THE SUPPRESSOR-MUTATOR SYSTuM OF CONTROL OF GENE ACTION IN MAIZE

Part I
1. Introduction
2. General Review of Mode of Operation of the Spm System
a). Basic Elements of System
b). Mode of Operation and Behavior of Spm
ec). States of a= and ag t
a). Types of change in action of Spm pther than that associated
with change in its cycle of action
e). Transposition of Spm
f). Modifiers of the Spm System
g). Resume of Mode of Operation of the Som System
Part II
m-l

The Origin and Behavior of a>

1. Origin of 2,27 and of its States

2. Discovery of an Independently Located Controlling Element Associated
with a, Expression *

3. Initital Evidende pf the Primary Mode of Operation of the Indevendently
Located Controll#hg Wlement, “Suppressor-Mutator (Spm)

4, Additional Evidence “upporting the Interpretation of the Primary Mode
of Action of Spm

5. Individuality of Expression of Each State of a, at when Two States
are Present in a Plant or Kernel

6. Establishment of the Validity of the Interpretation of the Mode of

Oper:tion of the Spm - a, Bot System

Part III

Transpssition of the Spm Element in the a m-1 Guitures

1
Part &¥ LL

Altered States of Spm gh
Part IW

The Modifier in the Spm System

Part V%
The Origin and Behavior of aya
Part VII
Proof of the Identity of the System Controlling both a, a1 and a at
Part VIII

- ~~ wey Le iw lee
Types of Mutants Produced by a= 1 ana ao 1 Mi fe Hie le,

Part IX

Alternating Cycles of Activity of Spm

Part X

Conclusions
(Brink et al, McClintock, 1966 a, b, c).

og L vA \

r ” f : a

a

fo.
if 7 YF,
THE suroaussgh wtonnyh ovsoun OF @NTROL CF GENE ACTION IN MATZE
fe i =
yy oy ParvT ©
The manner by which controlling elements in maize affect the

’

action of genes and induce specific changes in this,has been examined

2 wd cudtcall

by a number of investigators in recent years. Several systems, each
A

compysed of one or more controlling elements, have been identified,
ten doe thinwrorl
It is vesogaized that controlling elements are unit companenks apart from
“A
the genes even though they are carried by the chromosomes and reside at
specific sites in them, It was possible to det+errine this when it was
learned that these elements may undergo transposition from one siteto

another in the chromosome complement, It was further learned that

the mode of control of gene action was a reflection of the specificity

4

ne
of operation of the individual components of a system and the interactions

that occur between them, Furthermore, it was learned that one such
system would control” the action of different genes in like manner,

For induction of operation of a particular system at a known gene
locus, it is only necessary that one of its elements be inserted at a
site within or close to thélocus of the gene, This is shown in the

two known cases of control of gene action by the element Ac (Activator)

at the P locus in chromosome 1 and the Bz locus in chromesome 9

*
2

MOS tgs)
pr Af 0:

eg

ry.

i“

a y ~

of
$

Footnote:

Brink and his associates have used the term Modulator (Mp) instead

of Activator (Ac). This term was given to the controlling element at the
be

P locus before it was recognized to/the same Ac element that operates

in the Ds-Ac system, first reported in 197.)

A system composed of the above mentioned Ac element and a second
element, designated Dissoaiation (Ds), also has shown that the mode of
control of gene action resides in thease sme controlling elements, In
this system, it is the Ds element that becomes inserted at or close to
the locus of the gene whose action is being wm ntrolled, The operation

WEL
of this system at a number of different gene loci hosbeen-fattemsivel y

examined by the author (McClintock, 1947 to 1956) and santedependentix
aniaing case of ses OU the Bay locus in chromosome 1 nes“Geen
investigated by Nuffer (195 ). This system also has been used for
specifie purposes by a number of @her persons ( Brink et al.,
Fabage, Schwartz, Oliver, Notani, Peterson,
Dollinger, Sprague )e

The Suppressor-mitator (Spm) system is anothor eae in which the mode

of control of gene action by its componenet elements could be examined

*
“

 

at more than one gene locus,

 
} :
addi Inebulol dee Pavone

x
\
ak fia
¥

‘ : . al 5
Its maite=eé.op ration at tee two game Locapi Ay in chromosome 3 and vuld
“A ot ’

