If the synaptonemal complex (SC) is assumed to be a measure of exchange pairing during meiosis, then the time of appearance of this structure marks the earliest opportunity for exchange to occur. Recent studies report that the SC is seen during the meiotic S phase in Drosophila (R. F. Grell and J. W. Day, in Mechanisms in Recombination, R. F. Grell, ed.; J. W. Day and R. F. Grell, Genetics 83:67, 1976), wheat (H. A. McQuade and D. G. Pickles, Amer. J. Bot. 67:1361, 1980) and yeast (J. G. Petersen, L. W. Olson and D. Zickler, Carlsberg Res. Commun. 43:241, 1978). To define this period in the meiosis of Zea mays an electron microscopic study of microsporocytes in various stages of meiosis I was done. Meiotic stage of the anthers was determined by standard acetocarmine squash of one anther from each flower at the time of fixation and confirmed by thick sections for light microscopy.
Electron microscopic observation of thin sections immediately adjacent to the thick sections revealed synaptonemal complexes at pachytene, zygotene and leptotene. Complete complexes had previously been reported only as early as zygotene (C. B. Gilles, Carlsberg Res. Commun. 43:241, 1974). The complexes seen at leptotene were morphologically very similar to those seen at pachytene although the central element appeared less distinct (see micrograph). Short "complex-like" structures were seen at interphase with clear lateral elements. However, the central element was visible only rarely in interphase nuclei. The relative decondensation of maize interphase chromosomes probably obscures these structures.
C. M. McKinley and R. F. Grell
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