TRANSPLANTATION OF CELL NUCLEI
35
change persists in the absence of the initiating conditions. Among the
experiments which can be interpreted as cases of stable gene repression
are the results of transplanting endoderm nuclei in Rana pipiens (Section I I I , A, 1). While these experiments have clearly demonstrated that
transplanted nuclei have often undergone stable changes affecting gene
expression, these changes are often associated with chromosomal abnormalities which may have arisen during or after transplantation. We
know that stable nuclear changes of this kind are not always associated
with normal cell differentiation since they do not occur in nuclear
transfer experiments with Xenopus, and chromosome abnormalities do
not arise in normal development.
An example of a stable change in gene expression is the phenomenon
of paramutation described by Brink (1960). The expression of a gene
producing anthocyanin in the kernel and vegetative tissues of maize is
always changed when one of certain alleles are present in the same
chromosome set. Thus when the hétérozygote R
r B
8t
is crossed to a r
g r
9
plant, half the resulting progeny contain the alleles R
8t r
9
and are stippled
in colour; the other half are R
r r
g
and are not darkly mottled as would
be expected of R
r
but are weakly pigmented. This 'paramutant' effect is
specific in that it is always induced in R
r
by its alleles R
8t
and R
mb , and
R
r
always responds to these alleles in the same way. The changed form
of R
r
is fairly stable since it persists in subsequent generations except
for a general tendency towards reversion to the standard R
r
form. I t is
possible to imagine that changes in gene expression comparable to
paramutation might occur throughout development and might be
important in bringing about cell differentiation. If so, such changes
must differ from paramutational changes in two ways. As Brink (1960)
points out, they would have to be relatively unstable, always being
reversed at or before meiosis. The persistence of the paramutational
change at the R locus as first outlined would then be regarded as atypical
and only for this reason would the phenomenon have been detected.
I t would also be necessary to assume a more precise control of the gene
changes, since they could not promote cell differentiation unless they
occur at certain stages in the differentiation of some cell types but not
of others. In view of these points, it does not seem justified at present to
regard paramutation as exemplifying a kind of gene change that occurs
regularly during normal development.
Another type of stable gene change in maize is brought about by the
transposable elements described by McClintock (1956). Whereas paramutation involves two genes at the same locus on different chromosomes,
McClintock's controlling elements often involve genes at different loci on
the same chromosome. A peculiar property of these elements is their
tendency to move from one position in the chromosome set to another,
B2
A.M.4
35
change persists in the absence of the initiating conditions. Among the
experiments which can be interpreted as cases of stable gene repression
are the results of transplanting endoderm nuclei in Rana pipiens (Section I I I , A, 1). While these experiments have clearly demonstrated that
transplanted nuclei have often undergone stable changes affecting gene
expression, these changes are often associated with chromosomal abnormalities which may have arisen during or after transplantation. We
know that stable nuclear changes of this kind are not always associated
with normal cell differentiation since they do not occur in nuclear
transfer experiments with Xenopus, and chromosome abnormalities do
not arise in normal development.
An example of a stable change in gene expression is the phenomenon
of paramutation described by Brink (1960). The expression of a gene
producing anthocyanin in the kernel and vegetative tissues of maize is
always changed when one of certain alleles are present in the same
chromosome set. Thus when the hétérozygote R
r B
8t
is crossed to a r
g r
9
plant, half the resulting progeny contain the alleles R
8t r
9
and are stippled
in colour; the other half are R
r r
g
and are not darkly mottled as would
be expected of R
r
but are weakly pigmented. This 'paramutant' effect is
specific in that it is always induced in R
r
by its alleles R
8t
and R
mb , and
R
r
always responds to these alleles in the same way. The changed form
of R
r
is fairly stable since it persists in subsequent generations except
for a general tendency towards reversion to the standard R
r
form. I t is
possible to imagine that changes in gene expression comparable to
paramutation might occur throughout development and might be
important in bringing about cell differentiation. If so, such changes
must differ from paramutational changes in two ways. As Brink (1960)
points out, they would have to be relatively unstable, always being
reversed at or before meiosis. The persistence of the paramutational
change at the R locus as first outlined would then be regarded as atypical
and only for this reason would the phenomenon have been detected.
I t would also be necessary to assume a more precise control of the gene
changes, since they could not promote cell differentiation unless they
occur at certain stages in the differentiation of some cell types but not
of others. In view of these points, it does not seem justified at present to
regard paramutation as exemplifying a kind of gene change that occurs
regularly during normal development.
Another type of stable gene change in maize is brought about by the
transposable elements described by McClintock (1956). Whereas paramutation involves two genes at the same locus on different chromosomes,
McClintock's controlling elements often involve genes at different loci on
the same chromosome. A peculiar property of these elements is their
tendency to move from one position in the chromosome set to another,
B2
A.M.4