THE ROLE OF SECONDARY PIT CONNECTIONS IN RED ALGAL PARASITISM1

Abstract

ABSTRACTSecondary pit connections are common between cells of hosts and parasites in the widespread phenomenon of red algal parasitism. The DNA‐specific fluorochrome 4′,‐6‐diamidino‐2‐phenylindole (DAPI) reveals that in host‐parasite secondary pit connection (SPC) formation between the parasitic red algaChoreocolax polysiphoniaeand its hostPolysiphonia confusa,a nucleus and other cytoplasmic components of the parasite are delivered into the cytoplasm of a host cell. Host cells receive large numbers of parasite nuclei and these, apparently arrested in G1, are maintained intact in host cells for periods of several weeks. Within these enlarged, differentiated cells, starch accumulates and cytoplasmic organelles proliferate as the central vacuole decreases in size. Host nuclear DNA synthesis is stimulated in the infected host cell, resulting in an increase in the number of host nuclei, or an increase in DNA in each of the existing host nuclei (i.e. somatic polyploidy). Occasionally, infected host cells will recommence division and engender a new host branch. Microspectrofluorometry of nuclear DNA quantitatively confirms not only the identity and transfer of parasite nuclei to host cells, but also the transfer of parasite nuclei to other parasite cells. Measurements also reveal that the single nucleus ofChoreocolaxbecomes progressively more polyploid as cells become larger and more highly differentiated. Secondary pit connection formation betweenChoreocolaxandPolysiphoniaprovides the mechanism for the transfer of parasite genetic information (via the parasite nucleus and cytoplasm) into the host. The parasite nuclei may thereby control and redirect the physiology of the host for the benefit of the parasite.