4

A, in chromosome 5, hafe been compared; Both A, and A, are

associated with the production of anthocyanin pigmentation in vlant

yhoo

oy f
and kernel. a When a »lant or kernel is homoz. gous for recessive

pryuad “yfll®

allele, either @, OF &5, no anthocyanin appears in plant or kernel,

When Ag—and—s, are present, been the plant and the kernel are ddeply

gate oN wsly A yluk adhyy ap ipon. ccf

pigment, Of—the-tyo-eeses, the Spm system of control amass first at the

oxnurialioy

locus of A> and in the course of sessed: of instacility of expression of a

Lrawwuglion

gene associated with chkrorophyll development, In this stm, a number

aA

of plants of a culture were self-pollinated and on the ear produced by one

plant, kernels appeared that exhibited variegation for anthocyanin

0 owe QU aan aredee JRO va

pigmentation, Each showed areas of deep pigmentation, other areas of
“aw J

wd bacdkeowd Bok war

pale pigmentation amd—ett}} -ether—srersthet-were totally colorless,

Ung oittDy.p Pty oan

Kzominebten of the system responsible for the newly appearing guilt %tus

 

variegation commenced with plants grown from kernels on the above
As Osi, 2. WME Da yrsm cok, Kong cm il0 FY tiv
mentioned ear, It was soon realized that the varicration was initiated

hint God cesta te

by some modification e# the standard A, locus in chromosome 5 and—bhet—
A

this was associated withthe instability of iis expression, Tits
Vower fiw * ~ Pye Ly Pua Ww Alege: ut fa DD oO ”

3 could be exhibited implant or kernel whe the modified locus
A
was either homozygous or was heterozygous with the standard recessive,

Ade The modified As locus was then given the designation a, and

Study of it wes continued, th

LT | Yo clear impression’ of the mode of operation of the system
“ f 4 |
l responsible for variccation wes=geined-—fromthe Carly study of ao To
“Wunder Vapor Afowus palbuu zy Van alo wes Mot actly!
in the-eoyrse of this, howewer, the silks of an ear of a plant carrying
Duriuc, Tip pound,

the system responsible for control of gene action at as received

pollen from a plant that was homozygous for the standard recessive, ay»
carried in chromosome 3, and for the standard As, carried in chromosome 5,
In this cross, all of thekernels on the ear were expected to be fully
pigmented, This was found for all kernels except one, The exceptional
kernel exhibited varicgation for anthocyanin pigmentation, It had
spots of deep pigmentation in a colorless background, The plant grown
from it also exhibited varicgation for anthocyanin pigmentation and tests
conducted with it indicated that the variegation was associated with Al
modification of the standard Al locus that had occurred in a cell whose
descendents produced the female gametophyte in the ovule that gave rise
Lyons.

to the original variggated kernel. Th96 new modification, was than

given the designation amt and study of it was continued,
Karly examination of a, behavior suggested that a relatively

simple system was associated with control of gene action but as the

study progressed, it became evident that the system was far from being

a simple one, Nevertheless, some appreciation of its mode of operation
was gained during this period, At the time, study of ay Tl yee kad (On

G
discontinued as no clues had been grasped of the mode of operation of the
a

system associsted with it. After tne basic mechanism responsible for

control of gene expression at amt was recognized, study of at was

recommenced, with the expectation that some cintiante taht be found

.

toys aa th,
in the control systems associated with each, It was ‘saan. ‘realized that

fees
the mode of control of gene expression was essentially the—same for both

m=1 m=1

ay and a. ° With this in mind, further tests were conducted and

these, in turn, proved that gene action at both of these loci was controlleé

by the very same system of elements, By means of these concurrent

M=] m=1

studies of ay and a5 » confounding appects of behavior given by

each, when exemined alone, were resolved, It then became apparent that

lack of recognition of the cyclically occurring changes in action undergone

pvr

by the dominant element of the system, Suppressor-mtator (Spm), was the

, pine dene

mode of operation of the

 
  

These cycles of
of change in phnse of activity of Spm are responsible for many

different types of phenotypic expression that anpear within individual
plants or kernels and also for highly dqscrepant ratios of particular
phenotypes that appear in progeny derived from either self-pollination or

from test crosses of some plants,

jew
5 == anode o_O pousillow oy thy Zn Agito Re or

 

oo nn —
An outline of the mode of operation of the Spm wrote will be
given here in order that the following sections, each dealing with some
yy

particular aspect of this, may be read with appreeiation of the operation
A

of the system as a whole, The Spm system is considered to be composed,
around bbe
» of two hemi controlling elements, One of these is located

