Initial recognition of endosymbiotic diatom surface antigens by the largerforaminifer Amphistegina lobifera

Evidence suggests that all foraminiferal endosymbiotic diatoms share simila r surface molecules that act as molecular signals for their recognition and retention by their hosts. A 104 kDa polypeptide (CSSA, common symbiont sur face antigen) was found in the frustules of 11 symbiotic diatom species, bu t not found in the frustules of 5 species of diatoms that were;not symbiont s. Another polypeptide (66 kDa) was common to both symbiotic and non-symbio tic diatoms (CDSA, common diatom surface antigen). Antisera against these a ntigens were used in blocking experiments to see if these surface antigens were recognized by the host digestive processes. Symbiotic diatoms were dig ested by Amphistegina lobifera (a diatom-bearing host) when the algae first were incubated with antiserum against the CSSA (anti-CSSA) or polyclonal a ntiserum against the frustules of Fragilaria shiloi (anti-shiloi) (a common endosymbiotic diatom) before they were used as food. Treatment with antise rum against the other polypeptide (anti-66) did not affect ingestion or dig estion rates. This suggests that the 104 kDa polypeptide is a molecule rela ted to the recognition between the symbionts and their host, and the 66 kDa polypeptide is not. With the aid of indirect immunofluorescence techniques the 104 kDa polypeptide was localized on the surface of all the endosymbio tic diatoms we tested. The polyclonal antiserum against Fragillaria shiloi increased both ingestion and digestion of symbiotic diatoms more than the a ntibody against the CSSA alone. This suggests that there may be more surfac e antigens in the recognition system than just the 104 kDa polypeptide. Bot h antibody treatments reduced the uptake of symbiotic diatoms by the host w hich normally ate more symbiotic species than non-symbiotic ones. The resul ts of the experiments suggest that recognition begins in the granulo-reticu lopodial web where first contact is made between the foraminiferan and diat oms in the habitat.