NUCLEAR BEHAVIOR IN THE VEGETATIVE HYPHAE OF CERATOCYSTIS FAGACEARUM

Vegetative nuclear division in Ceratocystis fagacearum (Bretz) Hunt was found to differ from classical mitosis in that: (1) division always occurs perpeindicular to the longitudinal axis of the cell, (2) anaphase movement is unilateral anid unsynchronized, (3) a spindle occurs only between separating chromatids. Initerphase and prophase nuclei and nucleoli are morphologically similar to those in, higher plants. At metaphase the associated chromosomes form a bar of chromatini anid lie against the hyphal wall. Spindle fibers appear between separating chromatids, perhaps pushing them apart. When nuclear division is complete the nuclei become attenuated and migrate. Vegetative nuclear division in C. fagacearum may be an evolutionary form of classical mitosis. NUCLEAR DIVISION in the vegetative hyphae of fungi is poorly understood. Through improved techniques we now have detailed knowledge of meiosis in a number of fungi. Information on mitosis is fragmentary and conflicting. There are a number of published theories conconcerning asexual nuclear division in the fungi. The most common view is that division is by classical mitosis (McGinnis, 1953; Somers, Wagner, and Hsu, 1960; Ward and Ciurysek, 1961, 1962; Lu, 1962; Robinow, 1963; Hartman, 1964). Robinow (1963) clearly demonstrated classical mitosis in Basidiobolus ranarum Eidam. Several workers (Bakerspigel, 1.957, 1958, 1959a, 1959b, 1959c, 1959d, 1960a, 1960b, 1961, 1962; Robinow, 1957a, 1957b, 1962; Saksena, 1961), did not find spindles, metaphase plates, or centrioles as in regular mitosis. The nuclei simply elongate, constrict in the middle and then separate to form daughter tnuclei. Bakerspigel (1959a) found differences among species in the behavior of the nucleolus during nuclear divisions. In certain species the nucleolus divides along with the chromatin by elongation, constriction, and separation into two parts. In other species the nucleolus beconmes separated from the chromatin early in division. At the conclusion of division it either disappears or becomes fragmented. Dowding and Weijer, 1960; Weijer and Dowdinig, 1960; and Weijer, Koopmans, and Weijer, 1963, presented radically different interpretations of nuclear division in the vegetative hyphae of species of Neurospora and Gelasinospora. Studying living hyphae with phase-contrast microscopy, they found that the chromosomes are organized in a continuous ring in which no individual chromosomes can be identified. The ring duplicates and then breaks, giving rise to a two-stranded nuclear IReceived for publication January 15, 1966. Published with the approval of the Director of the Arkansas Agricultural Experiment Station. The help of Dr. Oval Myers, Jr., is gratefully acknowledged. filament. The two-stranded filament divides longitudinally, giving rise to two daughter nuclei each containing a nucleolus and a centriole. In stained preparations the same writers founid that distinct chromosomes are arranged in series on a filament that elongates and splits longitudinally, producinig two daughter nuclei. A special type of asexual nuclear division, believed to be characteristic of the Plasmodiophoromycetes, has been described and referred to as a "cruciform division" (Olive, 1953). A chromatin ring appears on an intranuclear spindle and surrounds the nucleolus. The nucleolus beconmes dumbbell-shaped while the chromatin passes to each pole. The nucleolus then constricts into two parts and one part passes to each nucleus. Turian and Cantino (1960), in studying Blastocladiella, reported a type of division in which a beaded chromnatin ring is fornmed, followed by formation of a prophase-metaphase crescent with both ring and crescent surrounding a spherical nucleolus. The chromatin separates to two poles and the nucleolus elongates (performing the function of a spindle) and constricts in the middle completing the division. The development of the perithecium of Ceratocystis fagacearumn (Bretz) Hunt has been described (Tiffany, 1956; Wilson, 1956a, 1956b). Details of meiosis are lacking. Except for a brief report (Aist and Wilson, 1965) of these findings, no information is available on the asexual nuclear behavior of C..fagacearum. The purpose of this paper is to describe the behavior of vegetative nuclei of C. fagacearurn as interpreted from study of stained preparations of the fungus. MATERIALS AND METHODS-Hyphae of both A and B compatibility types of C. fagacearum on agar blocks were placed on the surface of No. 1 coverslips. The agar blocks were then incubated aseptically on moist filter paper in petri dishes at 26 C for 3-10 days. The coverslips and adherent