The fine structure of secondary pit connection formation between the red algal alloparasiteHolmsella australis and its red algal hostGracilaria furcellata

Summary Pit connections (PCs) develop between the parasitic red algaHolmsella and its hostGracilaria. Only parasite cells initiate the formation of host-parasite pit connections. The parasite produces a small connecting cell (termed the conjunctor cell) which moves through the cell wall to fuse with either an adjacent host or parasite cell. The parasite secondary PC, which forms between the conjunctor cell and the parasite cell, is structurally different from a parasite primary PC, and has the distinct structure of a host-parasite PC. Only if the conjunctor cell fuses with another parasite cell will the former parasite-conjunctor cell PC be altered to a typical parasite-parasite PC. If the conjunctor cell fuses with an adjacent host cell the PC continues to develop as host-parasite. Occasionally a conjunctor cell fails to fuse with an adjacent cell (whether host or parasite), and the conjunctor cell and PC eventually breakdown in the cell wall. The parasite overcomes several barriers in order to infect the host, including the formation of host-parasite PCs which appear to be a necessary component of the parasiticHolmsella-Gracilaria association.