Meiotic chromosome behavior in anther-derived pollen plants of maize

Microsporocytes of five pollen-plants via anther culture of maize were examined by conventional aceto-carmine squash techniques. It was found that one was dihaploid (2n = 2x = 20), which was probably brought about by spontaneous chromosome doubling. Among the other four plants, one was aneuploid (2n = 2x 18); the other three were haploid (2n = x = 10).

During meiosis, the dihaploid plant consistently demonstrated regular chromosome behavior, such as 10 closely synapsed bivalents throughout pachynema and metaphase I. Neither univalents nor end-to-end associated bivalents were observed. The pollen grains produced by this plant were almost totally fertile and the seed-sets were about 100 percent. In the aneuploid, meiosis was extremely irregular. At pachynema, asynapses and foldback type non-homologous associations were persistently present. At anaphase I, fusions of all the chromosomes into a large chromatin body and laggards were frequently found. Furthermore, precocious splitting of univalents and irregular distribution of dyads also appeared at this stage. The pollen-grains of this plant were almost completely sterile, and only one well-developed kernel was obtained after repeated self-fertilizations.

For the three haploid plants (2n = x = 10), irregular meiosis was consistently observed. At pachynema, non-homologous associations of chromosomes by folding back were commonly present. Occasionally fusions of all chromosomes into a chromatin mass were also found. At diplonema and diakinesis, 10 clearly separate univalents were common configurations, even though on several occasions one or two secondary associations of heterologous chromosomes were seen. At anaphase I distribution of chromosomes varied a great deal--distributions such as 3 to 7, 2 to 8, 9 to 1 were all observed. A 5 to 5 distribution of chromosomes was sometimes found. However, at the same stage, 10 chromosomes grouped into three or four masses of chromatin were also present. At anaphase II, variations in chromosome distributions were ubiquitous--distributions such as 5 to 5, 4 to 4, 3 to 7 were all found. In addition, laggards, chromosome bridges, fusion of chromosomes into chromatin masses were also present. Another irregular division, with one cell having one chromosome, the second having two chromosomes and the third having seven chromosomes of a presumptive quartet, was also observed.

One (83-P-5) of the three haploid plants had about five percent of the microsporocytes with doubled chromosome number (2n = 2x = 20) at pachynema. Whether this change of chromosome number is due to natural doubling or due to fusion of microsporocytes is unclear.

Y.C. Ting and Margaret Yu


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