Continuation of Discussion of wx-m1; The Induction of Mutability at Selected Loci; The States of a1-m3
Series of lectures given by McClintock at the California Institute of Technology. The notes are partially written in McClintock's
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1954-02-22 (February 22, 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):
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)
The Origin and Behavior of c-m2; The Origin and Behavior of wx-m1 (February 18, 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 22, 1954
Continuation of discussion of wx-ml; the induction of mutability at selected loci; the states of a1-m3.
I. Review of previous discussion:
1. The C locus -- the origin of c-m2 from C in plants having Ac.
2. The types of mutations occurring at c-m2:
Class I: produces substance 1 in excess and substance 2. Color varies from very faint to quite dark -- a spectrum of mutations
in this class. Possibly the spectrum reflects amount of substance 2 produced.
Class II. Produces substance 2 in excess but substance 1 may be limited in those cases where color is light.
Some mutants -- plenty of both substance 1 and substance 2. These give rise to cells with exceedingly dark pigmentation --
as great as that produced by the normal C locus in 5 to 6 doses.
The normal C locus -- seems to produce insufficient amounts of substance 1. This may be the explanation of the quantitative
expression of dosages.
The hidden mutations -- those cases where change occurs at the c-m2 locus but gives insufficient color to be readily detected
in kernel. Detection comes from observations of effects produced on adjacent sector where visible mutation present.
II. The differences between c-ml and c-m2 quite obvious. c-ml produces only mutations to the normal C type. Only once has
a small sector on a single kernel shown a mutation resembling that of class I above and many thousands of kernels showing
mutations of c-ml have been examined.
If the units at the C locus -- presumably Ds in the case of c-ml, and something similar in the case of c-m2, -- were the same
in the two cases, we should expect to find some c-m2 like mutations arising from c-ml by some change in Ds that would simulate
or replace the former action. This is not observed.
Why do the two mutable C loci -- c-ml and c-m2 differ in their expression? What is controlling the expression of the genic
material when a change occurs at the locus, under the influence of Ac?
III. The types of changes that have been examined at the C locus:
(1) - c-ml from C Ac controlled
(2) - c-m2 from C Ac controlled
(3) c-m4 from C Ac controlled. This case when it first appeared gave mutations that resembled those produced by c-ml states
that show high frequencies of mutation to C and no breaks.
(4) c-m3. This arose from a normal C locus. It is not Ac controlled. The types of mutations: two only a) to the normal
C expression; b) to a pale producing mutant -- the sane pale color in all cases.
The recessives: Without Ac -- c-ml, c-m2, c-m4 are all stable recessives. The known recessive -- c -- is also capable of
showing color in the presence of a factor -- Blotch -- located in chromosome 6 by Dr. B.A. Emerson. The color produced very
clearly differs from that given by mutations of c-ml, c-m2, c-m3 or c-m4.
The common component of all these cases is -- a change at one position in the short arm of chromosome 9 -- corresponds with
the C locus. The changes in all cases affect the production of pigment in the aleurone layer of the kernel.
This indicates that there is something at this locus in the chromosome that is associated with the development of pigment.
Its mode of action, in this respect, however, can be controlled quite differently. This shown by the differences in the expressions
of this pigment in the cases described.
What is the nature of this control? What kinds of changes occur that result in specific types of genic action?
Possibly some answers to these questions can be had in examining the origin of other mutable genes, and the behavior of known
recessives to influences on them. The case of wx-ml will illustrate this.
IV. The origin of wx-ml -- previously discussed:
1. wx-ml was present in a single gamete among many thousands observed that were derived from plants carrying c-m2 Sh Wx and
2. This mutable locus -- proved to be Ac controlled.
3. The time and the cells in which mutations will occur -- like all other Ac controlled mutable genes, -- it responds to doses
of Ac, states of Ac, changes in states or doses occurring somatically in the same manner as other Ac-controlled mutables.
This proves by combinations of them in the same kernel -- like the cases of combinations of c-m2 and Ds, previously described.
4. The types of mutations occurring at wx-ml: A quantitative series, from a very small amount of amylose to the maximum amount
observed in normal material.
Continue with outline, page 4, February 18 discussion.
The induction of Ac-controlled mutability at selected loci.
I. From above discussions of origins and similarities of mutable loci derived from particular plants carrying known mutable
genes, one should be able to predict the origin of new mutable genes and their behavior.
a). If we assume that c-ml, c-m2, bz-ml, wx-m-l, 5,6,7,8, c-m4; arose from insertion of some controlling material at the locus
of the gene. If we know that this controlling material can be transposed from one location to another, then:
(1) In plants carrying both Ds and Ac, transposition of Ds could occur to various loci, among them, the locus of a known gene.
(2) By setting up the proper test conditions, such insertions, after they occur, should be detectible.
II. The nature of the tests conducted: The original test made as a "pilot" experiment as the frequency of occurrence
of transposition of Ds to a particular locus not known. The original test was small and no controls were done.
