WHEN THE VENERID CLAM TAPES DECUSSATUS IS PARASITIZED BY THE PROTOZOAN PERKINSUS SP IT SYNTHESIZES A DEFENSIVE POLYPEPTIDE THAT IS CLOSELY-RELATED TO P225

Molluscs, like other invertebrates, have primitive defense systems. Th ese are based on chemotaxis, recognition and facultative phagocytosis of foreign elements. Previously, we have described one of these system s: a cellular reaction involving infiltrated granulocytes against Perk insus sp, parasitizing the Manila clam Tapes semidecussatus, in which the parasites are encapsulated by a defensive host product, the polype ptide p225. The aim of this study is to determine the similarities bet ween the defense mechanisms of 2 venerid clams, T, semidecussatus and T, decussatus, when they are infected by Perkinsus sp. The hemocytes o f both species infiltrate the connective tissue, redifferentiate, and ultimately express and secrete the polypeptide which constitutes the m ain product of the capsule that surrounds the parasites. The main secr etion product of T, decussatus shows a high degree of homology to that of T. semidecussatus, since it has a similar electrophoretic mobility and the polypeptide is recognized by the polyclonal serum against p22 5 from T. semidecussatus, as confirmed by Western blotting and immunoc ytochemistry. In conclusion, we demonstrate the existence of 2 polypep tides that are closely related at the molecular and functional level, and are specific in the defense of some molluscs against infection by these protozoan parasites.