FIG. 14.11

What causes sectoring in corn kernels?

BACKGROUND In the late 1940s, Barbara McClintock discovered what are now called transposable elements, DNA sequences that can move from one position to any other in the genome. She studied corn (Zea mays). Wild-type corn has purple kernels, resulting from expression of purple anthocyanin pigment (Fig. 14.11a). A mutant with yellow kernels results from lack of purple anthocyanin pigment. McClintock noticed that streaks of purple pigmentation could be seen in many yellow kernels (Fig. 14.11b). This observation indicated that the mutation causing yellow color was unstable and that the gene could revert to the normal purple color.

HYPOTHESIS McClintock hypothesized that the yellow mutant color resulted from a transposable element, which she called Dissociator (Ds), jumping into a site near or in the anthocyanin gene and disrupting its function. She attributed the purple streaks to cell lineages in which the transposable element had jumped out again, restoring the anthocyanin gene.

EXPERIMENT AND RESULTS By a series of genetic crosses, McClintock showed that the genetic instability of Ds was due to something on another chromosome that she called Activator (Ac). She set up crosses in which she could track the Ac-bearing chromosome. She observed that in the presence of Ac, cells in mutant yellow kernels reverted to normal purple, resulting in purple sectors in an otherwise yellow kernel. From this observation, she inferred that the Ds element had jumped out of the anthocyanin gene, restoring its function. She also demonstrated that restoration of the original purple color was associated with mutations elsewhere in the genome. From this observation, she inferred that the Ds element had integrated elsewhere in the genome, where it disrupted the function of another gene.

CONCLUSION McClintock’s conclusion is illustrated in Fig. 14.11c: Transposable elements can be excised from their original position in the genome and inserted into another position.

FOLLOW-UP WORK McClintock won the Nobel Prize in Physiology or Medicine in 1983. Later experiments showed that Ds is a transposable element that lacks a functional gene for transposase, the protein needed for the element to move, and Ac is a transposable element that encodes transposase. Presence of Ac produces active transposase that allows Ds to move. Much additional work showed that there are many different types of transposable element and that they are ubiquitous among organisms.

SOURCE McClintock, B. 1950. “The Origin and Behavior of Mutable Loci in Maize.” Proceedings of the National Academy of Sciences of the USA. 36:344–355.