11. Controlling elements in maize cause breakage and rearrangements

15.11 Controlling elements in maize cause breakage and rearrangements

Key terms defined in this section
Acentric fragment of a chromosome (generated by breakage) lacks a centromere and is lost at cell division.
Controlling elements of maize are transposable units originally identified solely by their genetic properties. They may be autonomous (able to transpose independently) or nonautonomous (able to transpose only in the presence of an autonomous element).
Dicentric chromosome is the product of fusing two chromosome fragments, each of which has a centromere. It is unstable and may be broken when the two centromeres are pulled to opposite poles in mitosis.
Variegation of phenotype is produced by a change in genotype during somatic development.

One of the most visible consequences of the existence and mobility of transposons occurs during plant development, when somatic variation occurs. This is due to changes in the location or behavior of controlling elements (the name that transposons were given in maize before their molecular nature was discovered).




Figure 15.21 Clonal analysis identifies a group of cells descended from a single ancestor in which a transposition- mediated event altered the phenotype. Timing of the event during development is indicated by the number of cells; tissue specificity of the event may be indicated by the location of the cells.

Two features of maize have helped to follow transposition events. Controlling elements often insert near genes that have visible but nonlethal effects on the phenotype. And because maize displays clonal development, the occurrence and timing of a transposition event can be visualized as depicted diagrammatically in Figure 15.21.


The nature of the event does not matter: it may be an insertion, excision, or chromosome break. What is important is that it occurs in a heterozygote to alter the expression of one allele. Then the descendants of a cell that has suffered the event display a new phenotype, while the descendants of cells not affected by the event continue to display the original phenotype.


Mitotic descendants of a given cell remain in the same location and thus give rise to a sector of tissue. A change in phenotype during somatic development is called variegation; it is revealed by a sector of the new phenotype residing within the tissue of the original phenotype. The size of the sector depends on the number of divisions in the lineage giving rise to it; so the size of the area of the new phenotype is determined by the timing of the change in genotype. The earlier its occurrence in the cell lineage, the greater the number of descendants and thus the size of patch in the mature tissue. This is seen most vividly in the variation in kernel color, when patches of one color appear within another color.


Insertion of a controlling element may affect the activity of adjacent genes. Deletions, duplications, inversions, and translocations all occur at the sites where controlling elements are present. Chromosome breakage is a common consequence of the presence of some elements. A unique feature of the maize system is that the activities of the controlling elements are regulated during development. The elements transpose and promote genetic rearrangements at characteristic times and frequencies during plant development.




Figure 15.22 A break at a controlling element causes loss of an acentric fragment; if the fragment carries the dominant markers of a heterozygote, its loss changes the phenotype. The effects of the dominant markers, CI, Bz, Wx, can be visualized by the color of the cells or by appropriate staining.

The characteristic behavior of controlling elements in maize is typified by the Ds element, which was originally identified by its ability to provide a site for chromosome breakage. The consequences are illustrated in Figure 15.22. Consider a heterozygote in which Ds lies on one homolog between the centromere and a series of dominant markers. The other homolog lacks Ds and has recessive markers (C, bz, wx). Breakage at Ds generates an acentric fragment carrying the dominant markers. Because of its lack of a centromere, this fragment is lost at mitosis. So the descendant cells have only the recessive markers carried by the intact chromosome. This gives the type of situation whose results are depicted in Figure 15.21.




Figure 15.23 Ds provides a site to initiate the chromatid fusion-bridge-breakage cycle. The products can be followed by clonal analysis.

Figure 15.23 shows that breakage at Ds leads to the formation of two unusual chromosomes. These are generated by joining the broken ends of the products of replication. One is a U Vshaped acentric fragment consisting of the joined sister chromatids for the region distal to Ds (on the left as drawn in the figure). The other is a U-shaped dicentric chromosome comprising the sister chromatids proximal to Ds (on its right in the figure). The latter structure leads to the classic breakage-fusion-bridge cycle illustrated in the figure.


Follow the fate of the dicentric chromosome when it attempts to segregate on the mitotic spindle. Each of its two centromeres pulls toward an opposite pole. The tension breaks the chromosome at a random site between the centromeres. In the example of the figure, breakage occurs between loci A and B, with the result that one daughter chromosome has a duplication of A, while the other has a deletion. If A is a dominant marker, the cells with the duplication will retain a phenotype, but cells with the deletion will display the recessive a phenotype.


The breakage-fusion-bridge cycle continues through further cell generations, allowing genetic changes to continue in the descendants. For example, consider the deletion chromosome that has lost A. In the next cycle, a break occurs between B and C, so that the descendants are divided into those with a duplication of B and those with a deletion. Successive losses of dominant markers are revealed by subsectors within sectors.




Genes VII
Genes VII
ISBN: B000R0CSVM
EAN: N/A
Year: 2005
Pages: 382

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