74
EDWARD C. CANTINO AND JAMES S. LOVETT
itself, could explain the very sharp transitions in structure, function,
and composition which have been observed in synchronized cultures. On
the other hand, it is not unlikely that it may still play a partial regulatory
role in the phenomenon.
The second criterion of White and Sussman, though useful for screening
purposes, does not help to shed much light on the basic problem which
confronts us. It is obvious, of course, that some degree of chronological
co-ordination must exist between a control mechanism and its morphological expression. Nevertheless, in Blastocladiella
the changes in
cellular structures and activities which are correlated with the morphological event are so numerous that this criterion fails to provide
discrimination between factors which are intrinsic and those which are
extrinsic.
The third test of significance proposed by White and Sussman appears,
to our minds, to be potentially the most useful for an analysis of causal
mechanisms. The melanin pigment characteristic of a mature RS cell is a
case in point. When certain critically-controlled concentrations of
phenylthiourea are incorporated in the growth medium, formation of
melanin is completely suppressed while other visible attributes of RS
development are not affected (Cantino, 1953). Thus, here is a character
which appears only in response to the bicarbonate inducer but which,
nevertheless, appears to be completely unessential for the otherwisesuccessful completion of differentiation. Depression of melanogenesis in
cells made to undergo morphological reversal before 36 hours suggests that
this biochemical pathway not only requires the presence of bicarbonate
(and/or normal RS differentiation) for its induction, but for its maintenance as well. Synthesis of melanin is the only clear-cut example
available so far of a character which appears de novo as a result of
differentiation, yet is not essential to it. It is probable that the same is
true for synthesis of carotene. Its formation can be blocked in orange
cells of Blastocladiella
by exogenous diphenylamine (Cantino and
Horenstein, 1956a) and increased by actidione (Cantino and Hyatt,
1953a); however, although actidione similarly induces RS cells to
synthesize more carotene (Cantino and Horenstein, 1956b), the probable
inhibitory effect of diphenylamine thereon has not been tested.
3. Mutant Strains Incapable of BS
Differentiation
We have already touched upon the significance of a-ketoglutaric and
isocitric dehydrogenases in the bicarbonate trigger mechanism, as well
as changes in their activities during morphological reversal. The concept
that these enzymes are important was derived from a considerable
amount of more-or-less circumstantial evidence over a period of many
years (Cantino and Turian, 1959; Cantino, 1961a). More direct proof for
EDWARD C. CANTINO AND JAMES S. LOVETT
itself, could explain the very sharp transitions in structure, function,
and composition which have been observed in synchronized cultures. On
the other hand, it is not unlikely that it may still play a partial regulatory
role in the phenomenon.
The second criterion of White and Sussman, though useful for screening
purposes, does not help to shed much light on the basic problem which
confronts us. It is obvious, of course, that some degree of chronological
co-ordination must exist between a control mechanism and its morphological expression. Nevertheless, in Blastocladiella
the changes in
cellular structures and activities which are correlated with the morphological event are so numerous that this criterion fails to provide
discrimination between factors which are intrinsic and those which are
extrinsic.
The third test of significance proposed by White and Sussman appears,
to our minds, to be potentially the most useful for an analysis of causal
mechanisms. The melanin pigment characteristic of a mature RS cell is a
case in point. When certain critically-controlled concentrations of
phenylthiourea are incorporated in the growth medium, formation of
melanin is completely suppressed while other visible attributes of RS
development are not affected (Cantino, 1953). Thus, here is a character
which appears only in response to the bicarbonate inducer but which,
nevertheless, appears to be completely unessential for the otherwisesuccessful completion of differentiation. Depression of melanogenesis in
cells made to undergo morphological reversal before 36 hours suggests that
this biochemical pathway not only requires the presence of bicarbonate
(and/or normal RS differentiation) for its induction, but for its maintenance as well. Synthesis of melanin is the only clear-cut example
available so far of a character which appears de novo as a result of
differentiation, yet is not essential to it. It is probable that the same is
true for synthesis of carotene. Its formation can be blocked in orange
cells of Blastocladiella
by exogenous diphenylamine (Cantino and
Horenstein, 1956a) and increased by actidione (Cantino and Hyatt,
1953a); however, although actidione similarly induces RS cells to
synthesize more carotene (Cantino and Horenstein, 1956b), the probable
inhibitory effect of diphenylamine thereon has not been tested.
3. Mutant Strains Incapable of BS
Differentiation
We have already touched upon the significance of a-ketoglutaric and
isocitric dehydrogenases in the bicarbonate trigger mechanism, as well
as changes in their activities during morphological reversal. The concept
that these enzymes are important was derived from a considerable
amount of more-or-less circumstantial evidence over a period of many
years (Cantino and Turian, 1959; Cantino, 1961a). More direct proof for