A DISCUSSION OF SOME SPECIES OF OLPIDIOPSIS AND PSEUDOLPIDIUM

WHEN BOILED grass leaves are used as bait for the saprophytic chytrids in soil and water collections, Pythium is often found growing on the grass leaves. This method of collecting has proved effective in obtaining a number of species of Pythium and parasites of Pythium, which frequently are species of Pseudolpidium or Olpidiopsis. Pseudolpidium gracile Butler, the resting bodies of which lack companion cells, has been recollected and studied. In addition, two new species of Olpidiopsis, the resting bodies of which possess companion cells, have been found parasitizing Pythium. In a number of collections, however, the grass leaf bait yielded a species of Aphanomyces which was parasitized by what appeared to be Pseudolpidium Aphanomycis, in that all the resting bodies were without companion cells. These four species of Pseudolpidium and Olpidiopsis will be discussed in this paper. Recently taxonomic studies of the genus Olpidiopsis have been published by Shanor (1939, 1940) and McLarty (1939, 1941). Shanor (1939) suggests that the genus Pseudolpidium should be combine'd with Olpidiopsis for "none of the species originally placed in the genus when it was established can remain unquestionably as Pseudolpidium species. The continuance of the genus seems to rest on onlv two species, P. Pythii and P. gracile, both described by Butler. Even these two species need further critical study." McLarty (1941) merges the genus Pseudolpidium with the genus Olpidiopsis and amends Olpidiopsis "to include species with smooth and spiny zoosporangia which may form either sexual or asexual spores." Of Pseudolpidium Pythii and P. gracile McLarty states, "Very little is known of these species, although each has been observed since Butler's time. The resting spores of these species, as they are illustrated in the literature, appear very similar in appearance to spiny zoosporangia. Until they are re-collected and critically examined, these species may be transferred to Olpidiopsis as strictly asexual species." No opinion is expressed here as to the validity of the genus Pseudolpidium. The combining of Pseudolpidium and Olpidiopsis by McLarty (1941) on the basis of his having found both sexual and asexual spores in monospore cultures of his species, Olpidiopsis Achlyae, is justifiable. Nevertheless, the studies presented in this paper will emphasize the fact that in certain forms, formerly considered as species of Pseudolpidium and now to be placed in Olpidiopsis, according to McLarty, the lack of a companion cell on the resting spore is a constant and consistent characteristic of the species. In this connection two species, Pseudolpidium gracile Butler and P. Aphanomycis (Cornu) Fischer are now to be discussed. I Received for.publication May 1P, 1942. This work was supported by a grant from the Penrose Fund of the American Philosophical Society. Pseudolpidium gracile Butler.-A soil collection made by Dr. H. R. Totten at Key Largo, Florida, on December 27, 1940, yielded Pythium rostratum (fig. 15-20), parasitized by a species of Pseudolpidium (fig. 7 and 8). Because of the length of the spines and the yellowish color of the resting bodies, the parasite was identified as Pseudolpidium gracile Butler, though Butler (1907) lists Pythium rostratum as the host of Pseudolpidium Pythii. Pythium intermedium is given as the host of both P. Pythii and P. gracile, so it is possible that Pythium rostratum is likewise susceptible to both species of Pseudolpidium. Pseudolpidium gracile has been kept under observation in culture on its host since the date of its collection. Resting bodies are constantly produced in large numbers, but not a single resting body with a companion cell has been seen. The resting body is yellowish in color and covered with slender spines which attain a maximal length of 4 u (fig. 3). In shape the resting body varies from spherical to oval and measures 12 to 27 u in diameter, exclusive of the spines. Butler states that the wall of the resting body of his species is rather thin and in his figures of the resting body the wall does appear to be very thin. It is' for this reason that Shanor and McLarty have suggested that what Butler figures as resting bodies may be only spiny-walled zoosporangia. It is possible that Butler drew the resting bodies before the wall had attained its mature thickness for the contents of the resting body as illustrated by Butler appear to be undifferentiated. A mature resting body from the cultures of our P. gracile contains a single central oil globule surrounded by a peripheral layer of vacuolated protoplasm (fig. 3). Butler does not give any measurements of the thickness of the exospore and endospore walls of the resting bodies of P. gracile but measurements made on the resting bodies in our cultures show that the endospore may attain a thickness of 0.7 to 1.2 u while the exospore varies from 1.7 to 2.5 u in thickness. The development of the zoospotangia and the discharge of the spores do not differ from the descriptions of Butler. A period of vacuolization, during which one or two exit papillae appear on the sporangium, is terminated by the sudden disappearance of the vacuoles. Very soon after the disappearance stage the spores are cut out and discharged. The zoospores are biflagellate. The walls of some of the zoosporangia in older cultures are often covered with very short, fine spines (fig. 9). Pseudolpidium Aphanomycis (Cornu) Fischer.A culture of Aphanomyces cladogamous Dreschler was obtained from soil collected by Philip Couch near Petahatche, Mississippi. The A. cladogamous was parasitized by a species of Olpidiopsis which formed resting bodies that lacked companion cells (fig. 4 and 24). The identification of this species constitutes a difficult problem for two species of