Maize Transposable Elements

Barbara McClintock originally proposed that transposable elements explained the results of genetic analysis of variegated sectors of maize aleurone. The aleurone layer is part of the endosperm and is 3n due to double fertilization. Dosages affects exist in this 3n tissue because the female parent can contribute zero to four copies of an allele in 3n tissue. The female parent contributes 2n and the male parent contributes 1n.

Initially McClintock studied the bridge-break-fusion cycle in an abnormal chromosome 9 of maize. This bridge-break-fusion cycle was due to a chromosome that was dicentric. Dicentric chromosomes have two centromeres. The dicentric chromosome resulted in variegated sectors of purple and yellow aleurone tissue in the same kernel of maize. Later she discovered that the Ds allele, located on chromosome 9 could cause chromosome breakage. The Ds allele can initiate the bridge-break-fusion cycle on a chromosome that initially had only one centromere. As McClintock did a more in-depth genetic analysis, she discovered that a second element was required to cause chromosome breakage.

The Ac or Activator element was necessary to activate the Ds or Dissociation element. When the Ac element was absent from the nucleus, the Ds element was not activated. The Ds element induces a breakage at a point on the chromosome adjacent to its location. McClintock later discovered that both the Ds and Ac elements can relocate in the genome to either a different site on the same chromosome or to a non-homologous chromosome. Ds moves to a different site only when Ac is present in the genome. Ac is an autonomous element because it can move on its own initiative. Either the Ds or the Ac element can be transposed to a location within a gene, the effect is the same as a gene mutation to a different allelic form.