NON-FILAMENTOUS AQUATIC FUNGI
77
tion of the factors which lead directly to morphological irreversibility.
Unfortunately, this cannot be done ; we cannot provide an unequivocal
explanation for the rapid shift from a plastic system to one that is
essentially unidirectional. The quickness with which the change occurs
suggests that some pool or pathway reaches a critical upper threshold
level, or a lower limit. It still remains for us to find out exactly what it is.
Before leaving the subject, however,—such questions notwithstanding
—we should like to stress again the great value of this stage in ontogeny
for experimental analyses of differentiation. With a 32- to 34-hour RS
cell, we are provided with a morphogenetic system poised at a critical
juncture in its development. From this point, it can proceed in either of
two mutually exclusive directions, and the route actually taken is under
the direct experimental control of the investigator.
A similar situation exists for the OC cell. Before its development
becomes irreversible, we know that at or near its point of no return
synthesis of isocitritase commences while that of glucose-6-phosphate
dehydrogenase ceases, oxygen consumption falls sharply, and its linear
exponential growth rate is suddenly reduced (McCurdy and Cantino,
1960; Goldstein and Cantino, 1962); beyond this point, however, the
phenomenon remains essentially unexploited. Thus, both the OC and RS
cell of B. emersonii provides an experimental system with a built-in
morphogenetic equilibrium. With it, any factor which might contribute
to differentiation can be examined at all critical stages near the OC
or RS threshold. Furthermore, it is possible to study such cells in massive synchronized populations undergoing morphogenetic change in
either direction. We believe that this unique set of circumstances has
tremendous experimental potential.
F. Developments Beyond the Point of No Return
After 36 hours of growth, the developing RS cell is led unalterably
towards formation of a thick-walled, pigmented, dormant structure. The
period immediately beyond the point of no return, or the early maturation phase, superficially appears to be quiescent ; the only visible sign of
activity is gradual thickening of the sporangial wall and an increasing
degree of pigmentation. Upon closer examination, however, this
apparent inactivity turns out to be exceptionally deceptive. Although
our catalogue of the events which take place is far from complete, we
know enough concerning cellular activities during this developmental
phase to banish any thought that it is a system in repose. Although it
does not increase in size, the cell continues to amass pools of RNA,
chitin, lipid, melanin, polysaccharide, and total nitrogen for some 12
hours after it has passed beyond its point of no return (Lovett and
Cantino, 1960b; Cantino, 1961b; Cantino and Goldstein, 1961, 1962).
77
tion of the factors which lead directly to morphological irreversibility.
Unfortunately, this cannot be done ; we cannot provide an unequivocal
explanation for the rapid shift from a plastic system to one that is
essentially unidirectional. The quickness with which the change occurs
suggests that some pool or pathway reaches a critical upper threshold
level, or a lower limit. It still remains for us to find out exactly what it is.
Before leaving the subject, however,—such questions notwithstanding
—we should like to stress again the great value of this stage in ontogeny
for experimental analyses of differentiation. With a 32- to 34-hour RS
cell, we are provided with a morphogenetic system poised at a critical
juncture in its development. From this point, it can proceed in either of
two mutually exclusive directions, and the route actually taken is under
the direct experimental control of the investigator.
A similar situation exists for the OC cell. Before its development
becomes irreversible, we know that at or near its point of no return
synthesis of isocitritase commences while that of glucose-6-phosphate
dehydrogenase ceases, oxygen consumption falls sharply, and its linear
exponential growth rate is suddenly reduced (McCurdy and Cantino,
1960; Goldstein and Cantino, 1962); beyond this point, however, the
phenomenon remains essentially unexploited. Thus, both the OC and RS
cell of B. emersonii provides an experimental system with a built-in
morphogenetic equilibrium. With it, any factor which might contribute
to differentiation can be examined at all critical stages near the OC
or RS threshold. Furthermore, it is possible to study such cells in massive synchronized populations undergoing morphogenetic change in
either direction. We believe that this unique set of circumstances has
tremendous experimental potential.
F. Developments Beyond the Point of No Return
After 36 hours of growth, the developing RS cell is led unalterably
towards formation of a thick-walled, pigmented, dormant structure. The
period immediately beyond the point of no return, or the early maturation phase, superficially appears to be quiescent ; the only visible sign of
activity is gradual thickening of the sporangial wall and an increasing
degree of pigmentation. Upon closer examination, however, this
apparent inactivity turns out to be exceptionally deceptive. Although
our catalogue of the events which take place is far from complete, we
know enough concerning cellular activities during this developmental
phase to banish any thought that it is a system in repose. Although it
does not increase in size, the cell continues to amass pools of RNA,
chitin, lipid, melanin, polysaccharide, and total nitrogen for some 12
hours after it has passed beyond its point of no return (Lovett and
Cantino, 1960b; Cantino, 1961b; Cantino and Goldstein, 1961, 1962).