MORPHOGENESIS OF CROWN
GALL
73
Jinks (1964, p. 73) has proposed seven criteria that could be used to
distinguish nuclear from extranuclear inheritance: the heterokaryon
test, invasion, reciprocal differences in crosses, non-Mendelian segregation, somatic segregation, specificity of the mutation, and finding a cell
organelle involved. Most of these are only applicable to sexual reproduction, and the first two more specifically to fungi. The criteria which
could be tested in crown gall are the specificity of the mutation, somatic
segregation, and establishing the actual site of the change.
The specificity of mutation is a characteristic feature of crown gall.
In addition, the virtually 100% mutation rate agrees well with what is
known about extranuclear mutations. Somatic segregation can be studied
by cloning tumor cells to find out whether the resulting galls vary in
character. This is, indeed, the case in Hildebrandt's (1958) cloning experiments. However, the precaution of checking the chromosome complement of the clones has not been taken in this study. A further possibility
would be to treat tissue cultures of different types of crown gall with
acridine dyes or other substances known to act mainly as plasmatic
mutagens, in the hope of obtaining backmutations or of changing a slow
tumor into a fast one, or vice versa.
As pointed out earlier, electron microscopic studies have so far yielded
relatively few results on crown gall. No viruses have been observed
inside the cells, which as such does not exclude them as tumorigenic
agents. In addition, it may be remembered that bacteria have never been
found to be carriers of plant viruses. The possibility, nevertheless, remains that the bacteria in some way would activate latent viruses in the
host cells.
Braun (1954a) used to interpret his observations on the conditioning
of cells and their transformation on the basis of hypothetical cytoplasmic
entities, in other words, plasmagenes, of which a fast growing, tumor cell
would have more than a moderate or a slow growing cell. Whether the
increase in number of various particles in crown gall cells, described by
many authors (cf. Braun and Stonier, 1958; Kupila, 1963), could have any
relationship with Braun's units, is more than doubtful.
In a recent article Braun himself (1965) leans more to the side of the
dedifferentiation hypothesis. The mass of evidence accumulated by
developmental genetics shows that the development of an organism
depends on the activation and deactivation of genes in an orderly
sequence. If the induction of crown gall were analogous to dedifferentiation, it would involve an untimely and disorderly derepression of a
number of genes that regulate the growth substance metabolism. The
GALL
73
Jinks (1964, p. 73) has proposed seven criteria that could be used to
distinguish nuclear from extranuclear inheritance: the heterokaryon
test, invasion, reciprocal differences in crosses, non-Mendelian segregation, somatic segregation, specificity of the mutation, and finding a cell
organelle involved. Most of these are only applicable to sexual reproduction, and the first two more specifically to fungi. The criteria which
could be tested in crown gall are the specificity of the mutation, somatic
segregation, and establishing the actual site of the change.
The specificity of mutation is a characteristic feature of crown gall.
In addition, the virtually 100% mutation rate agrees well with what is
known about extranuclear mutations. Somatic segregation can be studied
by cloning tumor cells to find out whether the resulting galls vary in
character. This is, indeed, the case in Hildebrandt's (1958) cloning experiments. However, the precaution of checking the chromosome complement of the clones has not been taken in this study. A further possibility
would be to treat tissue cultures of different types of crown gall with
acridine dyes or other substances known to act mainly as plasmatic
mutagens, in the hope of obtaining backmutations or of changing a slow
tumor into a fast one, or vice versa.
As pointed out earlier, electron microscopic studies have so far yielded
relatively few results on crown gall. No viruses have been observed
inside the cells, which as such does not exclude them as tumorigenic
agents. In addition, it may be remembered that bacteria have never been
found to be carriers of plant viruses. The possibility, nevertheless, remains that the bacteria in some way would activate latent viruses in the
host cells.
Braun (1954a) used to interpret his observations on the conditioning
of cells and their transformation on the basis of hypothetical cytoplasmic
entities, in other words, plasmagenes, of which a fast growing, tumor cell
would have more than a moderate or a slow growing cell. Whether the
increase in number of various particles in crown gall cells, described by
many authors (cf. Braun and Stonier, 1958; Kupila, 1963), could have any
relationship with Braun's units, is more than doubtful.
In a recent article Braun himself (1965) leans more to the side of the
dedifferentiation hypothesis. The mass of evidence accumulated by
developmental genetics shows that the development of an organism
depends on the activation and deactivation of genes in an orderly
sequence. If the induction of crown gall were analogous to dedifferentiation, it would involve an untimely and disorderly derepression of a
number of genes that regulate the growth substance metabolism. The