Universidade Federal de Viçosa

A putative short paracentric inversion in chromosome 9 induced by B chromosome in maize

— Saraiva, LS; Carvalho, CR

The “high loss” phenomenon was discovered by Rhoades et al. (PNAS 57: 1626–1632. 1967), in which B chromosome induces break in knob-bearing chromosomes from the regular complement producing deficient chromosomes in maize. In this study Black Mexican Sweet Corn was used as the high loss stock with B chromosomes, and having the marker genes in chromosome 9 dominant Yg2, located close to the knob, which causes green seedling and plants and the dominant C allele that causes anthocyanin pigment in the aleurone. This stock was used to pollinate tester stock with recessive yg2, which produces yellow green seedling and plants and recessive c for colorless aleurone.

Pollen from plants with several B chromosomes, carrying a large knob terminating the short arm of chromosome 9 (9S) and homozygous for the dominant Yg2 C alleles, was applied to the silks of yg2 c tester plants. For genetic and cytological analysis exceptional yellow green progenies were selected which had a deficient chromosome 9.

The yg2 exception S-463-1 is an interesting case. It gave no pollen sterility and produced an ear with 154 C: 144 c. Thus, the deficient chromosome (C) was transmitted as female with the same frequency as the normal 9 (c). When this plant was used as the male in crosses with two c tester plants, it gave a total of 542 c kernels, but none had the C allele. Since there were no C kernels from crossing over when plant S-463-1 was used as male, the working hypothesis assumes a deficient chromosome with the breakpoint very close to the C locus. Twelve C kernels from plant S-463-1 were planted to obtain more genetic data and to locate cytologically the breakpoint in the deficient chromosome. From a total of nine ears there were 1116 C and 1098 c kernels, essentially 1C:1c, showing that in fact the frequency of female transmission of the deficient chromosome was not reduced in comparison with the normal chromosome 9. When these plants were used as the male in reciprocal test with c plants, the progenies consisted of 4021 c and 0 C kernels. There was no crossing over between the C locus and the breakpoint. However, cytological analysis showed that the deficient chromosome carried a short, knobless terminal segment which did not pair with the normal chromosome 9. The total length of the deficient chromosome appeared to be a little shorter than normal 9. In many pachytene cells from several plants, synapsis of the distal ends of the short arm was never observed.

The failure of the distal ends to pair may be caused by an inversion of 9S chromatin or by the translocation of a small piece of heterologous chromatin. Since the unpaired segment is very short, an unequivocal conclusion is not possible. Plants heterozygous for a short inversion will form neither dicentric bridges at anaphase nor loops at pachynema. However, it is tentatively concluded that the deficient chromosome 9 carries an inversion and not a translocated segment from a heterologous chromosome. This interpretation is based on the following observations:

  1. The deficient plants grew as well as normal yellow green individuals, giving good ears. If chromatin from a heterologous chromosome is attached to 9S, the deficiency in chromosome 9 would be greater than originally indicated. It is known from other yg2 exceptions that deficiencies in 9S of five or six chromomeres adversely affect growth and seed production;

  2. The unpaired terminal segment was never observed to pair with another chromosome, although it is true that pairing would rarely occur with a short translocated piece;

  3. The frequency of female transmission of the deficient chromosome was not reduced;

  4. Most of the translocated chromosomes induced by B chromosomes have a knob on the translocated fragment. Then it is probable that B chromosomes induced multiple breaks in chromosome 9, perhaps, three different breakage positions in the short arm.

Besides one break causing the loss of the terminal piece including the large terminal knob and the Yg2 locus, two more breaks probably occurred giving rise to the short paracentric inversion, that did not pair with the normal knobless chromosome 9 from the tester stock. That B chromosomes can induce more than one break in A chromosome, causing not only terminal deficiencies, was shown by Saraiva (Rev. Bras. Genet. 15: 623–630. 1992) who obtained 24 different non-reciprocal translocations involving different knobbed chromosomes.

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