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Robert J. Brooker - Genetica Esperimento di genetica 17.2 McClintock Found That Chromosomes of Corn Plants Contain Loci That Can Move Barbara McClintock began her scientific career as a student at Cornell University. Her interests quickly became focused on the structure and function of the chromosomes of corn plants, an interest that continued for the rest of her life. She spent countless hours examining corn chromosomes under the microscope. She was technically gifted and had a theoretical mind that could propose ideas that conflicted with conventional wisdom. During her long career as a scientist, McClintock identified many unusual features of corn chromosomes. She noticed that one strain of corn had the strange characteristic that a particular chromosome, number 9, tended to break at a fairly high rate at the same site. McClintock termed this a mutable site or locus. This observation initiated a six-year study concerned with highly unstable chromosomal locations. In 1951, at the end of her study, McClintock proposed that these sites are actually locations where transposable elements have been inserted into the chromosomes. At the time of McClintock’s studies, such an idea was entirely unorthodox. McClintock focused her efforts on the relationship between a mutable locus and its phenotypic effects on corn kernels. Chromosome 9 with a mutable locus also carried several genes that affected the phenotype of corn kernels. Each gene existed in (at least) one dominant and one recessive allele. The mutable locus was termed Ds (for dissociation), because the locus was known to frequently cause chromosomal breaks. In the chromosome shown here, the Ds locus is located next to several genes affecting kernel traits. ment of Ds occasionally causes chromosome breakage (Figure EG17.2.1). Keep in mind that the endosperm of a kernel is triploid because it is derived from the fusion of two maternal haploid nuclei and one paternal haploid nucleus (refer back to Chapter 2, Figure 2.2c). The kernel shown in Figure 17.10 was produced by a cross in which the pollen carried the top chromosome—CI Sh Wx Ds—while the two maternal chromosomes are C sh wx. This kernel is expected to be colorless, because the CI allele is dominant and causes a colorless phenotype. However, as the kernel grows by cell division, the movement of the Ds locus out of its original location may occasionally cause a chromosome to break, and the distal part of this chromosome is lost. This chromosome breakage may happen in several cells, which continue to divide and grow as the kernel becomes larger. This process produces a sectoring phenotype—patches of cells occur in the kernel that are red, shrunken, and waxy. By analyzing many kernels, McClintock was also able to identify cases in which Ds had moved to a new location. For example, if Ds had moved out of its original location and inserted between Sh and Wx, a break at Ds would produce the following combination: In this case, there are three genes that exist as two or more alleles: 1. 2. 3. C is an allele for normal kernel color (dark red), c is a recessive allele of the same gene that causes a colorless kernel, while CI is a third allele of this gene that is dominant to both C and c and causes a colorless kernel. Sh is an allele that produces normal endosperm, while sh is a recessive allele that causes shrunken endosperm. (Note: The endosperm is the storage material in the kernel that is used by the plant embryo to provide energy for growth.) Wx is the allele that produces normal starch in the endosperm, while wx is a recessive allele that produces a waxyappearing phenotype. During her intensive work, in which she studied corn chromosomes under the microscope, McClintock identified strains of corn in which the Ds locus was found in different locations within the corn genome. She could determine the location of Ds because the move- FI GU RE EG1 7 .2.1 T he se ct oring tra it in c orn k e rnels. Ge nes→ →T ra its This kernel is expected to be colorless, because the CI allele is dominant. On occasion, though, the movement of Ds may cause a chromosome break, thereby losing the CI, Sh, and Wx alleles. As such a cell continues to divide, it will produce a patch of daughter cells that are red, shrunken, and waxy. Therefore, this sectoring trait arises from the loss of genes that occurs when the movement of Ds causes chromosome breakage. Note: The movement of Ds does not usually cause chromosome breakage. In most cases, the movement of Ds out of a site is followed by nonhomologous end joining (described in Chapter 16) in which the chromosome pieces are connected together. © 2010 The McGraw-Hill Companies, S.r.l. - Publishing Group Italia Robert J. Brooker - Genetica This genotype would produce patches on the kernel that are red and shrunken but not waxy. In this way, McClintock identified 20 independent cases in which the Ds locus had moved to a new location within this chromosome. Overall, the results from many