Parasitism of Dispira Cornuta

Abstract

Dispira cornuta van Tiegh., the fungus used in this study, is one of the less known, so-called “conidial” species of the Mucorales. It was found initially by the author associated with an unidentified species of Mucor growing on hog dung from Waltham, Mass. The fungus previously had been reported only twice from France, once from England, and once from North America, and hence had been considered a rare species. The writer has collected it six times since learning the requirements of the fungus. Three species, Dispira cornuta, D. americana and D. circinata, have been reported but an intensive study of the writer's fungus on its natural hosts and in pure culture for over two years has shown the range of variation in structural features to include the various characteristics emphasized as distinctive in the previous descriptions and illustrations. It seems justifiable to assume, therefore, that the three species are in reality reducible to one. D. cornuta was found to be parasitic on members of the Mucorales only although a long list of fungi representing not only other orders of the Phycomycetes, but also Myxomycetes, Ascomycetes, Basidiomycetes, and Fungi Imperfecti were tested as possible hosts. Within the Mucorales, the fungus was found to parasitize representatives of all genera tested although it was not equally parasitic on the different species used. The fungus is equally parasitic on the male (minus) and the female (plus) strains of such heterothallic fungi as Rhizopus nigricans, Cunninghamella echinulata, Blakeslea trispora, and Absidia glauca. In comparison with other parasitic members of the Mucorales, D. cornuta has a more extensive range of hosts than either Parasitella or Chaetocladium and one which is fully equal to that of the several species of Piptocephalis. Neither D. cornuta nor Piptocephalis were parasitic on Coemansia reversa, a little-known conidial fungus which by some has been considered to be a member of the Mucorales because of its aberrant methods of growth and of conidial formation, and by others a member of the Ascomycetes. In the case of the genus Mortierella, it was found that D. cornuta readily attacked Mortierella I (an undetermined species of Mortierella seemingly new); was only weakly parasitic on Mortierella III, and apparently utterly unable to parasitize four other species of this genus. On species of Mucor and on Mortierella I known to be susceptible to it, Dispira was never able to develop when the hosts were grown on agar media containing dextrose, but grew luxuriantly when the hosts were grown on proteose-peptone agar, an instance of the alteration of the susceptibility and resistance of a host through its nutrition, not as yet reported among the fungi. The morphological details of the relation of the parasite and the host were studied and it was found that in the presence of suitable hosts the spores of D. cornuta swelled, produced one or more germ tubes which elongated, became septate, and branched, ultimately reaching the hyphae of the host. If the hyphae of the host were young, thin-walled, and growing actively the parasite penetrated, establishing haustoria within. These developed, only in the vegetative, creeping hyphae of the host, never forming in sporangia, sporangiophores or in the aerial hyphae. When the vegetative hyphae of such hosts as Sporodinia grandis became mature, the hyphae of the parasite were no longer able to penetrate and establish haustoria within them. The general significance of some of the aspects of the parasitism of D. cornuta is considered in relation to other pertinent cases of parasitism reported in the literature, and its importance is discussed.