TRANSPLANTATION OF CELL NUCLEI
37
sexual maturity and allows conjugation. Since the sexually mature
state persists after macronuclear regeneration, the latter must be stable.
The basis of the change is not known, but it can be understood if the
macronucleus is supposed to consist of subunits each of which is comparable to a diploid set of chromosomes. Nanney (1963) suggests that
the postulated subunits of the macronucleus may differ genetically and
the ratio of one type to another may alter during the divisions occurring
after conjugation. When one type of subunit greatly outnumbers another, then a particular phenotype would be expressed. This suggestion
attributes macronuclear differentiation to an increasing imbalance
among its subunits.
The macronucleus of ciliates is mainly responsible for the development and physiological function of the cell whereas the micronucleus is
mainly concerned with reproduction. In this respect the nuclei of ciliates
can be compared to the somatic and germ-cell nuclei of animals. However, there are two reasons for believing that the somatic cell nuclei of
vertebrates do not differentiate like the ciliate macronucleus. First, no
stable differentiation has been revealed by the transplantation of
endoderm nuclei in Xenopus, though the latter achieves an experimental
situation closely comparable to macronuclear regeneration in ciliates.
Second, polyploidy of the kind which seems to arise during macronuclear differentiation in ciliates is not regularly associated with cell
differentiation in multicellular organisms.
Conclusion
The experiments discussed above appear to constitute the clearest
reported examples of stable gene changes in development and of nuclear
differentiation. They are of great importance in having helped to elucidate the kind of factors which influence gene expression, but the special
properties of each case make them unlikely to exemplify mechanisms
generally important in the promotion of cell differentiation.
C. Reversible Gene Activation and Repression
The differentiation of intestine cells in Xenopue is brought about by
an entirely reversible activation or repression of genes (Section III, A,
1). The activity of the genes is evidently controlled by the nature of the
chromosomal environment. The same conclusion can be drawn from
several quite different kinds of experiments to be described now.
1. Puffing in Polytene Chromosomes
Cytological evidence of reversible gene activation has been provided
by the beautiful work of Beermann and associates. This work has been
carried out on the salivary gland polytene chromosomes of Chironomus.
37
sexual maturity and allows conjugation. Since the sexually mature
state persists after macronuclear regeneration, the latter must be stable.
The basis of the change is not known, but it can be understood if the
macronucleus is supposed to consist of subunits each of which is comparable to a diploid set of chromosomes. Nanney (1963) suggests that
the postulated subunits of the macronucleus may differ genetically and
the ratio of one type to another may alter during the divisions occurring
after conjugation. When one type of subunit greatly outnumbers another, then a particular phenotype would be expressed. This suggestion
attributes macronuclear differentiation to an increasing imbalance
among its subunits.
The macronucleus of ciliates is mainly responsible for the development and physiological function of the cell whereas the micronucleus is
mainly concerned with reproduction. In this respect the nuclei of ciliates
can be compared to the somatic and germ-cell nuclei of animals. However, there are two reasons for believing that the somatic cell nuclei of
vertebrates do not differentiate like the ciliate macronucleus. First, no
stable differentiation has been revealed by the transplantation of
endoderm nuclei in Xenopus, though the latter achieves an experimental
situation closely comparable to macronuclear regeneration in ciliates.
Second, polyploidy of the kind which seems to arise during macronuclear differentiation in ciliates is not regularly associated with cell
differentiation in multicellular organisms.
Conclusion
The experiments discussed above appear to constitute the clearest
reported examples of stable gene changes in development and of nuclear
differentiation. They are of great importance in having helped to elucidate the kind of factors which influence gene expression, but the special
properties of each case make them unlikely to exemplify mechanisms
generally important in the promotion of cell differentiation.
C. Reversible Gene Activation and Repression
The differentiation of intestine cells in Xenopue is brought about by
an entirely reversible activation or repression of genes (Section III, A,
1). The activity of the genes is evidently controlled by the nature of the
chromosomal environment. The same conclusion can be drawn from
several quite different kinds of experiments to be described now.
1. Puffing in Polytene Chromosomes
Cytological evidence of reversible gene activation has been provided
by the beautiful work of Beermann and associates. This work has been
carried out on the salivary gland polytene chromosomes of Chironomus.