crosses were consistent with the idea that Ds can transpose itself throughout the corn genome. McClintock also found that a second locus, termed Ac (for activator), was necessary for the Ds locus to move. Researchers later discovered that the Ac locus contains a gene that encodes an enzyme called transposase, which is necessary for Ds to move. We will discuss the function of transposase later in this chapter. Some strains of McClintock’s corn contained the Ac locus, while others did not. During her studies, McClintock noticed a particularly exciting and unusual event. By making the appropriate cross, she sought to produce kernels with the following genotype: To further study this phenomenon, McClintock carried out the experiment shown in Figure EG17.2.2. As seen here, a cross produced kernels that were not entirely colorless. In most cases, red sectoring occurred because the Ds element left the C gene in a few cells during the growth of the kernel. However, in the ear of corn shown in Figure 17.11, one kernel was completely red. This suggests that the Ds element transposed out of the C gene prior to kernel growth. In this case, the Ds element transposed out of the C gene during the formation of the haploid male gametophyte that produced the pollen nuclei that fertilized this particular red kernel. Therefore, all of the cells in this kernel would have a red phenotype. (Note: Every kernel in an ear of corn is equivalent to a distinct offspring, because each is produced from the union of haploid cells from the male and female gametophytes.) THE HYPOTHESIS The transposition of the Ds element into the normal C gene prevents kernel pigmentation. When the Ds element transposes back out of the C gene, the normal C allele is restored, which gives a red phenotype. THE DATA The kernels were expected to be red. Because the strain also contained the Ac locus, breakage would occur occasionally at the Ds locus to produce colorless patches. Among 4,000 kernels, she noticed one kernel with the opposite phenotype—a colorless kernel with red patches. This observation suggested that the inherited genotype in this case, which produced a colorless phenotype, was mutable to become C. How did McClintock explain these results? She postulated that the colorless phenotype was due to a transposition of Ds into the C gene: When Ds was located within the C gene, it inactivated the C gene, thereby resulting in the colorless phenotype. However, McClintock proposed that when Ds occasionally transposed out of the gene during kernel growth, the C allele would be restored and a red patch would result. In this case, the formation of patches, or sectoring, was due to the movement of Ds out of its location within the C gene and the rejoining of the two ends, not simply due to chromosome breakage. According to this hypothesis, the red phenotype should be associated with two observations.First, Ds should have moved to a new location. Second, the restored C allele should no longer be mutable because the mutable locus had been removed. Strain From parental cross (see step 2) From red kernels (see step 3) Kernel Phenotype Colorless background with red sectoring Red kernels Location of Ds Within the C gene Mutability? Yes, red sectoring occurred instrains containing Ac. Ds had moved out of the C gene to another location. No, the C gene was stable; no sectoring was observed. INTERPRETING THE DATA By conducting the appropriate crosses, McClintock found that in the progeny of a solid red kernel, the Ds locus had moved out of the C gene to another location (see the data of Figure 17.11). In addition, the “restored” C gene behaved normally. In other words, the C gene was no longer highly mutable, and the kernels did not show a sectoring phenotype. Taken together, the results are consistent with the hypothesis that the Ds locus can move around the corn genome by transposition. When McClintock published these results in 1951, they were met with great skepticism. Some geneticists of that time were unable to accept the idea that the genetic material was susceptible to frequent rearrangement. Instead, they believed that the genetic material was always very stable and permanent in its structure. Over the next several decades, the scientific community came to realize that transposable elements are a widespread phenomenon. Much like Gregor Mendel and Charles Darwin, Barbara McClintock was clearly ahead of her time. She was awarded the Nobel Prize in 1983, more than 30 years after her original discovery. © 2010 The McGraw-Hill Companies, S.r.l. - Publishing Group Italia Robert J. Brooker - Genetica Starting material: The male pollen of the corn plant was homozygous for a chromosome in which the Ds element had moved into the C gene: CDsC Sh wx. The male plant also contained the Ac locus. The plant contributing the female gamete was homozygous for a chromosome carrying c sh Wx. FI GURE EG1 7 .2 .2 Evide nc e for tra nsposa ble e le m e nt s in c orn. © 2010 The McGraw-Hill Companies, S.r.l. - Publishing Group Italia