Rozella andRozellopsis: Naked endoparasitic fungi which dress-up as their hosts

Abstract

This monograph surveys the literature on all species of the zoosporic fungal generaRozella (Chytridiales, Olpidiaceae) andRozellopsis (Lagenidiales, Olpidiopsidaceae), which are obligate intracellular parasites of other zoosporic fungi. The special characteristic of these parasites is that their zoosporangium (ZS) fills completely a “compartment,” such as a hyphal segment or a protuberance, in the host thallus. However, the truly distinctive feature of the zoosporangia ofRozella andRozellopsis appears to be the absence of a cell wall of their own. Thus far, this feature has been demonstrated only inRozella allomycis (A. A. Held, Canad. J. Bot.58: 959–979, 1980). However, the fact that in all other species the compartment of the host thallus is occupied just by a single ZS and is separated from the rest of the thallus by one or more septa indicates strongly that the wall which surrounds the ZS belongs entirely to the host. It is argued that since zoospore release is explosive, it is necessary that internal pressure be maintained temporarily in the ZS, and a container made of the host’s cell wall can provide the needed mechanical support only to one naked parasite ZS. (If several naked ZS. filled the container, after the first one released its zoospores the remainder might be unable to do so.) The septation, which the parasite apparently evokes in the host, delimits a ZS which in most cases resembles that of the host, and this phenomenon is referred to as host mimicry. In fact, this septation may in turn bring about the reproductive phase of the parasite. So-called monosporangiate parasite species, which form solitary, nonconjoined ZS, occur in hosts whose own ZS are single (monocentric chytrids) or nonconjoined (e.g.,Pythium spp.), whereas polysporangiate species, which form linear sori of conjoined ZS, occur in hosts which tend to form similarly arranged ZS or gemmae on their own (Allomyces spp., Saprolegniaceae). The naked parasite apparently also stimulates the host wall to form a discharge apparatus-a papilla or tube through which the parasite zoospores are released.