A search for meiotic mutants is being undertaken among plants of the M3 generation following treatment of pollen with EMS. To date, sporocytes of 27 mutants which exhibited ovule abortion and/or abnormal kernel development have been examined. Of these, at least two appear to be new mutants.
One affects meiosis in several ways and is variable in its expression. Two or four univalents are sometimes present during diakinesis, prometaphase I and metaphase I. Whether the univalents result from the failure of homologous chromosomes to pair or from desynapsis is not yet known. Lagging chromosomes (usually 1 or 2, but sometimes 3 or 4) have been observed at both meiotic anaphases and telophases.
Occasionally, the axis of one or both of the metaphase II spindles is oriented in the same direction as, instead of at right angles to, that of the metaphase I spindle. As a result, quartets of the type shown in Figure 1, or linear quartets (Figure 2) are formed. Some quartets contain three normal microspores and one without a nucleolus, or two binucleolar microspores and two without nucleoli. Other quartets contain one, two, or more apparently normal nucleoli in addition to the four which are usually present (e.g., Figure 3). Clusters of five to eight microspores which have arisen from a single sporocyte are frequently observed (Figures 4 and 5). In some clusters, each cell has a nucleolus (Figures 4 and 5) while in others some cells may lack a nucleolus. The cells comprising the abnormal quartets and groups of more than four microspores are often unequal in size (Figures 2, 3, 4 and 5). Pollen and ovule abortion and the proportions of normal and defective kernels on ears are variable.
The presence of more than four nucleoli per quartet or group of microspores suggests that an extra replication of the nucleolus organizer, and possibly of the whole chromosome complement, has occurred at some stage. Attempts to determine whether the extra cells in clusters such as those in Figures 4 and 5 arise from supernumerary mitoses and if so, whether each cell receives a complete set of chromosomes, have been hampered by a tendency of cells which are near the end of the second meiotic division to fall apart when squashes are made; it may be necessary to resort to sectioning in order to examine these stages more fully. No mitoses have been observed during or after the late quartet stage. It has also still to be established whether there is more than one mutation affecting meiosis in the mutant stock or not.
The second mutant has not been studied in detail. It is completely male sterile and has poor seed set. Meiosis proceeds normally through leptonema to the early synizetic stage. Thereafter, a long stage follows during which the chromosomes become indistinct and appear to fuse with the nucleolus. Tassel samples with later stages are not yet available. The meiocytes may degenerate or they may undergo a modified type of meiosis similar to that shown by sticky chromosome homozygotes; however, there are features which differ from Beadle's description of the latter mutant.
P. M. Nel* and M. G. Neuffer
*The courtesy of the Department of Genetics and Development during a sabbatical tenure at the University of Illinois is greatly appreciated
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