Serosal cells of Biosteres longicaudatus (Hymenoptera: Braconidae): Ultrastructure and release of polypeptides

Abstract

AbstractAs a prelude to a study in vitro of the function of the embryonic serosa of the parasitic wasp Biosteres longicaudatus (Braconidae), the ultrastructure of serosas of different ages reared in vivo and in vitro were compared. The evidence suggests that the serosal capsule consists of one to three cell layers. The innermost (internal cells) which line the serosal capsule and the outermost (external) cells which are bathed by the host's hemolymph are secretory. Large, coated vesicles in the internal cells increase in number and size with age and, likely, take up and transport molecules into the serosa. Multivesicular bodies, Known for their enzyme‐degradative function, occur in external cells and are eventually extruded into the surrounding environment. Distinctive electrondense, rod‐shaped particles appear in external cells within 8 h after larvae hatch, increase in number with larval age, and occur at the bases of microvilli. The latter appear electron dense with age and eventually they and the lobulated microvilli in internal cells fragment into the surrounding environment. To determine whether parasites and/or their serosas release substances into the host Anastrepha suspensa (Diptera: Tephritidae), hemolymph from unparasitized and superparasitized (> 1 parasite/host) pharate pupae was analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS‐PAGE). Serosas and/or parasite larvae were incubated in artificial media and each of these was subjected to SDS‐PAGE. A polypeptide, approximately 24 kilodaltons (Kd) occurred in the hemolymph of 24‐h‐old superparasitized pharate pupal hosts but not in the control. A similar polypeptide was observed in medium cultured with parasite larva and serosa as well as serosa alone, but was not in their respective control media. This approximately 24‐Kd band in SDS‐PAGE gels corresponds to a band in the serosa homogenate and may be identical to it. Serosas and parasite larvae in vivo and in vitro have similar protein profiles. Based upon these ultrastructural and electrophoretic studies, it appears that the serosa of B. longicaudatus has a synthetic function, as has been reported for the extra‐embryonic membrane of other parasitic Hymenoptera. It may sequester and degrade molecules from the host hemolymph and likely release newly synthesized as well as degraded products into the host.