“\

at or close to the gene whose action is being controlled, and in the cases

under discussion, at the locus of A, in chromosome 3 and of A

Srourwatebe

chrompsome 5, The other controlling element, Spm, is located wlsewhere
A

tr Darah J Why

haoLetienelament undergo¢é$ transposition

D in
in the chromosome complement,
from one location to another, in-the complement. On different occasions,
8 transposition has resulted in insertion of it at a site sufficiently

e¢ lose to a known gene marker to allow ready detection of the new location
by means of linkage studies with the markert and its insertion at and

transposition from spaeific sites in chromosomes 3, 5, 6, and 9 have been

examined,
It has not yet been proved that a transposible element resides
at the locus of the gene whose action is being controlled by the Spm

system, ‘That this is te be expecte@g is indicated by evidence of the peesen
OL Ly a ody wat ater a re EA ER AUM

presence of such an element at the gene locus obtained from study of
A

anotner two element control system, Ds- Ac, It could be shown that the
Ds element of this system is inserted at or near the locus of the gene

whose action is being eonteottes by the system, and that Ac resides
BU Re ae av
elsewhere in the complements | Ds i lmowr-te=be_a transposible element,

Again, the fact that amt ar@se in a plant in which the a Mod som

2

m-1

System was operating, and that the behavior of both at and a.

are impressi¥ly alike, strongly supports the assumption that theseme

cud ak Fo thy ome aloud vn btllCeuy

Q transpésible element resides at each of these two loci, and—thet eae@h
A

at quay

1

le Conniaitr ut, Thou, Uiset 2000,

arose from insertion of this element, first from an unknown location in
the chromosome complement to the locus of A, and subsequently to the locus
of A of tly poops -
1° A) Thode chum aud belavee, { wp Sun Liteuoel
In each of the two cases under discussion, modification of gene
action probably occurred as an immediate consequence of insertion of Thy

cubeme anewtllowed — gq

tts controlling element at the locus, In both cases, thts appoars to

 

have effected a reduction in capacity of the gene to contribute to
“P

 

have
g-~ Im plants and kernels that axa A, and A, » anthocyanin pigment

intensity is deep either when Spm is present or when it is apsent,

In contrast to this, pigment intensity is reudced in those kernels and
Croc hekgur £OLO- oul, wine Aes)
plants that are homozygous for the original st=te of either a,
T= suc .
as or are heterozygous fer-ene—of-—tkhese_and the corresponding recessive
\b
pp pod ‘wi Spa
allele, whenever Spm hecabsent or r,inactive.q When, however, * is present

m1
or

foe
{ Koepemee, The constitutions of th 1ese plants would be:

| m=] m=], mL m-1 ,

| for a, 3 AjA, or Ap/an3 a, ~/a4 or a, Ja, Cul

4 : me m1, m-1 ~ i

\ bor ar 7 AyA, or Aj/ays ag [a2 or a5 » Le J
in Jhentey, ove of

and activef both plantq and kernelg ae these seme ie

anthocyanin pigment formation seen beanie in well defined areas,

a

1. Within-_these-sreas;—pisment—apperas, _hree

distinetiy different types of events are responsible for the appoarance

 

The first of them is associated with some
wo, * i. @,a%

either

of these pigmented ALEAS 6

x
ME eee ee event occurring at the locus of the gene concerned, >,
\
Sy edrs da Lout Cave

or ast as the case may be, Fees results in reestablishment of
A

m1
ay
the full or near fvll capacity of the locus to contribute to anthocvanin

prvduebin  mubtalur - RLF

formation, The’ events occurg in individual cells whose progeny cells

ove na

may then exhibit intense pigmentation, When thee occurs in a cell of

the germ line of a plant, gametes may be formed that carry the locus

ope A, Buto eS wednvndan2d
with restored activity, and OA a Mule may be detected the progeny.
annals) muxaurr- Vale overt olay wil dweo

