|McClintock and the Ac/Ds Transposable Elements of Corn||
. McClintock discovered transposable elements by analyzing genetic stocks of corn that were phenotypically unstable. In particular, she was analyzing genes that control the color of the aleurone layer of the endosperm. Remember that this tissue is triploid (3n). The genes that she was following were located on the short arm of chromosome 9 of corn and were involved in the development of the color of the seed. The genetic map of this region and the allelic designations follow.
___________________________________ C Bz DsCI = dominant allele that prevents color from being expressed in the aleurone layer
C = recessive allele that leads to color development in the aleurone layer
Bz = dominant allele that produces purple aleurone color
ds = a genetic location where chromosome breakage occurs
Homozygous stocks were created and CC bzbz -- females (without ds, denoted by the dash) were mated with CICI BzBz dsds males. The aleurone layer of the endosperm would thus have the genotype CICC Bzbzbz --ds. Because of the presence of the inhibitor allele, the aleurone layer was expected to be colorless. For many of the kernels this was the case but a few kernels had dark brown colored sectors on an otherwise colorless background.
How could this have occurred? McClintock concluded that in some manner the CI and Bz alleles were lost because chromosome breakage had occurred at the Ds locus. But why the sectoring? This breakage apparently did not occur during gamete formation, but had occurred after fertilization and during the development of the seed. This breakage and loss of genes occurred in a single cell, but all cells that developed from mitotic division of that cell did not contain the inhibitor gene, so the color expression was controlled by the bz allele in those cells.
____________________________// C bzMale gametes:
__________________________________ CI Bz dsThe following is the expected chromosomal composition of triploid endosperm. Because of the dominant CI allele the endosperm should be colorless without breakage
____________________________// (from female) C bz ____________________________// (from female) C bz ___________________________ (from male) CI Bz dsBut if breakage at ds occurred, then the genotype of the endosperm would be:
____________________________// (from female) C bz ____________________________// (from female) C bz _______// (from male)and any cells with this genotype would be dark brown in color.
Breakage at ds had been established by McClintock prior to performing these experiments. The designation ds was short for dissociation or a locus were breakage of chromosomes occurred. But after crossing with a number of different genetic stocks, she realized that Ds alone could not induce the breakage. A second factor, Ac, short for activator, was also necessary. (Thus, some genetic stocks contained Ac whereas other stocks did not contain that locus.) This system is called a two-element system and historically has been called the Ac/Ds system.
Additional genetic stocks were analyzed by McClintock and she determined that in the presence of Ac, Ds could move locations as well as cause breakages. She was able to isolate a corn line where Ds had moved into the normal Bz allele and caused a mutation in that gene. But as was mentioned this only occurred when Ac was present. Furthermore, when this new line was used and Ac was present, the Ds element was shown to move out of the Bz locus and reversion to the original phenotype was detected. This mutated allele was designated bzm1. But in the absence of Ac, bzm1 was a stable allele. Another unstable Bz allele was found that contained an Ac insertion and was designated bzm2. One difference between this allele and bzm1 was its higher rate of transposition and reversion back to the original phenotype
. So what conclusions can be drawn from these experiments and observations: