THE HSP70 PROTEIN IS INVOLVED IN THE ACQUISITION OF GAMETE SELF-STERILITY IN THE ASCIDIAN CIONA-INTESTINALIS

In the hermaphrodite ascidian Ciona intestinalis, gamete self-incompat ibility is a mechanism that prevents self-fertilization and is based o n the ability of the oocyte vitelline coat to distinguish and accept o nly heterologous spermatozoa. The onset of self-sterility occurs durin g oogenesis and involves or is controlled by the follicle cells. Gamet e self-nonself discrimination, a process that can be likened to an imm une recognition event, represents a useful model with which to study t he evolution of self-nonself recognition, Hsp70 genes, which belong to the major histocompatibility complex (MHC) class III, are supposedly ancestors of the MHC class I and II genes, and chaperonins are known t o be involved in antigen processing and presentation, We have isolated and characterized an hsp70 gene (Cihsp70) that is constitutively expr essed during oogenesis in the follicle cells of previtellogenic and vi tellogenic oocytes, Using a polyclonal antibody against Cihsp70 protei n, we demonstrate that the expression of Cihsp70 is required for the s witch from self-fertility to self-sterility. The functional involvemen t of Cihsp70 in gamete self-nonself recognition provides evidence for an ancestral MHC-like system in protochordates.