The Origin and Behavior of c-m2; The Origin and Behavior of wx-m1
Series of lectures given by McClintock at the California Institute of Technology. The notes are partially written in McClintock's
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1954-02-18 (February 18, 1954)
Original Repository: American Philosophical Society. Library. Barbara McClintock Papers
Reproduced with permission of the Barbara McClintock Papers, American Philosophical Society.
Medical Subject Headings (MeSH):
Crossing Over, Genetic
Introduction to Study of Variegation in Maize (January 7, 1954)
The Discovery of Ds. Its Behavior and its Relation to Mutable Loci (January 11, 1954)
The Genetic Location and Cytological Behavior of Ds [14, 18, 21 January 1954]
The Inheritance Behavior and the Mode of Action of Ac (January 21, 1954)
Continuation of Inheritance Behavior in Ac (January 25, 1954)
Continuation of Transposition of Ac; and its Relation to Somatic Sectoring and to Controls of Mutation of Mutable Genes (January
Changes in State of Ac -- the Effects; Breakage-fusion-bridge Cycles in Endosperm; Transposition of Ds (Begin) (February 1,
Continuation of Transposition of Ds; Locations of Ds in Short Arm of Chromosome 9. The Missing Regions; Why. The States of
Ds (February 4, 1954)
The Mechanism of Transposition of Ds; Origin and Behavior of cm-1 (February 8, 1954)
Review of Previous Discussions [11, 15 February 1954]
Conclusions on c-m1; Outline of bz-m1; Begin c-m2 (February 15, 1954)
Continuation of Discussion of wx-m1; The Induction of Mutability at Selected Loci; The States of a1-m3 (February 22, 1954)
The Breakage-fusion-bridge Cycle and the Origin of Instability at Gene Loci (February 25, 1954)
Spread of Mutation Change Along Chromosome. The a1 m-1; a1 m-2 Control Systems (March 1, 1954)
The Interpretation of Origin and Behavior of the Controlling Systems (March 4, 1954)
Box Number: 2
Folder Number: 1
February 18, 1954
The origin and behavior of c-m2; the origin and behavior of wx-ml.
I. Review of previous discussion.
1. Attempted to show the action of Ac on two different loci it controls when these are located in two different chromosomes
in the same nucleus.
a). Both respond alike in any one kernel to doses of Ac -- 1, 2 and 3 Ac.
b). Somatic changes in Ac leading to formation of sector with altered dosage action -- reflected in the mutation or break
behavior at both loci.
2. Question would appear: Does Ac control not only the time during development when changes at c-ml or Ds will occur, but
also the cell in which it will occur?
a). The test of this: c-ml wx / c-ml wx / c Wx Ds, Ac Ac ac. The sectors in which the Ac action had altered examined to
give patches of C and patches of wx. Do these patches coincide? Do they represent events occurring to c-ml and to Ds in
the very same sell?
b). How this could be told:
Assume, during development, mutation to c and break at Ds occur in the same cell:
All ceils arising from this one would be C in genotype
All cells arising from this one would also be wx in genotype.
Sector would be formed from this cell: The outer part of this sector terminates in the aleurone layer -- C color. Underneath,
the starch in the cells of endosperm would stain wx:
[Diagram = "Diagram of the sector"]
c). Would expect coincidences to appear in high numbers, only if the states were reciprocal in this case -- the state of c-ml
must give a very high frequency of mutations to C and few if any breaks. That of Ds must give a high frequency of breaks
to eliminate the Wx. Deviations from these two types of behavior would not give a coincident C wx sector.
d). In the test made, a high coincidence of C aleurone with underlying wx starch. This indicates that both c-ml and Ds are
altered in the same cell at the same time.
3. Conclusions: The coincidence observed suggests that Ac effects the expression of Ds, wherever it may be located and regardless
of numbers, in the same cell at the same time -- within limits difficult to determine.
The time during development and the cells in which Ac-controlled changes at the loci it controls is not a random event. Some
conditions must arise in these cells, under the influence of Ac, that results in changes at both c-ml and at Ds-standard location.