Tests of such peceeny indicate that thi@ modttitesticon-nesetes in stability
aM wo Justhcr 2 Ua en Up ackiny eesun, cle the
of gene expression tam in the presence and/absence of Spm,

ited mex rnd wa Thy Of peat. of.
, bhe second and third tepeeoot event /pive ;
Racy untied 2 Cuber (oucbyya , yD maliny: aL wep aya OO Gaup ~

aupaaming azeas bet, the cuaticraducneitiedter eba teaiteorent: artthe

 

quel consequences of tke evembs-avre skep distinctly different. the first of
frou ine Lr alin ty Besley wile Pruner Yuniod
these is associated with a SOmetcallyoseuemime trans osition of Spm MW
oO POWAY A Lb V 6A aby | ‘et bay UU
Weteh, through Subsequent segregation of chratids at A mitotic anaphase,

wit wa Up aoe — ck UW) Tn austen
MUL emer £ from b’moikhe. The progeny cells arising from one in which
pal anes’ coe
Spm has been removed, exhibit the reduced grade of pigment intensity
A A
WA

that ec}

 

apn ca rance—of whole plants or kernels in which

ViePsy tod a Orta. jail wo ay aay 2,20, oul I
s — 1S a
pm is azosent Erephenet SRUVESE r 8 ont ire-oneny
Lewes, w Progelay bo LOD Ov Lure potlen prodliucsd wali
Kemnels—aad-pionts derived from such an area, is —quite—siable. res.
w CLbe
Spite aiecht) mo further change | occurs at the Locus of —bhe—-SOReed EOLA
Vrlpae Shun iy OS dun Wrobel wm ope rubsteucet ADP,

The third type of event gives rise to pighented aras exnibiting

  

* CY
the very same phenotype as that just destribed, However, the cause edt
$s

LO

this is not removal of Spm from the nucleus but rather its complete

4p
inactivation without change im i0cavian. The phenot*pic expression Mull
this inactivation pmedwees is stable as long as Spm remains in sas tho
inactive vhase, Return to the active phase in some subsequent cell
again will initiate not only suppression of gene action in the immediate

QUrd mor

progeny cells, but also it will initiate ageén the same round of events
just described, Consequently, within some of the large lightly pigmented
are.s of a plant or kernel, smaller, non-pigmented areas may be formed and
within them, in turn, both deeply pigmented and lightly pigmented areas
may appear, Whether or not a variegated pattcrn of the type just
described will avpear following a particular inactivation of Spm, and
awa should it apgpesr, just what type of pattcrn of wariegation will be
exhibited, depends upon the duration of the inactive phase of Spm,
Following some inactivations, the inactive vhase may persist through
many cell or even plant generaticns. Following others, the duration
of the inactive phase may be short with frequently occurring returns to
the active phsse in some cells that woe a few cell generations
removed from that in wnich inactivation occurred, And, a large range

of difference in duration of either tee inactive or the mpactive phase

of Spm has been noted,
11

On the basis of the above description of events occurring at the

m1 m=] yy a Abe ’

locus of either ay or a. and , to the Spm element, it is evident that

O
af) variegated pattern in, plant} may be simple and readily interpreted if Wo

slommdd wl vr one Wud aun “| Love, decradn ,
apm weth~a—lLong duration efmisies active phase. ia=—present and if transpositior

| > thy plaut

of it is infrequent or does not occur during dyeing developmental stages.
“4

In contrast, the pattern may be very complex if the active phige of Spm

oudin Leblousd \ an outine hod -< omen doef,

is of short duration or if trenspositions of it occur in a number of cells

rather early in development.
va o. pewter as oud

When two or more active Spm elements are present, the freauency of
A

occurrence of the mutation-inducing events at either aot or a,

remains the same, In other words, the pattern of variegation produced

warty “cubation. Prucintes, Layeu¥s increased
ber suchceverts is wn apficbed by/dose of the Spm element, In contrast,

the pattern of variegation induced by trensposition or/ehence in phase of
Spm is much altered by increased dose of Spm. The number of pigmented
areas attributable to these events is decreased as the Spm mumber
increases, For this reason, the number of Spm elements in a plant or
kernel la-eble—+to effectcin a marked way the over-all pattern of

variegation, mhaerved However, in any one plant or kernel thet exhibitalt

only that pattern of variegation attributable to ma tion at the gene

locus it is not possible to knaw without test whether this pattern is
L2

ge ss wanlog Ss pourtu

produced because two or more/Spm elements are present or because of the
“A A

presench of only one Spm element which has a very long duration of its
abso ih Vutel -
i if ; fs Lie \ . +r
active a ok Vial Gutr {our oy Wo Cus pen AW, Los,

The time of change in phzese of activity of Spm during development

forty = Ww
of plant o» kernel also pri }-he—mediftertten of patterns of

variegation, With the original state of both amt and ast, mutations

Ui Wwe Orble
to a full or near full Axpression of gene action may occur early in
“A

