"The Suppressor-Mutator System of Control of Gene Action in Maize" (Part V, "Origin and Behavior of a2m-1")
Part of a longer manuscript draft that reflected McClintock's increasingly complex understanding of the suppressor-mutator
as represented in her Carnegie Institution reports after 1956.
NOTE: Pagination is not sequential; several page numbers are duplicated. In addition, page numbers become erratic after page
206; subsequent pagination is extrapolated.
NOTE: Annotations are written in McClintock's hand.
Number of Image Pages:
5 (364,130 Bytes)
1959-10 (October 1959)
Original Repository: American Philosophical Society. Library. Barbara McClintock Papers
Reproduced with permission of the Barbara McClintock Papers, American Philosophical Society.
Medical Subject Headings (MeSH):
Gene Expression Regulation
The Suppressor-Mutator System of Control of Gene Action in Maize (October 1959)
Box Number: 3
Folder Number: 1
Part V. Origin and Behavior of a2m-1
In the progeny derived from self-pollination of a plant that had undergone the chromosome type of breakage-fusion-bridge cycle
in early development, an unstable expression of a gene concerned with chlorophyll development appeared. The locus responsible
for this was designated luteus-mutable although the location of it in the chromosome complement was not known. During the
early study of lum, a number of sister plants carrying it were self-pollinated. Some of these plants were C/c in constitution
and a ratio of three fully pigmented to one colorless kernel was expected to appear on their ears. This ratio was found among
the kernels on ears of all but one of these plants. On the ear of the exceptional plant, plant number 3908-5, in addition
to fully pigmented and colorless kernels there were some kernels that had spots of deep anthocyanin pigmentation in a colorless
background and the pigment type in them suggested that a gene locus resembling either A1 or A2 in action was responsible for
this. On this ear there were 214 fully pigmented kernels, 105 totally colorless kernels, and 35 kernels that had pigmented
spots in a colorless background. The pattern of variegation among the latter kernels was not alike. The majority of them
had some large areas of deep pigmentation and a number of smaller spots of this. Some kernels, however, had only small spots
of deep pigmentation and among them, the number of spots was not the same.
The following winter, five plants were grown from the variegated kernels on the ear just described under culture number 4062A
and four plants were grown from the colorless kernels under 4062B. Four of the five plants in 4062A were self-pollinated,
crossed to plants homozygous for c, and to plants carrying other gene markers. Similar tests were conducted with three of
the four plants in B of culture 4062. On the self-pollinated ear of the plants in 4062B, all kernels were totally colorless.
Two of these plants were C/C and one was C/c. Among the four tested plants in 4062A, derived from variegated kernels, three
were C/C and one was C/c (plant 4062A-1). The types of kernels that appeared on the self-pollinated ears of these plants
are given in table 1. Fully pigmented kernels, variegated kernels and colorless kernels appeared on all ears. These kernel
types suggested that the gene responsible for instability of expression of anthocyanin pigmentation was undergoing mutation
to give either a full expression of the gene or a null expression of it.
The following summer, plants were grown from more of the variegated kernels on the ear of plant 3908-5 and also from kernels
on the ears produced by crosses of plants 4062A-3 and A-4 and 4062B-1. The variegated kernels were graded according to the
pattern of pigmented spots, from those that had a number of large pigmented areas to those that had only a few small pigmented
spots. The plants derived from the variegated kernels were themselves variegated. Note was taken of the type of this in
each plant. It was obvious from these observations that some component was controlling the pattern of mutation and that this
component was undergoing change during plant development. Clearly defined sectors appeared in these plants in which the rate
of mutation was distinctive, each sector exhibiting one particular type of this. The differences were expressed in number
of pigmented streaks in a non-pigmented background, and in the size of these streaks. These plants were tested in various
ways including crosses with plants derived from the totally colorless kernels on the self-pollinated ear of plant 4062B-1
and to plants homozygous for the standard recessives of a1 in chromosome 3, a2 in chromosome 5, and a3 in chromosome 10.
These tests revealed that the locus responsible for the variegation expression was A2 in chromosome 5. Some change had occurred
at this locus in a cell of a plant carrying lum, and this was responsible for the subsequent unstable expression of A2. The
modified locus was therefore given the designation a2m-1.
Study of a2m-1 was continued for several more growing seasons but from the tests of it that were made, no clear interpretation
of the mode of operation of the system responsible for control of its expression. During this period, the kernel carrying
a1m-1 appeared on an ear of a plant in these a2m-1 cultures. Examination of its behavior, outlined in sections 2, 3 and 5,
made evident the system that was responsible for control of its expression. Realizing that a1m-1 had arisen in a plant in
which the a2m-1 system was operating and that it was possible that both a1m-1 and a2m-1 might be controlled by the same system,
study of a2 m-1 was recommenced in the summer of 1955. Evidence obtained during this summer made it clear that a2m-1 also
was controlled by a Suppressor-mutator system but this Spm in the a2m-1 cultures was behaving in a manner distinctly different
from the Spm element in the a1m-1 cultures. Some of the crosses made during the summer of 1955 indicated, however, that a2m-1
would respond to the Spm element of the a1m-1 cultures in the same manner that a1m-1 responds to it. Tests were then commenced
to establish the common type of response of a1m-1 and a2m-1 to the Spm element and the evidence of this is reviewed in detail
in part VI of this report. There is no longer any doubt that both a1m-1 and a2m-1 belong within the same Suppressor-mutator
system. The question arises, then as to why this system was not realized in the earlier study of a2m-1. There were two main
reasons for failure to recognize this. One of them involves cyclically changes in phase of action of Spm which was occurring
in the a2m-1 at times during development of the plant that obscured recognition of the presence of Spm. The other was the
appearance of a state of a2 m-1 that differed from all other isolated states, either of a1m-1 or of a2m-1. It gives rise
in both plant and kernel to variegated patterns of anthocyanin pigmentation but in contrast to other states, these do no result
from mutation at a2m-1. When attention was directed to the possible control of a2m-1 by an Spm element, and thus, when tests
to determine this were conducted, the nature of this state was quickly revealed. Subsequently, it proved to be a most important
state for examining the cyclically occurring changes in phase of activity of Spm.
The early tests of a2m-1 behavior and some of those conducted in 1955 and later will not be reviewed here. Nevertheless,
all of them have been useful in interpreting a2m-1 behavior in one way or another. Many of them illustrate change in phase
of activity of Spm but until this behavior was recognized, the tests with a2m-1 lacked the preciseness that is required to
establish this. Therefore, in this section, only some selected tests of a2m-1 will be discussed.