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 genetic recombination. 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
![The life cycle of an angiosperm.](javascript:dispOrigImg("38618/Double-fertilization_FULL.jpg", "The life cycle of an angiosperm.", "Y", "Figure 1", "1221766469284-8921219686_1", 'Y', "© 2008 by Sinauer Associates, Inc. All rights reserved. Used with permission.", '900', '1012', "http://www.sinauer.com"); "The life cycle of an angiosperm.")
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 (Figure 1). 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.
Figure 2: Variation in kernel phenotypes is used to study transposon behavior.
Kernels on a maize ear show unstable phenotypes due to the interplay between a transposable element (TE) and a pigment gene.
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 (Figure 2). 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, as outlined in Table 1.
Table 1: Maize Genes Studied by Barbara McClintock
| Gene | Description |
| C' | Dominant allele on the short arm of chromosome 9 that prevents color from being expressed in the aleurone layer of the maize kernel, causing a so-called "colorless" phenotype (which is actually white or yellow in color). This is also known as the inhibitor allele. |
| C | Recessive allele on the short arm of chromosome 9 that leads to color development. |
| Bz | Dominant allele on the short arm of chromosome 9 that leads to a purple phenotype. |
| bz | Recessive allele on the short arm of chromosome 9 that leads to a dark brown phenotype. |
| Ds | Genetic location on the short arm of chromosome 9 at which chromosomal breakage occurs. |
| As | A factor of unknown location (at least when McClintock was conducting her research) that impacts the expression of Ds. |
Adapted from McClean, 1997
McClintock bred females that were homozygous for C and bz and that lacked Ds (denoted CCbzbz--, where the dashes indicate the absence of Ds alleles) with males that were homozygous for C', Bz, and Ds (denoted C'C'BzBzDsDs) to yield heterozygotes with an aleurone layer that had the genotype C'CCBzbzbz--Ds. (Remember, in double fertilization, the sperm provides one set of alleles, and the egg provides two.) Because of the presence of the dominant inhibitor allele C', the offspring kernels were expected to be colorless, no matter what their genetic makeup at the Bz/bz locus. Upon crossbreeding, many of these kernels were indeed colorless. However, McClintock also observed many kernels with colorless backgrounds and varying amounts of dark brown spots or streaks, and she concluded that individual cells in those kernels had lost their C' and Bz alleles because of a chromosomal break at the Ds locus. Without either the C' allele (to prevent color expression) or the Bz (purple) allele, the cells that had experienced a breakage at the Ds locus ended up with some brown coloring.
Within the affected seeds, the amount of colored streaking or spotting depended upon when during seed development the somatic cell mutation at Ds occurred. If this mutation occurred early in development, then, as the one mutant cell continued to divide, more cells in the mature kernel would have the brownish phenotype, and the spot or streak of color on the kernel would be larger. On the other hand, if the mutation occurred later in development, the spotting would be smaller, because the kernel would undergo less cell division prior to maturity.
McClintock also performed additional experiments to demonstrate that the phenotypic effect of Ds depended upon the presence of another element, which she called Ac. McClintock had trouble mapping both the Ac and Ds elements, however, noting that they changed their positions on the chromosome in different maize plants. In fact, further experiments showed that Ds didn't just break chromosomes, but it could actually move from one chromosomal location to another. When Ds inserts itself into the Bz allele, for example, it causes a mutation in the Bz gene (but only when Ac is present), thereby destroying the ability of the Bz gene to produce any pigment at all. Ds can also excise from the Bz allele (again, only in the presence of Ac), causing Bz to revert back to its purple or brown phenotype. Again, the amount of purple or brown depends upon when during development Ds is inserted or excised. If excision happens prior to fertilization, then the affected kernel will be entirely purple or brown, depending upon the Bz/bz genotype.
Years after McClintock discovered the Ac/Ds system, scientists were finally able to study both TEs in much more molecular detail. Today, we know that Ac elements are about 4,500 base pairs long and are similar in structure to other DNA transposons.
McClintock and Epigenetics
Barbara McClintock was also the first scientist to correctly speculate on the basic concept of epigenetics—and she did so before the molecular structure of DNA was even discovered. Mainly, she recognized that genes can be expressed and silenced during mitosis in genetically identical cells. In a 1951 paper, McClintock described this idea as follows:
"[T]he progeny of two (such) sister cells are not alike with respect to the types of gene alteration that will occur. Differential mitoses also produce the alterations that allow particular genes to be reactive. Other genes, although present, may remain inactive. This inactivity or suppression is considered to occur because the genes are ‘covered' by other nongenic chromatin materials. Gene activity may be possible only when a physical change in this covering material allows the reactive components of the gene to be ‘exposed' and thus capable of functioning."
Thus, in these few sentences, McClintock summarized epigenetic regulation by way of chromatin remodeling, a concept not formally described until more than 40 years later.
References and Recommended Reading
Coe, E., & Kass, L. B. Proof of physical exchange of genes on the chromosomes. Proceedings of the National Academy of Sciences 102, 6641–6646 (2005)
Creighton, H. B., & McClintock, B. A correlation of cytological and genetical crossing-over in Zea mays. Proceedings of the National Academy of Sciences 17, 492–497 (1931) (link to article)
Feschotte, C., et al. Plant transposable elements: Where genetics meets genomics. Nature Reviews Genetics 3, 339–341 (2002) doi:10.1038/nrg793 (link to article)
McClintock, M. The Order of the Genes C, Sh and Wx in Zea Mays with Reference to a Cytologically Known Point in the Chromosome. Proceedings of the National Academy of Sciences 17, 485–491 (1931)
McClintock, B. Mutable loci in maize. Carnegie Institution of Washington Yearbook 50, 174–181 (1951) (link to article)
McLean, P. McClintock and the Ac/Ds transposable elements of corn, www.ndsu.nodak.edu/instruct/mcclean/plsc431/transelem/trans1.htm (1997)