 

plant or kernel development if a fully active Spm element wks present Ww

at the beginning of development, Such mutations give rise to large,

4

Lua, OM
deeply-pigmented areas in beéh plant amd kernel, However, if an

wna,
inactive Spm is present that returns to the active phase in some cells
mgr bE pracy” N
tees only late in development, only small areas ame pmodueed in which
A $
>

04? PLarditee
it is active, These areas are charactorized by the pregemee of small

3.
spots of deep pigmentation appessime in a non-pigmented background, ys
Ghyte
We oQuoe”
Since mutation to full capacity of gene action Sam occurs only sagem Spm
a we mp noBuito “th Dubwe pave
is active, delayeft change in—phase—of-setivity-of Spm ttould give rise
land Wadan Broun wo obi
Uiie, Shese—comithbens only to small spots of deep pigmentationy within
pee } bate Dae ALUN ,

the restricted aress in which it is cctivgh it is evident, thea that the

tyne of variegation exhibited by a plant or kernel may be. very CBE a

if the Spm element in it is undergoing change in phase of activity at
A

13
at diffcrent timeds during development, It is largly because of this
that the early study of Bo mel lead to no clear understanding of the
system involved in its control, although other conditions, which will be

fatty
mentioned shontly, likewise contributed to this, In contrast,

appreciation of the system controlling a,™t behavior was more readily 2
‘\
ulcer
atgained, The reason for tris is now evident, In the early studies of

investigated
it, the Spm element present in the plants/had a very long duration of it

Ey:

active phase, °o few changes to the inactive phase occurred during

db
development that these did not obscure the generally consistent ww

a
And, It was this consistent pattern. ot behavier tL Ayo

pattern of hemevier Ahat allowed interpretations to be drawn and~tasteef
A

Voce Eur Cue sadtup

c) Bere States of a and a. m= .

anyjedatin \luveg
One of the most oe alee aspects of the bemesbear of a controlling °.

element at a gene locus is the modification cf&ti-oae induced by he that

nowt mM ww SAP 4 puecbadion
will—sive—wise km ee either in,frequency oe ea
A to
meterttron abthe locust the gona, tim typé of mutation, or both in)

Lhe pallies. panecalion reacting Hoe few toteo “ow onan Cot ae sy.
type—and -frecueney of maitation i n eomparigon to that waich was sppearing S
~

Meee

before tni& modification occurred. Stmee €ach such modification is

wos STasn sgver.timn Long Pepusd seed wif Dy ho aval

heritable in that & continueg$ to produce its own particular pattern |
A

Vorefene , oun aus. modifica a

and—tese. of mutation in subsequent generations, 4

 

n
Ly

ono Tee
and with respect to a mod and Ao 1 these are termed changes in state

1

M1 m7]
of ay or a5 e

Study of several different control systems indicates that all
dak on2
manifestaticns of change in action of the gene, ofa heritable tap,
whether it be a stable mutation or an altered state, are expressions of

modification induced by the controlling element of the system aadoccallag

resides _at—the—loous—ef the gene. In a two element system such as Ac-Us,

bods srtoation ante Put
Dt-aj, En-palegreen, etc., Hae modification . 6

=

ef-kne response of tha clement at—the<gene—tesus to the presence of the
independently located element of the system, which is Spm in the cases

. . " . m-1 m-1
under consideration, Therefore, a change in state of a, or a,
will occur only when this element is present and active and only at
those times during development when mtation may also occur, A change
in state is, then, only one of several possible consequences of a single

response of the element associ=ted with the gene to the prescnce of the

corplementayy element of the system,

SM wre al wed
From, censidenation—of origing of new states, piven-eboye, it is

Dayan

etean that isolation of a newly altered state for subsequent examination

requires that it arise in some cell of the germ lineand be present in a

gamete, therefore, the chance ff isolation of a new state depends
15

largly upon the time during development of aplant when the controlling

element associated with the gene locus is responsive to the independently

(walldus Ran (aT *

located element of the system and, indeed, time of this is one the

i ’ a wvbah

distinctive expressions of] State. qf, ina plant, the statdis guek that

it responds relatively early in development and if such responses occur
in sporogenous cells, a number of gametes may be produced each of which
cavries either a mutation or an altered State, Both types of change

may be detected in the progeny of this plant, and each may thereby be
isolated for subsequent study. If, on the other hand, the state is one
in which the element associated with the gene locus responds to the
independently loceted element only very late in development of the plant,

few or no gametes may be produced that carry either a mutation or an

From new ue ato? ed)

