Download 17.2 McClintock Found That Chromosomes of Corn

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