Comparison of univalent behavior in desynaptic, asynaptic and several trisomics

Evidence has been obtained that univalents in microsporocytes from plants homozygous for desynaptic represent true desynapsis, following (at least sometimes) events of crossing over. Pachytene in such material appears normal (or nearly so), as first reported by Nelson and Clary (J. Heredity 43:205-210, 1952). Recent observations in this laboratory show that, at diakinesis, matching and adjacent or aligned univalents (presumed homologues) may demonstrate equational separation of heterozygous knobs. This suggests that the desynaptic mutant may represent a lesion in the normal sister chromatid cohesiveness thought to be needed for maintenance of chiasmata until metaphase-anaphase I. The asynaptic mutant, on the other hand, has been reported to represent variable failure of synapsis, although regular synapsis was found in centromere regions and with lesser frequency in distal regions (Miller, 0. J., Genetics 48:1445-1466).

Observations have been made in this laboratory of univalent behavior at metaphase I, anaphase I and prophase II in microsporocytes of plants which were trisomic for chromosome 5, 9, 10 or a maize-Tripsacum interchange chromosome, as well as of plants which were homozygous for asynaptic or desynaptic. In all cases, univalents may either move intact to a pole or lag and orient late on the metaphase I plate, with sister centromeres directed to opposite poles; in the latter instance, univalents of desynaptic cells tend to separate freely, sending a chromatid to each pole at anaphase I, while such univalents of asynaptic cells tend to remain in the plate region, at least until very late anaphase I or early telophase I and sometimes apparently indefinitely, while showing considerable decondensation. These laggards generally show apparent stickiness of sister centromere regions, but sometimes of distal or other regions. Univalents of all the trisomics studied show more variable behavior than the synaptic mutants, sometimes separating freely after lagging and sometimes apparently sticking together and remaining in the plate region until very late. Equational separation of the chromatids of one of the chromosomes of trivalent configurations at early anaphase I was also observed. At prophase II in the trisomics, some cells contain 10 normal dyads plus two monads (thought to have arisen from the premature separation of the chromatids of a univalent which had been distributed intact to a pole at the first division).

These observations are consistent with the suggestion that the normal development of meiotic sister chromatid cohesiveness (finally reversed normally for the last remaining regions, i.e. sister centromeres and adjacent zones, at early anaphase II) is dependent upon normal synapsis. A function of the fully elaborated synaptonemal complex, then, might be somehow related to the development of sister chromatid cohesiveness.

Electron microscopy of the synaptonemal complex in normal, desynaptic and asynaptic cells is in progress. The quantitative relationship of synaptic failure frequency to premature separation of sister chromatids is also being explored.

Marjorie Maguire

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