Self/non‐self recognition in sponges

A variety of procedures have been utilized to study the sponge ability to discriminate self‐ from non‐self. In adult sponges, the histocompatibility reaction has traditionally been tested by transplantation techniques or parabiosis. These consist of pushing into contact different species (xenograft) or conspecific individuals (allograft) with their outer pinacodermal layers intact. A fast reaction can be observed when fragments of the inner part of the sponge are tied together. Lack of compatibility generates signals suppressing or activating both exopinacocytes bounding the contact surface and discrete mesohyl cell types (archaeocytes, collencytes, gray and spherulous cells). These cells are involved to different extents according to the species (active migration along the contact surface, cytotoxicity, phagocytosis, synthesis of a collagen barrier). In sponge cell aggregates, species‐specific sorting‐out expresses the recognition reaction of the mixed cells. Glycoconju‐gates are key molecules for self/non‐self cell discrimination. In the model derived from Geodia cydonium cells, galectin molecules in the presence of Ca2+ form a bridge between the intercellular multiprotein complex aggregation factor (AF) and the cell surface‐associated aggregation receptor (AR). The AF is a soluble and diffusible macromolecule that may play an important role also in self recognition mechanisms occurring in sponges in toto. A new paradigm has recently been suggested for cell recognition: interactions between the most peripheral cell surface specific pro‐teoglycans (glyconectins). These molecules have been purified from three marine sponge species (Microciona prolifera, Hali‐chondria panicea, and Cliona celata). This mechanism of self recognition could represent a remarkable event for the appearance of multicellularity.