22
J . B. GURDON
3. Back Transfers from Foreign Egg Cytoplasm
Amphibia
Making use of Briggs and King's nuclear transfer technique, Moore
(1958) showed that nuclei which have replicated in the egg cytoplasm
of a different species undergo a stable change affecting their function.
Rana sylvatica eggs were fertilized with Rana pipiens sperm and then
enucleated. The resulting embryos which contained pipiens haploid
nuclei in sylvatica cytoplasm did not develop beyond the blastula stage.
Nuclei were then taken from these 'hybrid' blastulae before they became arrested in development and were transplanted back to enucleated
Rana pipiens eggs. These 'back-transfer' embryos did not develop
further than the early gastrula stage, whereas many of the control
transfers (two nuclear transfers to Rana pipiens eggs only) developed
normally. This experiment shows that pipiens nuclei are changed in
some way as a result of replicating in sylvatica cytoplasm, though it
does not show what kind of change is involved nor whether it is comparable to that reported by Briggs and King, as described above.
Moore (1960) then carried out a series of back transfers in which a
'hybrid' blastula derived from a pipiens nucleus and sylvatica cytoplasm
provided nuclei for serial transplantation back to enucleated pipiens
eggs. Except for a few larvae in the first back-transfer generation, all
serial transfer embryos were arrested at or before the gastrula stage.
The induced nuclear change was therefore irreversible under the experimental conditions.
A cytological analysis of back transfer embryos derived from pipiens
nuclei and sylvatica cytoplasm has been made by Hennen (1963).
Following back transfer of pipiens nuclei she obtained many different
developmental abnormalities ranging from arrested blastulae to abnormal tadpoles, as well as one normal tadpole. The normal tadpole
had the usual 26 chromosomes, but all the abnormal embryos had
aneuploid chromosome sets often with minute and ring chromosomes.
In general the more severe were the chromosome abnormalities,
the earlier in development did the embryo concerned become arrested.
This correlation suggests that the chromosomal abnormalities may be
the cause of the developmental arrest. Hennen found that chromosome
abnormalities sometimes occurred in homospecific transfers (pip.-+pip.),
but occurred much more frequently in androgenetic hybrids (pip. x (syl)),
and in back transfers. As she points out, these results suggest that
chromosome changes occur at random following nuclear transplantation, and that the chance of their occurrence is very greatly increased
when nuclei replicate in foreign cytoplasm.
Work of a comparable kind done on two species of Xenopus by
J . B. GURDON
3. Back Transfers from Foreign Egg Cytoplasm
Amphibia
Making use of Briggs and King's nuclear transfer technique, Moore
(1958) showed that nuclei which have replicated in the egg cytoplasm
of a different species undergo a stable change affecting their function.
Rana sylvatica eggs were fertilized with Rana pipiens sperm and then
enucleated. The resulting embryos which contained pipiens haploid
nuclei in sylvatica cytoplasm did not develop beyond the blastula stage.
Nuclei were then taken from these 'hybrid' blastulae before they became arrested in development and were transplanted back to enucleated
Rana pipiens eggs. These 'back-transfer' embryos did not develop
further than the early gastrula stage, whereas many of the control
transfers (two nuclear transfers to Rana pipiens eggs only) developed
normally. This experiment shows that pipiens nuclei are changed in
some way as a result of replicating in sylvatica cytoplasm, though it
does not show what kind of change is involved nor whether it is comparable to that reported by Briggs and King, as described above.
Moore (1960) then carried out a series of back transfers in which a
'hybrid' blastula derived from a pipiens nucleus and sylvatica cytoplasm
provided nuclei for serial transplantation back to enucleated pipiens
eggs. Except for a few larvae in the first back-transfer generation, all
serial transfer embryos were arrested at or before the gastrula stage.
The induced nuclear change was therefore irreversible under the experimental conditions.
A cytological analysis of back transfer embryos derived from pipiens
nuclei and sylvatica cytoplasm has been made by Hennen (1963).
Following back transfer of pipiens nuclei she obtained many different
developmental abnormalities ranging from arrested blastulae to abnormal tadpoles, as well as one normal tadpole. The normal tadpole
had the usual 26 chromosomes, but all the abnormal embryos had
aneuploid chromosome sets often with minute and ring chromosomes.
In general the more severe were the chromosome abnormalities,
the earlier in development did the embryo concerned become arrested.
This correlation suggests that the chromosomal abnormalities may be
the cause of the developmental arrest. Hennen found that chromosome
abnormalities sometimes occurred in homospecific transfers (pip.-+pip.),
but occurred much more frequently in androgenetic hybrids (pip. x (syl)),
and in back transfers. As she points out, these results suggest that
chromosome changes occur at random following nuclear transplantation, and that the chance of their occurrence is very greatly increased
when nuclei replicate in foreign cytoplasm.
Work of a comparable kind done on two species of Xenopus by