1. The nature of the pilot experiment:
a). Number of plants carrying Al Sh2 / A1 Sh2 in chromosome 3 and A2 Bm1 / A2 Bm1 in chromosome 5 short arm grown. Each plant
had Ds in one chromosome 5 long arm and Ac at an unknown location in the chromosome complement:
b). Pollen from two sources used on these plants:
(1) al sh2 No Ac; neither recessive mutates in presence of Ac
(2) a2 bml No Ac; neither recessive mutates in presence of Ac
2. The results:
a) 72 ears from cross (1). All kernels with normal appearing aleurone color except 1. This kernel showed colorless background
with colored spots.
b). 120 ears from cross (2). All kernels normal in color with exception of three. These three found on three different ears.
Each showed a colorless background with colored spots or areas.
3. The tests of the single kernel from cross (1). It proved to be a mutable gene -- designated a1-m4. It was Ac controlled.
Manner of control exactly like that of all other Ac-controlled mutables examined:
a). Without Ac -- completely coloress, stable.
b). With Ac: mutations of two main types: pale color, and full Ac1 color.
The differences in expression of these two classes of mutants:
[Diagram = "Pale, No diffusion rims"]
[Diagram = "Full A1, Diffusion rims"]
c). Mutations in the plants -- similar; will be considered shortly.
d). The germinal mutations -- like the mutations in kernels to give sectors. Pales; full A1.
e). The changes in state: Few changes in state noted. One type appeared on several occasions: colorless with only a few small
spots of the full Ac type -- diffusion rims.
The meaning of changes in state at the mutable al locus will be presented shortly in the examination of a1-m3, another Ac-controlled
4. The tests of the three variegated kernels from cross (2).
a) The female parent:
A2 Bm10 Ds Pr
A2 Bm10 ds pr
b). The three kernels removed, plants grown from them. Tests conducted to determine the nature of the instability expressed
in the kernels.
(1) One plant: In the tests of this plant, the chromosome 5 contributed by the female parent was not transmitted to the following
generations. Something wrong with it. No cytological examination made. This mutable condition lost, therefore.
(2) Plant 2: Proved to have a new mutable a2 locus. This arose in the A2 Bm1 carrying chromosome. In this plant, something
wrong at the Pr locus or close to it -- not male transmissible. The crossovers recovered -- carried a2-m4 Bm1 ds pr.
(a). a2-m4: Proved to be Ac controlled, and again this expression the same as that shown for all other Ac-controlled mutable
loci with respect to time of occurrence of mutations as an expression of Ac dose and state. The nature of the mutations --
many very pale mutations, few like full A2 with diffusion rims:
[Diagram = "Low rate of visible mutations in original state."]
The range of pales -- very light to quite dark; no diffusion rims. From frequencies observed in this initia1 state, many
hidden mutations suspected. They do not give any perceptible amounts of color.
(3) Plant 3: A new origin of mutability at A2 in the A2 Bm1 chromosome. This mutable is not Ac-controlled. Designated a2-m3.
It mutable behavior very different from that of the Ac-controlled types. Many of the mutations to full A2 expression and
some to pales of varying intensity. Many early occurring mutations in the plant; many changes to stable a2. Many changes
in state occur, and these take place early in development of plant. Plants may be highly sectorial for many changed states.
Can be extracted from these plants and maintained as altered states.
5. Conclusions: Although test is small, and no control, it is clear that new Ac-controlled mutable loci may be obtained and
at loci previously selected for this. The mechanism of control can be anticipated but the types of mutations that will occur
cannot be anticipated. This will be shown in the examination of a1-m3, an Ac-controlled mutable locus derived from a normal
A1 in plant carrying Ds and Ac.
The States of a1-m3
I. The origin of A1-m3.
Constitution of plant:
Tests being conducted to investigate the behavior of Ds in the Pr chr. Ear; self-pollination of this plant: Some variegated
kernels appeared; type of variegation suggested change at the A1 or A2 locus.
Crossed to a1/a1 plant: Showed that the locus involved was A1. No mutable present at this locus in sister plants.
II. The subsequent tests: This new mutable proved to be Ac-controlled. The expression of mutations differed markedly from
those of a1-m4 just described.
1. Must be emphasized: The time of occurrence of mutations completely controlled by Acand its does and state. The type of
mutations controlled by conditions present at the a1-m3 locus.
2. The original state: Rather dark pale background with very deep A1 spots or areas, each showing diffusion rim.
3. Changes in state occurred frequently in comparison to the infrequence of such occurrences in a1-m4.
4. The types of change in state are important. Effects seen in both the kernel and the plant.
III. The types of states: See chart.
1. Origin of a1-m3 in A1/A1 plant. One of the A1 loci affected. This plant carried Ds in chromosome 5 and Ac in unknown
2. Original state -- dark pale with no Ac; Mutations with Ac to full A1, to lighter pales, to colorless.
3. In course of study, a number of different states isolated. Each characterized by the types of mutations that occur in
the presence of Ac.
4. The relation of one state to that of the known a1-Dt condition. The changes in state observed in this a1-Dt association
-- by Nuffer.
5. The state of the normal recessive, a1. What type of mutation phenomenon would be expected if a new Dt were made:
6. The state of the mutable loci -- very important aspect of the control system. What differences exist among these states
that serve to produce these effects?