Development of Rozella in Allomyces: a single zoospore produces numerous zoosporangia and resistant sporangia

Abstract

Single thalli of Allomyces arbuscula were inoculated with single zoospores of an intracellular parasite, Rozella allomycis. Infection caused segmentation of numerous host hyphae which usually comprised a sector emerging from one limb of the arbusculate thallus. Most segments contained Rozella zoosporangia (ZS), but some basal segments contained resistant sporangia (RS). The spread of Rozella appeared to be centrifugal–acropetal and to follow the host's branching. Ultrastructural observations indicated that Rozella grew inside its host as a mycelium which lacked cell walls and became segmented by host septa. In broth-grown hosts, secondary infection of previously uninfected hyphae by zoospores released from ZS produced mostly RS.Thus the "polysporangiate" growth of some Rozella species, in which numerous ZS occur in linearly conjoined series and which has been juxtaposed to "monosporangiate" growth of others, can arise from single zoospores. The discussion considers taxonomic changes in Rozella and emphasizes developmental similarities between R. allomycis and Allomyces, as well as environmental effects on formation of ZS versus RS. A report that R. allomycis is heterothallic and produces RS only through appropriate pairing of mating types has not been confirmed.