altered state, Wik the former state, xhaagexkoxmkher states Gould | anigse
bone from ob peo
oud! with appreciable frequencies whereas wkk the later state they would apie

wi WO jnocoe inertia Cousitine rakabiuc to
woth very rarely or not at all, Considering the Sectors asscocicted
dak odbupn a vou atolls quale uw

wrth these origin and isolation, itis evident that no conclusions arr
JA “

regarding relative stabilities of the two states are justified from

PequLI ee. ANU’ gh anu ak ato wr poceng Tits

camparisons of hie type. With the latter state, as many changes of state

le
may Ceued with the former but there is no way of detecting and isolating

04 vdigae thaterp : ho ho,
whe those mooduees Be bu theletter, ‘Therefore, in this study, ait eath new

A. » ee Ne : .
Co, wi Loe ABU. wR ae . » v : aa
16

state was derived from one in which response of the element asseeiettd
A

With—the-perie Ieous to an active Spm occurred relatively early in
plant development,

As stated above, mutation to a stable expression of gene action and to

OW

change in state ave the consequence of response of the gene-associated
element to Spm. Methods are available for determining the relative
frequency of occurrence of different types of mutation to a stable gene
expression but it is difficult if not impossible to estimate accurately

the relative frecueney of occurrence of modifications effecting change in

the a mano! Ubu
state, This is because some_of them give ayiyes of expression, ‘dest
from
differs only slightly from that/whieh it originated, and it is often
aud. =o  andluseliot fous,

difficult to detected cases nents, (AThose that effect a marked change

mw py are On the other hand;) readily recognized, amd Qt is e#idens with both

JN obra
Ma] _ app caranes ctendloots

ay and a5" 7 that the mezuxxremre of ean states is considerably

frequent
less{¥han the Os Op aSbeaking « stable

gene expression,
17

From the original statebt am, whose gene~-associated controlling
element may respond to an active Spm very early in development, 18

independently arising changes of state were selected, Each was
those
recognized in an individual kernel appearing on AH eas that developed

when pollen of the original plant carrying at

was placed on silks e€
eans-of plants that were homozygous for the stahdard recessive, a

wba
\> Guus atodte w the prrtuey Spur.

A Observatinns were made of the variesated pattern in the plant derived

1

from each and each, in turn, was crossed to plants homozygous for ay

in order to examine the expression of the modified a,™t locus in resto
progeny. Subsequent fests were then conducted with the plants derived

from selected kernels in th@progeny in order to establish themode of

athe ond wrote hath ty prounve du oro Spy,

expression in subsequent generations

A : )
wlov y ‘ ~ 3
Whth regard to_oipmcosion of variegation, the 18 selected cases of

0 od yg

change in state of a, "| _-sne—r8-selectedwescss could be SLE MD
five main groups. The @ in the first group were charactcrized by some

early occurring mutations to full (or near full A, expression and very

aud an ate Span vo. gud fen thus

many late occurring mutations ee this types In contrast, thé original

owed lanl

state gives rise to many early occurring mutations to various levels of

A
bet ® Qk

A, ex-ression, and t& fewer later occurring ones, inthe rbseree
18

at
In the absence of Spm, the expression of the GE cored states in the
first group diffcre@l., With each, pigment developed slowly in the plant
until it is quite intense but in the kernels, orf of the se two is

charact rized by the appcarance only of faint pigmentation wh-reas the

pour, Hd Thr de qvuds woe wee fc uuowtad
hw otnen . giuns wise to deeply pigmenbed_kesnets, In this regard, the

original state expresses a medium grade of pigment intensity in the kernels
in the absence of Spm,
Fue dourd bo j.lacsd wa a
There-were § altered states iacte second group. These weno
characterized by production, in the presence of %D m, of late and very
frequently occurring mutations in both plant and kernel, which usually
gave the full or near full expression of pigment intensity. Again,
subdivisions of them could be made becatise of differences in degree of
pigment intensity that were expressed in kernels in the absence of Spm.
QL “our of the 18 selected altered states fall within a third group.
These were characterized by the appearance, when Spm was present, of

muntaul wiles Wok are apretl Heseung Wo syrusebian - Ann oilers, LAsQuUty

matetieons—thes occurre#@ late in development of both plant and kernel, part

au, Lyi ty Mure dn Daten
tour, the frequency of this wes much less than that guiasem by members of the
A ;

group just described, Here, also, they differe@ among themselves with

regard to intensity of pigment appearing in sme kernels when Spm wee 4

absent.
19

QQ
There ware two altered states in the fourth group. Very few

Gwe paounliy,» are 2A
mutations to an Ay : in either plant or kernels

dotho wer uh ioe rLinslns dur clweyp auall,

Seed
and all of them arése late in development 4 The remaining 5/cases f@ll
within a fifth group. In the presence of Spm, nearly all of the mutations

maxexka expresséd only low levels of pigment intensity ahd the type of

With to 4 lino absly ,

pigment appears to differp from that produced by Ae thm—_seme—ot—_thent.