II. Review - continued:
1. Attempt made to show that the control of when and in what cells Ds breaks or mutations at c-ml will occur depends upon
the state as well as the dose of Ac. Used "stabilized" Ac state as an illustration.
2. General conclusions:
a). Ac controls when and where changes will occur at the various loci it controls -- cm-l, bz-ml, Ds at any location. This
by dose of Ac, and by state and dose of Ac.
b). The consequence of a change at an Ac-controlled locus depends on the state of that locus itself:
3. The question of Dr. Lewis: Does the addition of Ds loci to the nucleus have any effect on the action of these loci -- that
is, modify Ac action? Answer: No effects have been noted of the addition of Ds to the action of Ac. Appear to be independent.
4. Discussed briefly the origin and behavior of bz-ml. Much like that of c-ml.
5. Began the discussion of c-m2; its origin.
III. Continue account of c-m2, page 3 of Feb. 15 outline.
IV. General conclusions from discussions of c-ml and c-m2.
1. Mutations of c-ml: To original C type action. Intermediate alleles not produced.
2. Mutations of c-m2: Exhibit a variety of phenotypic actions quantitative and qualitative differences in action exhibited
by the various different mutants.
3. What does this mean with regard to the genic action at the normal C locus
4. Does the action of c-m2 indicate the C locus is compound?
5. If not, what is the nature of the change, or series of changes at the locus of c-m2 that is responsible for the variety
of different types of action?
6. How may we interpret genic action at a single locus? Does some type of control exist at the locus which alters the type
and degree of genic action? Can we explain the behavior of c-m2 on some basis other than that it reflects a compound gene?
Is there some better interpretation and how can it be tested?
7. Before attempting to find answers for these questions, it would be better to wait until more of the actions at mutable
loci have been considered.
The origin and behavior of wx-ml
I. The origin:
c sh wx ds ac females x (c-m2 Sh Wx)/(c-m2 Sh Wx) Ac/Ac male
Kernels: Nearly all c to C Sh Wx.
One kernels: c-C Sh; wx to Wx. Wx spots in a wx background.
Aleurone layer sliced off and starch in endosperm cells below stained with I-KI solution: Areas of different intensities of
blue staining in a red-staining background.
II. Plant from this kernel grown. Pollen examined:
1. Pollen: How produced; haploid condition; starch in grain;
Appearance with wx; Appearance with normal Wx -- deep blue.
with I-KI solutions.
Pollen of particular plant: Most grains wx.
Few grains staining blue with I-KI
Intensity of stain varied from pale lavender to intense blue.
Indicated that a mutable wx locus present.
Tests, indicated below, showed that mutable wx carried by the c-m2 Sh chromosome. Origin, then, in the c-m2 Sh Wx parent
2. The crosses of this plant to those that were c sh wx ac:
a). Linkage relations of c to C and wx to Wx kernels indicated the presence of a mutable wx in the c-m2 Sh chromosome.
Constitution of plant: (c-m2 Sh wx-ml)/(c sh wxs) Ac/ac
Appearance of wx to Wx in 1/2 of the kernels, the linkages with the c to C variegated kernels, etc., suggested Ac control
of mutations at wx-ml.
3. The cross by c sh wxS ac also showed evidence of Ac control through dosage action -- speckled for c to C mutations; spk
for wx to Wx; Early mutations of c to C also early mutations of wx to Wx alleles.
4. Plant crossed by pollen from a C Sh wx Ds ac tester plant:
a). wx to Wx, and speckled pattern, only in those kernels that were also C to c variegated due to Ds breaks in C Sh wx Ds
b). Sector with changed Ac action -- affected in same manner both the C to c areas (breaks in C Sh wx Ds chromosome) and the
wx to Wx mutations.
