THE DEVELOPMENT AND CYTOLOGY OF THE EPIBIOTIC PHASE OF PHYSODERMA PULPOSUM

LINGAPPA, YAMUNA. (U. Michigan, Ann Arbor.) The development and cytology of the epibiotic phase of Physoderma pulposum. Amer. Jour. Bot. 46(3): 145-150. Illus. 1959.-Physoderma pulposum, a chytrid parasite on Chenopodium album L. and Atriplex patula L., has a zoosporangial epibiotic phase. The latter consists of extramatrical sporangia and intramatrical bushy rhizoids, both enclosed in large protruding galls. The sporangia are subspherical, up to 350,u in diameter, and may produce hundreds of planospores. If planospores settle on the host surface, they develop narrow germ tubes which penetrate the epidermal cells and develop into rhizoids. The planospore body, however, remains on the host surface and develops into a mature epibiotic sporangium in -about 20-25 days at 16?C., 12-15 days at 20-25?C., or 6-8 days at 30?C. During development, its nucleus and daughter nuclei divide mitotically with intranuclear spindles until the sporangium contains several hundred nuclei. This is followed by progressive cleavage which delimits the planospore rudiments. When mature sporangia are placed in fresh water, the planospores are quickly formed within 1 hr. at 25?C. and begin to swarm within the sporangia. They escape in large numbers through an opening formed by the deliquescence of a papillum in the sporangial wall. The planospores are subspherical or elongate, 3-5 x 4-6,u, and each has an eccentric orange-yellow refractive globule and a flagellum 18-22, in length. The electron micrographs of the flagella indicate that the flagella are absorbed from tip backward during encystment of the planospores. By periodic inoculation of the host plants with planospores from epibiotic sporangia, as well as from germinating resting sporangia, generation after generation of epibiotic sporangia have been obtained for 4 years. This proves the existence of a eucarpic, epibiotic, ephemeral zoosporangial phase in P. pulposum. Field observations on the duration and sequence of development of the fungus indicate that the endobiotic resting sporangial phase always follows the epibiotic phase. The results of infection experiments also indicate that the epiand endobiotic phases belong to one and the same fungus, P. pulposum. THE OCCURRENCE of epibiotic and endobiotic phases of Physoderma pulposum Wallroth (1833), their relationship, the presence of sexuality, and host reaction to infection were briefly reported by the author (1958). As emphasized in the paper just cited, Schroeter (1889) created the genus Urophlyctis on the basis of occurrence of epibiotic sporangia and as a result of a mistaken concept of zygomycetous sexuality in P. pulposum. However, the desirability of maintaining Urophlyctis as a separate genus from Physoderma was seriously questioned by several mycologists (cf. Karling, 1950). After careful study of the literature, Karling (1950) concluded that all known distinctive attributes of Urophlyctis were of doubtful diagnostic value as generic characters and he merged it in 'Received for publication Nov. 21, 1958. 2 Part of a Ph.D. thesis submitted to the Graduate School, Purdue University, Lafayette, Indiana. The author is grateful to her major professor, Dr. J. S. Karling, for valuable guidance and encouragement during the present investigation. She is also thankful to Drs. G. B. Cummins and C. L. Porter for encouragement. Thanks are due to Messrs. D. J. Mason and J. W. Greenawalt for help in taking electron micrographs. This study was supported by a Cancer Research Fellowship of the Indiana Elk's Association. the older and valid genus Physoderma. Nevertheless, as pointed out by Karling (1950), the possibility exists that more valid distinctions may be found when all species have been fully studied. With this in mind, a comprehensive study of the life-history, development and sexuality of P. pulposum was undertaken, especially because it had been the type species of Urophlyctis. The present paper concerns only the development of the epibiotic phase, but the author proposes to present other aspects of the study in subsequent papers. However, in the interest of clarity, fig. 1 and its legend have been prepared in summary of the complete life cycle of P. pulposum as at present understood. The synonymy of various reproductive bodies is indicated by parentheses in the legend. Motile cells of the fungus are referred to as planospores, those that show sexual tendencies (irrespective of whether or not they fuse to form zygotes) as gametes, and asexual ones as zoospores. Inasmuch as stray and unfused gametes are not distinguishable from the zoospores when both are sedentary, they are referred to as planospores which is a noncommital term. The following account relates to the development of planospores into epibiotic sporangia. [The Journal for February (46: 65-144) was issued February 25, 1959] AMERICAN JOURNAL OF BOTANY, Vol. 46, No. 3, March, 1959.