Direct cytological evidence for true terminalization of chiasmata

Recent reports in the literature which suggest that chiasmata do not terminalize appreciably between early diplotene and metaphase I in several organisms have raised the question whether the true positions of chiasmata actually change with stage advancement, or only appear to shift toward the distal ends of bivalent arms (because of strong prophase condensation) in maize. The positions of knob-bearing regions relative to chiasmata at diakinesis in material heterozygous for knobs provide relevant information.

Microsporocytes from a plant heterozygous for six or seven knobs, and homozygous for one were studied in systematically scanned acetocarmine squash preparations. Bivalents with equational separation of a heterozygous knob in the presence of a distal chiasma were frequently observed. This is taken as evidence that in such cases a crossover has occurred proximally to the knob, and the resulting chiasma has been terminalized through the knob. Equational separation of a heterozygous knob-bearing region was not found in the absence of a distal chiasma. Disjunctional separation of heterozygous knobs in diakinesis bivalents was also observed. This is interpreted in several ways, depending on the absence or presence (as well as position) of a chiasma: either no crossover has occurred in the arm in question (in the absence of a chiasma there), or crossing over occurred distally to the knob (in the presence of a distal chiasma), or crossing over occurred proximally to the knob and has not yet terminalized through it (in the presence of a proximal chiasma).

The plant studied was heterozygous for a reciprocal translocation. Pachytene translocation configurations indicated the presence of heterozygosity for a conspicuous knob, and diakinesis quadrivalent translocation configurations gave particularly clear demonstrations of equational separation of the knob-bearing regions in the presence of a distal chiasma.

Marjorie Maguire


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