ULTRASTRUCTURE OF THE METABASIDIUM OF AURICULARIA FUSCOSUCCINEA

Maturation of the metabasidium of Auricularia fuscosuccinea was followed with light microscopy and transmission electron microscopy. The basidium was divided into four compartments by septa which developed centripetally as in hyphae. Each septum formed a septal pore apparatus with imperforate pore caps. A band of electron-dense material was situated in the middle of the septal pore. There was a large increase in the volume of cytoplasm, excluding vacuoles, in each compartment during sterigmal outgrowth. Compartments were evacuated in basipetal sequence and vacuole enlargement began at the base of a compartment only when sterigmal formation was well advanced. The septal pore apparatus was intact until late in maturation of a compartment when septal swellings occluded the pore. The metabasidial wall was differentiated from those of other hymenial and subhymenial cells. The pattern of basidial maturation is compared with that in other phragmobasidiate and holobasidiate fungi. Use of the septal pore apparatus for phylogenetic and taxonomic purposes is discussed, as is the concept of primary and adventitious septa. NUMEROUS THEORIES of evolution within the Basidiomycota have been proposed, some of the more recent being those of Savile (1955, 1968), Olive (1957), Bessey (1961), Gaumann (1964), Shaffer (1975) and Oberwinkler (1977b), bu.t many basic questions remain unresolved; e.g., is the phragmobasidium or the holobasidium more primitive? How have the transitions between the various basidial types been achieved? Basidiomycetes are numerically poorly represented in the fossil record and both rusts and polypores are present in the oldest finds (Tiffney and Barghoorn, 1974). However, the numerous extant genera intermediate between the major orders provide hope that the evolutionary relationships eventually may be revealed (Rogers, 1971; Donk, 1972; Oberwinkler, 1977a). Ultrastructural data may help solve this evolutionary puzzle (Oberwinkler, 1978; Patton and Marchant, 1978a; Tu and Kimbrough, 1978). The phragmobasidium has been little studied at the ultrastructural level. The Uredinales, one of several orders with an auricularioid basidium, possesses a number of features, such as the structure of the septal pore (Littlefield and Bracker, 1971) and the spindle pole body I Received for publication 29 October 1979; revision accepted 31 March 1980. I thank Dr. Bernard Lowy for the species identification, Margaret Birse for technical assistance, Audrey Engels for preparation of photographs, Dr. Edson C. Setliff and an anonymous reviewer for helpful suggestions, and Dr. Esther G. McLaughlin. Supported by a grant from the Graduate School, University of Minnesota, and NSF Grant DEB 7823392. or nucleus-associated organelle (Heath and Heath, 1976), which suggest that they may be intermediate between the basidiomycetes and their presumed ancestors, the ascomycetes. Thus, studies on fungi with auricularioid basidia may be crucial to an understanding of basidial evolution. The Auriculariales contains a great diversity of basidiocarp forms and probasidial morphology (Shaffer, 1975) as well as saprobic and parasitic species. Some members of this order are difficult to separate from the Uredinales and Septobasidiales and a few are difficult to delimit from the Tremellales (Donk, 1972). The auricularioid basidium is also characteristic of the Ustilaginales (Ustilaginaceae). The parasitic and saprobic species of the Auriculariales may contain transitional forms to the orders with auricularioid basidia as well as to several other orders of basidiomycetes. We know little about the parasitic species. An initial ultrastructural report on the basidium of Auricularia fuscosuccinea (Mont.) Farl. revealed some features in common with members of the Tremellales, Tulasnellales and Dacrymycetales (McLaughlin, 1979). Thus far there is little ultrastructural evidence to connect A. fuscosuccinea to the other orders with auricularioid basidia. Besides the initial ultrastructural report on basidial septa and sterigmal initiation (McLaughlin, in press), micrographs of the septal pore apparatus in several species of Auricularia (and Hirneola) have been published (Moore, 1978; Patton and Marchant, 1978a; Tu and Kimbrough, 1978). This report on a sa-