Barbara McClintock and the Discovery of Jumping Genes (Transposons)

Some of the most profound genetic discoveries have been made with the help of various model organisms that are favored by scientists for their widespread availability and ease of maintenance and proliferation. One such model is Zea mays (maize), particularly those plants that produce variably colored kernels. Because each kernel is an embryo produced from an individual fertilization, hundreds of offspring can be scored on a single ear, making maize an ideal organism for genetic analysis. Indeed, maize proved to be the perfect organism for the study of transposable elements (TEs), also known as "jumping genes" which were discovered during the middle part of the twentieth century by American scientist Barbara McClintock. McClintock's work was revolutionary in that it suggested that an organism's genome is not a stationary entity, but rather it is subject to alteration and rearrangement—a concept that was met with criticism from the scientific community of the time. Eventually, however, the significance of McClintock's work became widely appreciated, and she was awarded the Nobel Prize in 1983.

McClintock and the Origins of Cytogenetics

Barbara McClintock began her scientific career at Cornell University, where she pioneered the study of cytogenetics—a new field in the 1930s—using maize as a model. Indeed, the marriage of cytology and genetics became official in 1931, when McClintock and graduate student Harriet Creighton provided the first experimental proof that genes were physically positioned on chromosomes by describing the crossing-over phenomenon and geneticrecombination. Although Thomas Hunt Morgan was the first person to suggest the link between genetic traits and the exchange of genetic material by chromosomes, 20 years elapsed before his ideas were scientifically proven, largely due to limitations in cytological and experimental techniques (Coe & Kass, 2005). McClintock's own innovative cytogenetic techniques were what allowed her to confirm Morgan's ideas, and these techniques are thus among her greatest contributions to science.

Discovering TEs through Experimentation with Maize

As previously mentioned, McClintock is best known not for her innovations in cytogenetic techniques, but rather for her discovery of transposable elements through experimentation with maize. In order to understand McClintock's observations (and logic) that led to her discovery of TEs, however, it's first necessary to be aware that the phenotypic system that McClintock studied—the variegated color pattern of maize kernels—involved three alleles rather than the usual two. Think of every maize kernel as essentially a single individual, originating as an ovule that undergoes (or has undergone) double fertilization. During double fertilization, one sperm fuses with the egg cell's nucleus, producing a diploid zygote that will develop into the next generation. Meanwhile, the other sperm fuses with the two polar nuclei to form a triploid endosperm. As a result, the colored (or colorless, as the case may be) tissue that makes up the aleurone (or outer) layer of the endosperm is triploid, not diploid.

McClintock worked with what is known as the Ac/Ds system in maize, which she discovered by conducting standard genetic breeding experiments using plants with an unusual phenotype. Through these experiments, McClintock recognized that breakage occurred at specific sites on maize chromosomes. Indeed, the first transposable element she discovered was a site of chromosome breakage, aptly named "dissociation" (Ds). Although McClintock eventually found that some TEs can "jump" autonomously, she noted that the movements of Ds are regulated by an autonomous element called "activator" (Ac), which can also promote its own transposition.

Of course, these discoveries were preceded by extensive breeding experimentation. It was known at the time from previous work by Rollins A. Emerson, another American maize geneticist, that maize had genes encoding variegated, or multicolored, kernels; these kernels were described as colorless (although they were actually white or yellow), except for spots or streaks of purple or brown. Emerson had proposed that the variegated streaking was due to an "unstable mutation," or a mutation for the colorless phenotype that would sometimes revert back to its wild-type variant and result in an area of color. However, he couldn't explain why or how this occurred. As McClintock discovered, the unstable mutation Emerson puzzled over was actually a four-gene system.