance.
ABSTRACT
Having shown earlier that the larva of C. inanitus is essential in inducing the precocious onset of metamorphosis in polydnavirus/venom containing S. littoralis, we here analysed release of proteins by parasitoid larvae and their effects on host development. Parasitoid larvae released proteins in vivo and in vitro in a stage dependent manner. An approximately 212 kD protein was released from the mid 1st instar onwards and additional smaller proteins could be associated mainly with the 2nd and 3rd instar. When parasitoids were implanted into S. littoralis larvae, parasitoid-released proteins were seen 6 hr later. When parasitoids were removed from hosts, parasitoid-released proteins persisted in the host haemolymph for some time. Injection of antiserum against parasitoid-released proteins after removal of the parasitoid larva accelerated the disappearance of the 212 kD protein and reduced the number of larvae entering metamorphosis precociously. Repeated injections of concentrated parasitoid medium into polydnavirus/venom containing larvae caused a reduction of the head capsule width and formation of miniature 6th instar larvae; this effect was not seen in the absence of polydnavirus/venom. These observations suggest that proteins released by the parasitoid might play a role in modifying host metamorphosis in the presence of polydnavirus/venom, and the temporal appearance of the 212 kD protein makes it the most interesting candidate for being involved in such an effect.
