TRANSPLANTATION OF CELL NUCLEI
23
Gurdon (1962b) agrees with that done on Bana in that Xenopus laevis
nuclei are irreversibly changed after replication in Xenopus tropicalis
egg cytoplasm. However, in contrast to the results obtained with Bana
the nuclear changes induced by foreign cytoplasm in Xenopus appear,
rather surprisingly, to cause arrest at a particular stage in development
(Fig. 5, A-L). Thus Xenopus laevis nuclei which have replicated in
Xenopus tropicalis cytoplasm and which are then transferred back to
laevis cytoplasm only support development up to the early neurula
stage (Fig. 5, Gr-L). This stage is characteristic for back-transferred
laevis nuclei since no back transfers ever developed beyond this stage,
and some of those which stopped development before it were shown by
serial transplantation to contain nuclei capable of promoting development to exactly this stage. The specificity of these changes is also shown
by other combinations of nucleus and cytoplasm from laevis and
tropicalis, such as laevis nuclei in tropicalis eggs, and back-transferred
tropicalis nuclei; each combination appears to be characteristically
arrested at a different stage. We do not know whether these Xenopus
nuclear changes are associated with chromosome abnormalities, but
these experiments do suggest that Xenopus eggs contain speciesspecific substances which cause stable changes in nuclei replicating in
them.
Moore (1962) has summarized the essential points in his hypothesis
that these nuclear changes result from the uptake of foreign deoxyribonucleic acid (DNA) precursors. This hypothesis assumes that amphibian
eggs contain DNA which is taken up by nuclei during cleavage without
first being broken down. It is supposed that nuclei would be unable to
obtain their own 'species' of DNA in foreign egg cytoplasm, and would
undergo gene or chromosome changes as a result of taking up foreign
DNA precursors. In normal development nuclei would not be changed
since they could take up their own 'species' of DNA. If this hypothesis
is correct it describes a situation which would not bring about gene
changes in normal development.
Amoeba
A small percentage of viable clones are obtained from enucleated
Amoeba discoides after injection of an Amoeba proteus nucleus ; nuclei
from these clones can then be transplanted back to enucleated proteus
after any desired number of divisions (Danielli et al., 1955). Proteus
nuclei which have replicated in discoides cytoplasm will reactivate
enucleated discoides individuals in about 40% of cases, whereas normal
proteus nuclei do so in only about 1% of cases (Danielli, 1958). It was
also found that discoides cytoplasm with which proteus nuclei had
divided many times behaved as if it had changed in the proteus direction,
23
Gurdon (1962b) agrees with that done on Bana in that Xenopus laevis
nuclei are irreversibly changed after replication in Xenopus tropicalis
egg cytoplasm. However, in contrast to the results obtained with Bana
the nuclear changes induced by foreign cytoplasm in Xenopus appear,
rather surprisingly, to cause arrest at a particular stage in development
(Fig. 5, A-L). Thus Xenopus laevis nuclei which have replicated in
Xenopus tropicalis cytoplasm and which are then transferred back to
laevis cytoplasm only support development up to the early neurula
stage (Fig. 5, Gr-L). This stage is characteristic for back-transferred
laevis nuclei since no back transfers ever developed beyond this stage,
and some of those which stopped development before it were shown by
serial transplantation to contain nuclei capable of promoting development to exactly this stage. The specificity of these changes is also shown
by other combinations of nucleus and cytoplasm from laevis and
tropicalis, such as laevis nuclei in tropicalis eggs, and back-transferred
tropicalis nuclei; each combination appears to be characteristically
arrested at a different stage. We do not know whether these Xenopus
nuclear changes are associated with chromosome abnormalities, but
these experiments do suggest that Xenopus eggs contain speciesspecific substances which cause stable changes in nuclei replicating in
them.
Moore (1962) has summarized the essential points in his hypothesis
that these nuclear changes result from the uptake of foreign deoxyribonucleic acid (DNA) precursors. This hypothesis assumes that amphibian
eggs contain DNA which is taken up by nuclei during cleavage without
first being broken down. It is supposed that nuclei would be unable to
obtain their own 'species' of DNA in foreign egg cytoplasm, and would
undergo gene or chromosome changes as a result of taking up foreign
DNA precursors. In normal development nuclei would not be changed
since they could take up their own 'species' of DNA. If this hypothesis
is correct it describes a situation which would not bring about gene
changes in normal development.
Amoeba
A small percentage of viable clones are obtained from enucleated
Amoeba discoides after injection of an Amoeba proteus nucleus ; nuclei
from these clones can then be transplanted back to enucleated proteus
after any desired number of divisions (Danielli et al., 1955). Proteus
nuclei which have replicated in discoides cytoplasm will reactivate
enucleated discoides individuals in about 40% of cases, whereas normal
proteus nuclei do so in only about 1% of cases (Danielli, 1958). It was
also found that discoides cytoplasm with which proteus nuclei had
divided many times behaved as if it had changed in the proteus direction,