MEIOSIS IN THE OOMYCETES: I. A MICROSPECTROPHOTOMETRIC ANALYSIS OF NUCLEAR DEOXYRIBONUCLEIC ACID IN SAPROLEGNIA TERRESTRIS

A cytological and cytochemical examination of Saprolegnia terrestris Cookson was undertaken to resolve the conflict of opinion regarding the life cycle of fungi in the Saprolegniaceae, i.e., whether meiosis is zygotic or gametic. Microspectrophotometric analysis reveals that pre-divisional oogonial and antheridial nuclei are 4C; a subsequent reduction division in the sex organs produces 1C male and female gametes. These division products coexist in the developing oospore and presumably fuse to produce the diploid zygote. Analysis of the somatic hyphal nuclei shows a 2C DNA content, with the characteristic skew and bimodal character indicative of predominantly G1, and some S-period and G2 types. The evidence is completely consistent with gametic meiosis, i.e., a predominant diploid vegetative phase and short haploid phase, and refutes the notion of zygotic meiosis. Further ramifications of the work are discussed along with preliminary results of related work in progress. THERE is an increasing body of evidence indicating that meiosis occurs during gametogenesis in members of the Saprolegniales and Peronosporales (Trow, 1895, 1899, 1904; Stevens, 1899; Sansome, 1961, 1963, 1965; Sansome and Harris, 1962; Barksdale, 1966, 1968). Collectively this evidence is in favor of such an interpretation. However, no report has been presented which is, in itself, entirely convincing. The occurrence of gametic meiosis in members of the Saprolegniales was first reported by Trow (1895, 1899, 1904) and in members of the Peronosporales by Stevens (1899). Two years later Stevens (1901) stated that a reduction in chromosome number during gametogenesis was improbable in those fungi he had studied. In the next 50 years numerous reports were made of the occurrence of one mitotic division rather than two divisions of meiosis in the sex organs of the Oomycetes (see Barksdale, 1968). Schrader (1938) and Ziegler (1953) claimed that meiosis occurred in the germinating oospore of members of the Saprolegniaceae. In spite of the generally accepted view of zygotic meiosis in this group of fungi, the question was again raised through the efforts of Sansome (1961, 1963, 1965) and Sansome and Harris (1962). The observations of these investigators on the gametangial nuclei of several species of Oomycetes may be summarized as follows: (1) Two successive divisions occur with an associated reduction in nuclear size. (2) The number of IReceived for publication 17 February 1969. Research supported by Office of General Research, University of Georgia, Athens 30601. 2Present address: Division of the Sciences, Kirklani College, Clinton, N.Y. 13323. nuclei in the gametangia following the second division are approximately four times the number of nuclei undergoing the first division. (3) Chromosomal configurations with loops are present in early anaphase figures. (4) Irregularities in chromosomal behavior occur such as early separation or failure of pairing. (5) Typical metaphase figures are present only in the gametangia. (6) Multiple associations of more than two chromosomes (e.g., trivalents and quadrivalents) occur. (7) A bridge and fragment are present occasionally in first division anaphase nuclei. Clearly this evidence is substantial. Nonetheless the nuclei of these fungi are small, and as a result the photomicrographs presented in support of these observations are not convincing. Supporting her own contentions, Sansome (1963, 1965) aptly points out that Ziegler's (1953) illustrations of the metaphase nuclei in the oogonia of Achlya recurva show considerably larger chromosomes than the supposed meiotic metaphase chromosomes in the oospore drawn at the same magnification. Further doubt is cast upon Ziegler's (1953) observations since the zygote nucleus in sectioned material is clearly illustrated by Ziegler (p. 62Fig. 8, Fig. 24; p. 64: Fig. 34) as being situated within the ooplast (Moore and Howard, 1968), a vacuolar-like region of the oospore, rather than within the organelle-containing cytoplasm. Mullins and Raper (1965) present genetic evidence which suggests that meiosis is gametic in Achlya ambisexualis and Dictyuchus monosporus. However, as pointed out by Mullins and Raper (1965), their results are also consistent with a zygotic meiosis followed by the survival of a single meiotic product.