Break-bridge-fusion cycle due to Ac-Ds

Variegated kernels have sectors that differ. For example, a purple-yellow variegated kernal would ahve endosperm with purple spots on an otherwise yellow kernel. One way that the Ds-Ac transposable elements can cause variegated sectors is by the action of the Ac element which causes a chromosome break at the position of the Ds element.

The Ac element may be located on a non-homologous chromosome and cause a portion of the short arm of chromosome 9 to break off. This results in an acentric fragment. The sister chromatids of the remaining portion of chromosome 9 may then fuse.

After double fertilization, the endosperm tissue develops through repleated mitotis of somatic cells. The dissociation event results in a deficiency for the I factor which is distal to the Ds location.

The deficiency in the I allele results in purple aleurone color, provided the A₁, A₂, C, R, and Pr loci carry dominant alleles at other loci. Some cell lineages are deficient for the distal portion of chromosome 9 and have purple aleurone. Cell lineages that do not have a break due to the Ac-Ds elements will be yellow sectors. A break-bridge-fusion cycle is initiated, due to the fusion of the sister chromatids.

The Ds element is transposable and can relocate to another position of the short arm of chromosome 9. Perhaps Ds transposes to the right (proximal) to the I factor.

In the presence of the Ac element, the Ds element can dissociate or break at this new location.

The result is a break-bridge-fusion cycle for endosperm tissue which undergoes repeated mitotis of somatic cells.

Suppose the F₁ plant I Ds Sh₁ Bz Wx/C ds sh₁ bx wx is testcrossed to a female plant which is C ds sh1 bz wx/C ds sh₁ bz wx. The endosperm tissue is 3n. Assume that the male plant is homozygous for the Ac factor. Then the Ds element will interact with the Ac element to cause a chromosome break between the I and Sh₁ loci. Fusion of sister chromatids creates a dicentric chromosome which breaks at various locations during anaphase of mitosis. Variegated sectors of the kernels result.

Prophase

Anaphase

One cell might receive the duplicate segment for Bz Sh₁ Sh₁ Bz Wx and cells from this lineage would be non-shrunken (Sh₁) with normal starch (Wx). Cells derived from the other lineage would only have the Wx allele and would be deficient for Bz Sh₁. Due to the deficiency, these cells would have the shrunken phenotype, purple endosperm, bronze color. The Bz and Sh1 alleles are deficient and would have the bz, sh₁ phenotype.