Structure and development of hyphal septa in Allomyces

Development of hyphal septa (pseudosepta) in Allomyces macrogynus begins with the formation of five or more discontinuous pieces of wall material that project inward from the hyphal wall. Lateral fusion of these projections leaves a central pore in the septum that is later filled in by centripetal deposition of wall material. However, lateral fusion of the projections is not complete; peripheral pores remain in the rim of the mature septum. The position of cytoplasmic microtubules corresponds to the position of actively moving cellular particles and organelles. Allomyces reticulatus and A. arbuscula have similar septa. PREVIOUS STUDIES on the physiology and cytology of Allomyces have been concerned primarily with the reproductive aspects of this fungus, such as zoospore ultrastructure, sporangial and gametangial development, pheromones, and life cycles (Emerson, 1941; Kochert, 1978; Lange and Olson, 1979; Griffien, 1981; Olson, Eden and Lange, 1981). The structure ofthe vegetative portion ofthe fungus has received relatively little attention. Since the ecological success of Allomyces, as with any other fungus, generally depends in part upon the success of its vegetative phase, a better understanding of the structural and functional aspects of the thallus itself would be helpful in understanding its biology. Little is known about septa in the zoosporic fungi, and all studies to date have been concerned with the septa at the base of sporangia orgametangia(Powell, 1974;Morrison, 1977; Taylor and Fuller, 1980). The septa of Allomyces are taxonomically important within the Blastocladiales. Allomyces is separated from other members of the Blastocladiales by the presence of septa in the hyphae (Sparrow,1960, 1973; Emerson and Robertson, 1974). These septations are commonly referred to as pseudosepta because they incompletely separate adjacent cells in the vegetative hyphae. Despite their taxonomic significance, published illustrations of hyphal septa in Allomyces are lim1 Received for publication 1 November 1984revision accepted 8 April 1985. We thank R. L. Blanton, C. Haigler, and J. Taylor for their helpful suggestions. We also thank R. Berman and J. Taylor for providing the cultures of Allomyces reticulatus. This paper represents a portion of a dissertation submitted by R. J. Meyer to the Department of Botany of the University of Georgia, Athens. 2 Present address: Department of Botany, University of Toronto, Erindale Campus, Mississauga, Ontario L5L 1C6, Canada. ited to the drawings of Barrett (1912) and the micrograph of Adrian Couch (in Coker, 1930). In a detailed consideration of septa, Barrett ( 19 12) stated that "in older parts of the plant they reach their most perfect development, and then only incompletely separate the protoplasm of the adjacent segments.... They are first seen as separate processes or thickenings protruding inwardly from the wall at the constrictions. These processes increase in length, probably by accretion, until on meeting at the center of the cell, a fusion takes place and a definite central plate results." This is the generally accepted view of how septum development proceeds and what the mature septum looks like in Allomyces. However, there has been some suggestion that variation does exist in the structure of the septum within the genus Allomyces. Emerson and Robertson (1974) reported that they never observed septa with central plates and radiating strands in A. reticulatus Emerson et Robertson. Rather, they observed that A. reticulatus had septa that were "simply ring-like thickenings." At times these rings were incompletely formed and extended "only part way around the hyphal wall." Since there seemed to be variation in the structure or interpretation of septa in diffierent species of Allomyces and since septal development has never been examined ultrastructurally, a reexamination of the hyphal septa in Allomyces was undertaken. MATERIALS AND METHODS-Light microscopy-Living material of A. macrogynus Emerson et Wilson, A. arbuscula E. J. Butler, and A. reticulatus Emerson et Robertson (Emerson cultures Burma 3-35, PI1, and Cal70, respectively) were examined with both bright field and diffierential interference con-