aah Cocasionaligmutant area ‘appeared that exhibitéd the full Ay type

a aul. aera Lous dud he Orta abweny Wa omat.
xmomexknemx the st&te& within this
vot ovtt-

group differ from one another beth in the time of occurrence of mutation,

   

“in the presence of Spm, > Uo
A end also in the degree of pigment production in the absence ef Sam,

anthocyanin
With one, no/pigment is produced in either plant or kernel in the absence

of "om. In this respect, the phenotype is auite similar to that
appearing in plants and kernels that are homozygous for the standard ay

recessive,

. rs
a s,

oo .
From the total of 18 examined) ceses—ef altered state, 6 have been

widely used over a period of years in study of the Spm svstem,

Photographs of kernels, figures >» Wilustrate the the phenotype each

produced in both) the presence and absence of Spm,
eu A
20

m1

Detection and analysis of altered states of a,

wasuncomplicated

because, as mentioned earlier, the Spm element in the original plant,

and in the progeny derived from it, had an active phase of long duration.

. it
‘lad kwex undergone frequent change in phase of activity, variegation

quant Lubioh,

patterns and inheritance behevior would have been so complex that no

4

certain conclusions govld have been drawn from them of the precise nallnpd

Ma
state of a, 1 that might be present in a particular plant or kernel,

Amare) Conedutinn m1
Such were the clneumetanees encountered in early study of ao .
A

Variegation patterns and inhsritance behavior were so complex that

even though different states were recognized, a precise characterization

Wo loco? rollakion
of each could not be formulated ae the eontributien of cyclical change

in activity phose of Spm to altered expression of variegation and BO

puoauies notung phaoly jlo m yeoctun, Ud ih 20

e had not yet been It was not until

these states were exanined in the pre senceof an Spm element with axkang
an active phase of long duration that it was nossible to charactcrize them

precisely. It was then found that their types were similiar to those

Me]

that had appeared in the ay cultures, There was one, however, that

m=

1 . Initially, its

was quite uhlike any of the selectcd states of a

behavior was an additional cause of confused impressions of the mode of
ye ce

21

operation of the system res~onsible for control of gene action at a, +

that were generated early in the study. Subsequently, it proved an
invaluable tool in analysis of this system, In the absence of Spm, or
when it is inactive, plants and kernels having this state are intensely
pigmented and the degree of this is nearly equivalent to that produced
wh the standard Aj,is-present. When an Active Spm is present, all
pigment formation is suppressed in the kernels except in those cells

in which Spm has become inactive or has been removed by trang osition,
No mutations to give a stable A, expression have yet been identified,
Thus, the variegation patterns appearing when this state is present are
reflections solely of modifications occurring to the Spm element itself,
either transposition of it or change in phase of its activity, and the use

fov Amipsbvyatiog

of this state jm-analusis-ef cycles of activity of Spm will be described
A

in one of the sections of this revort,

Some definite conclusions may be drawn regarding the significance

oli
Chey
of state in conv rol of variegation expression, For this purrose, all

examined states of either ayn or as wd that give rise to stable

wa VER Ck wh Lurid,
mutations »iteh—ase expressed oe. type of anthocyanin pignent®tiddation ond

by-intensity of this, may be grouped together, and this includes all of
1)

them except the one just mentioned above, It +8 concluded that in the
22

presence of an active Spm element, both the time of occurrence of et

ud ut tad OP"

mutation during development of plant or kernel and the type or—thts—that

Mmay—eesws, are expressions of the state of the ay a or the a, mom

Rue Aptian wb be eywit ett atcaeraslaan fe
that may be present in a particulsr plant or kernel, few-inenemsed dosed

of Spm ,de=—met=altor—thts—exreseten, Again, the type and intensity of
anthocyanin pigment formation in kernel and »lant in the absence of Spm

or when it is present but inactive, is also an expression of state,

ern ma
af .

