Apparent chiasma maintenance and terminalization across a region homoeologous for a segment derived from Tripsacum

In maize microsporocytes heterozygous for a chromosome 2 interchange chromosome which carries a homoeologous Tripsacum segment substituted for the distal half of its short arm, normal synapsis usually occurs at pachytene throughout the bivalent. Crossing over, however, is almost entirely restricted to the homologous proximal region of the arm, where it apparently occurs frequently (Genetics 45:195-209, 1960; Genetics 45:651-664; J. Heredity 53:87-88, 1962). Acetocarmine squash preparations of anthers from heterozygous plants were systematically scanned, and diakinesis stage cells were classified for presence or absence of a chiasma in the knob-carrying arm of chromosome 2 as well as for the position of the chiasma (when present) within this knob-carrying arm. The chromosome 2 bivalent was usually readily identifiable at diakinesis because of its terminal knob and relatively large size. Of 123 diakinesis cells so classified, 6% showed no chiasma in the knob-carrying arm, 33% were found to have a chiasma a substantial distance from the knob, 58% showed a chiasma immediately adjacent to the knob, and 3% had an apparently terminal chiasma, in the knob region. Thus in a very large proportion of the cells a chiasma presumed to have resulted from a necessarily more proximal crossover event (in the region of homology) was found to be located immediately adjacent to or at the knob by diakinesis. In these cases the amount of chromatin between the chiasma and the knob was so scant as to make the interpretation unreasonable that differentially, strongly condensed, more distal chromatin rendered the apparent position of the chiasma to be falsely nearly terminal. It is difficult to avoid the interpretation that many chiasmata had terminalized across most of the region where the two chromosomes of the bivalent shared only a homoeologous relationship, and that loss of chiasmata formed in the proximal half of the arm was rare at most. Such loss would produce a chromosome 2 bivalent with the ends of the knob-bearing arm free and with equational separation of the terminal knob at these ends. No such bivalent was seen.

Marjorie Maguire

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