[Diagrams = "Diagrams of types"]
5. Subsequent tests: no doubt about the Ac control of mutations of wx-ml.
Begin here. Feb. 22
III. The nature of the mutations occurring at wx-ml.
1. The quantitative nature of the action of the normal alleles, Wx and wx.
a). wx wx wx endosperm -- starch is amylopectin. Stains red with I-KI
b). Wx wx wx endosperm 22% of starch is amylose, 80% amylopectin. Stains blue with I-KI
c). Wx Wx wx endosperm 25% amylose. Stains blue with I-KI
d). Wx Wx Wx endosperm 28% amylose. Stains blue with I-KI
e). Physical appearance of the different kernels above: wx wx wx is like candle wx in appearance; Wx -- more translucent.
2. the known wxa mutant from S.A. -- one dose, 0.65% amylase. Stains very faint lavender. Physical appearance, waxy.
3. Mutants produced by wx-ml. On criteria of both staining and physical appearance, many different levels of production of
All different types can appear in same endosperm:
c wx/c wx/c wx-ml. 1 Ac:
[Diagram = "Diagram of types"]
4. The germinal mutations. How obtained?
Female x Male
c wx x c wx-ml Ac/ac
a). Types of germinal mutations: Whole kernels with mutation to one of the alleles of Wx. Stain from faint lavender to very
deep blue. Physical appearance follows the staining reaction -- from waxy to very translucent.
b). The low amylose allele: Show faint staining. Majority are associated with altered growth conditions in endosperm -- outer
region of endosperm seems rough and wrinkled. This condition not seen in the kernels with the higher alleles of Wx. Condition
expressed as a dominant.
b). In above cross, two types of kernels with respect to stabilities: those that show no change in staining throughout the
kernel and those with areas of altered staining reactions -- wx, light lavender or dark blue.
c). The two types associated with presence and absence of Ac. Stability of mutant when Ac absent; When Ac present, further
mutations occur, although, as with the pinks from c-m2, the rate much reduced over wx-ml or c-m2.
(1). When Ac present, mutations at wx-ml to various alleles of Wx -- quantitatively expressed. When Ac absent, no change
at the locus.
(2). Germinal mutations occur to various alleles. These stable in absence of Ac but unstable when Ac present.
(3). Behavior of wx-ml quite similar to c-m2 in essentials; quantitative expressions; two types of mutants; relative stability
of the germinal mutants with Ac in comparison with either original, that is, c-m2 or wx-ml rates.
6. The additive effects of doses of the mutants with low amlyose production.
a). Breakage-fusion-bridge cycle:
Female, wxs x male, chromosome 9 with broken end and low allele of Wx from wx-ml:
b). The dosage action of the lower alleles in 1, 2 and 3 doses: No Ac:
(2) The change in physical appearance with increased doses:
wx wx wx
Wx-allele, wx wx
Wx-allele, Wx-allele, wx
Wx-allele, Wx-allele, Wx-allele
c). Chemical analyses of amylose content of the various alleles and doses of them: (Made by Sprague and Brimhall, Iowa State
[Table = "% amylose starch]
IV. General Conclusions:
1. In plants with c-m2 and normal Wx, and Ac, change occurred at Wx locus to produce the wx-ml condition.
2. Mutations occur when Ac present -- at times dependent upon dose of Ac and its state. No mutations without Ac.
3. The types of mutations in both c-m2 and wx-ml much alike although the reactions effected obviously not the same -- anthocyanin
vs. starch structure.
4. The unit responsible comparable to Ds but not necessarily Ds as in other cases reported. May be similar type of unit,
however. Responds to Ac.
5. Two types of mutants at wx-ml. (a) Those altering amount of amylose produced, and those (b) giving a change in the structure
of the kernel -- a morphological alteration related to (?).
6. The types of mutations occurring at wx-ml are not typical for all mutable conditions arising at normal Wx locus. Some
behave like c-ml, that is, wx to full Wx; Their origins independent in each case.
7. It is not the mutations of the gene that are being expressed in all these cases (c-ml; c-m2; bz-ml, wx-ml) but changes
in the gene controlling materials that are associated with them or become associated with them. These effect how the genic
substance at the locus can act. Changes in them control the type of change in genic action. When and where these changes
will occur depends on Ac.