Zz Aa! FQq url Ot Tas Ting,
ot Bre Yifferent states here pbeon. useful in this study. It—has—beex

A Pausg.to ue
. y Mart i

necessary to select particular stateq for mammixyxkaxef specific punposes
A

ie of analysis, and the reasons for this will bemade apparent in later

Munpuns fr meburtun © plrbr sl. praplun

» . sections when the purposes are discussed,

. . some
Q In the absence of “pm, all states ave quite stable and mamx of them

have been carried through many vlant generations without ghving any

indication of change, Those states that give rise to early occurring

on oabwe giro

mutaticn in the presence of Spm give rise in its presence to some nue
A

oul

altered states thas) may be recognized in individual progeny derived a
A

from plants having-ene—such—stete, In contrast, those states that

are charactcrized by very late occurring mutations do not give rise to

or must do so very rarely Ate QXALE
altered states in the presence of Spmy In studies, sf two of them, Ou
A
* Condit od with,

carried through 9 generations of plants, not one case of altered state

_ has been encountered,
wo
23

ad). Types of change in action of Spm other than those related to its

cyeles of activity.
In addition to the activity cycles undergone by Spm, mentioned earlicr,
another type of modification oceurs to it that effects a marked alteraticn

in pattern of mutation produced by different states of a md. or at,

1

These modification arpear to arise in individual cells in which a fully
active Spm element had been present. The descendent cells than express the
presence of the modified Spm element in them by exhibiting markedly altered
patterns of mutation, Evidence now available does not suggest that the
responsible event is associated with change in location of Spm in the

chrom some complement, for it has been determined in several cases that an

Spm element whose locstion is known may undergo such a modification without
alot ne
conincident transposition,

The modifications of Spm underconsideraticn, effect: its capacity to
A :

* 1
i" goths, ub
of ets, jan

or amt and to induce mutation to
&.

suppress gene action at either at

stable alleles, When one euch Spm eldment is present, anthocyanin pigment,

resembling that aprearing in the absence of an Spm elethent,is produced in
plants but it develops at a very slow rate, Ho pigment may appear in young
Prqiats Ewords

plants but it develops gradually as the plant, nesckes maturity. In kernels,

the capacity of such an Spm elehemt to suppres gene action likewise is
2h,

reduced but the degree of this is less marked than in the plant. The
dirference may be related to the more limited time that is available,for
accumulation of pigment in the kernels Its rapid maturity stops the
process, Whatever may be the caume, very little or sometimes no pigment

of the type produced in the absence of Spm abjears in kernels having one
such modified Spm element, However, spots of pigment derived from mutation
at either a, mri or ans as the case may be, do appear but the number of
them is much lower than that produced with a fully active Spm element,

For each state, the reduction is expressed as a proportion of that given
when a fully active Spm is present, Both the suopressive and mutative
capacity of Spm appears to be weakened, and for this reason, such

modified Spm elelents are designated Spm-w in contrast to the sowcalled
standard Spm element that gives full suppression of gene action of amt
or a, and a constant and predictable vattern of mutation with any one
State of amt and with all but one exceptional state of a ml,

In subsequent discussions that deal with Spm-w, it will be necessary to
distinguish it from the standard Spm element. Therefore, in these

discussions, the standard Spm element will be tuxkounkest symbolized as

Spm= s e
aythp

25

When Spm-s and Spm-w are present in the same plant or kernel, the actu
Spm-s element is dominant, In a plant having one es each of these two

types of Spm element, expected patterns of segregation of the two occur

w Apeedtod pofihww

at meiosis and each is recovered in the progeny. When the, locationgy of
: “A _

eeah in the chromosome complement da known, bhe—two_appeaminthe—prereny
appre. us tho mocery

in_ expected proportions—end each in its respective location in the

 

complement, barring a few cases of transvosition of one or the other

that may have occurred, Thewemem, Gin an extensive study in which

comparisons were made of the effect of one

 

isolate of Spm-w on mode of expression of four of the described states of
a, mt and of one state of asst, no evidence of transposition of the Spm-w
element was obtained in tests conducted through five plant generations and
in different genetic backgrounds,

Spm-w may express the same cyclée of change in phase of activity as

does Spmes,. “t may also undergo subsequent modification that effects a

return to higher levels of capacity to suppress gene action and induce

m1 m-1

mutation at ay and ao . Again, the event responsible for this
occurs in an individual cell whose vrogeny ex-ibit the effect of its

increased expressivity. The rate of return to or towards Spm-s has not

been the same with all Spm-w isolat<«s, With the one mentioned above that