The Cyst of Oxytricha bifaria (Ciliata: Hypotrichida). I. Morphology and Significance

Abstract

Before actual encystment, the volume of Oxytricha bifaria falls to about 20% of normal (47,700 im3 vs. 234,000 ,m3). Once it has attained this minimal volume, the rounded cell first produces a jelly coat and then a rather homogeneous, thick cyst wall (-4 jm). Within 36-48 h, a definitive morphology is achieved and the cyst resembles a ball (-45 Aum in diameter) regularly covered by about 90 finger-like protrusions (~7 Am in height), most of which are distributed regularly along lines that are almost parallel. The possible functional significance of this morphology is discussed. Oxytricha bifaria Stokes, 1887 is a fresh-water hypotrich ciliate, approximately 120 ,tm long, which reproduces vegetatively by transverse binary fission. Depending on various internal and external conditions, Oxytricha also can display conjugation, cannibalism, and encystment. During conjugation, two cells belonging to complementary mating types interact with, meet, and fertilize one another (Ricci, 1981); cannibals are gigantic cells (220-230 ,um long) that feed on other cells, and whose formation is induced by overcrowded conditions (Ricci & Riggio, 1984); the cysts are small (diameter, 45 ,um) subspherical structures that allow resting cells to withstand severe and/or unfavorable conditions. The wide range of variation in shape and physiology of the cells, along with the presumably sophisticated codes which control them in Oxytricha, suggests that the genetic memory of the species is involved in controlling conjugation, cannibalism, and encystment. Thus, 0. bifaria lends itself well to investigations of cell differentiation (Ricci & Banchetti, 1982), namely as an organism capable of expressing several biological states differing from one another in morphology, physiology, and function. The natural encystment capacities of ciliates are well known. The major features of cystic morphology have been studied and described for many different species; a number of reviews are available (Corliss & Esser, 1974; van Wagtendonk, 1955). Ultrastructural studies have been conducted for some species. In particular, the cyst of 0. fallax has been described in detail by Grimes (1973). The cyst of 0. bifaria has been studied by Kay (1945), but several discrepancies (diameter of 45 Am vs. 70 tm reported by Kay) suggested that it might be useful to study its morphology ex novo. We used Nomarsky interference phase-contrast and scanning electron microscopy to study the cyst formation and morphology of 0. bifaria; the results 1 Preparations for SEM observations were carried out in the Institute of Zoology, University of Siena, and the authors are deeply indebted to Dr. M. Mazzini for his kind assistance and helpful instructions. The authors gratefully acknowledge Dr. R. Nobili for his useful criticisms. This paper was supported by a grant from the Consiglio Nazionale delle Ricerche. TRANS. AM. MICROSC. SOC., 104(1): 70-78. 1985. ? Copyright, 1985, by the American Microscopical Society, Inc. This content downloaded from 207.46.13.149 on Mon, 03 Oct 2016 06:10:15 UTC All use subject to http://about.jstor.org/terms VOL. 104, NO. 1, JANUARY 1985 presented in this paper in turn led to a study of the ultrastructure of the cyst (Verni et al., 1984) and of its cytochemical composition (Rosati et al., 1984). MATERIALS AND METHODS Clones C1, C6, and L1 of Oxytricha bifaria, which spontaneously produces cysts, were maintained in a lettuce medium inoculated with Enterobacter aerogenes according to the technique described by Ricci et al. (1980). To determine the age of the cysts within 12 h, the mass cultures were kept in depression slides and transferred daily into new depressions, after being examined so as to isolate cysts as young as possible. Two groups of cysts were studied: (1) cysts 0-48 h old (this period of time was chosen after observing that nearly two days are necessary for a cyst to form and to complete its development); (2) cysts about 14 days old (this age was chosen to permit the study of resting cysts exhibiting their definitive morphology). In vivo observations of encysting and encysted cells were made with a Leitz Orthoplan photomicroscope equipped with Nomarski's interferential contrast. A micrometric slide (Zeiss, 5 + 100/100 mm) was used to measure the size of different structures. The preparation techniques for scanning electron microscopy (SEM) were those routinely used for ciliates (Luporini & Dallai, 1980).