NON-FILAMENTOUS AQUATIC FUNGI
53
division, the cell of Blastocladiella
provides numerous parameters:
volume changes and subsequent alterations in allometry, increase in
complexity of its rhizoidal system, protoplasmic migration, centripetal
development of a single, inner cross-wall, thickening and/or pigmentation and pitting of the thallus wall, and finally, progressive cleavage of
the protoplast during genesis of spores. The use of cultures of this sort
for studies of growth and differentiation will be further emphasized in
the following pages.
D. Environmental Control of Exponential Growth
Let us return, now, to the spore germination stage. Initiation of
exponential growth follows on the heels of disintegration of the nuclear
cap and dispersal of its contents throughout the body of the germinating
spore. As the nucleus of the spore divides, the DNA/cell doubles, the
RNA/cell is reduced by an equivalent amount, and a binucleate
germling is produced (Turian and Cantino, 1959a). Thereafter, the
germling can develop along either of two major morphogenetic pathways
(Fig. 2) ; one of these (the OC path) is markedly affected by visible
illumination (Cantino and Turian, 1961).
For studies of the role of RNA and DNA in the initiation and control
of ontogeny, these early moments in the life of Blastocladiella
may
represent the stage par excellence; now the spore, previously so selfsufficient, is suddenly transformed into a germling, with reorganization
of its internal architecture in a state of flux, and dependent for survival
upon exogenous nutrition and profoundly subject to environmental
variations.
The developmental path taken by the germling can be induced and
maintained by either providing or withholding exogenous bicarbonate,
whereby either RS or OC cells, respectively, are formed (Fig. 2)f. Let us
consider, therefore, the exponential phase of growth along both pathways.
t Whether or not the immediate causal factor for this effect is solely bicarbonate,
carbonate, C0 2 , or a combination thereof, and to what extent other factors are involved
became a focal point for some criticism in the past (cf. Cantino, 1956; Cantino and
Turian, 1959). However, the agitated, synchronized, single-generation cultures of
Blastocladiella used in recent years have been grown at constant pH and with heavy,
continuous aeration (thus insuring a constant partial-pressure of C0 2 ). Under these
conditions, a decrease in the optimum concentration of exogenous bicarbonate needed for
RS formation results in genesis of only OC plants (Lovett and Cantino, 1960b), while
an increase in bicarbonate so accentuates formation of RS cells—and 'overdrives' the
mechanism, so to speak—that RS cells with abnormally thickened walls are formed
(Lovett and Cantino, 1960c). Thus, it is clearly not the partial pressure of C0 2 that is
involved. It would be important to know precisely, of course, the relative contribution of
each of these molecular species, and there is no reason why experiments cannot be done
to establish this. But, for the purposes of our discussion in this paper, the question is
essentially an academic one.
53
division, the cell of Blastocladiella
provides numerous parameters:
volume changes and subsequent alterations in allometry, increase in
complexity of its rhizoidal system, protoplasmic migration, centripetal
development of a single, inner cross-wall, thickening and/or pigmentation and pitting of the thallus wall, and finally, progressive cleavage of
the protoplast during genesis of spores. The use of cultures of this sort
for studies of growth and differentiation will be further emphasized in
the following pages.
D. Environmental Control of Exponential Growth
Let us return, now, to the spore germination stage. Initiation of
exponential growth follows on the heels of disintegration of the nuclear
cap and dispersal of its contents throughout the body of the germinating
spore. As the nucleus of the spore divides, the DNA/cell doubles, the
RNA/cell is reduced by an equivalent amount, and a binucleate
germling is produced (Turian and Cantino, 1959a). Thereafter, the
germling can develop along either of two major morphogenetic pathways
(Fig. 2) ; one of these (the OC path) is markedly affected by visible
illumination (Cantino and Turian, 1961).
For studies of the role of RNA and DNA in the initiation and control
of ontogeny, these early moments in the life of Blastocladiella
may
represent the stage par excellence; now the spore, previously so selfsufficient, is suddenly transformed into a germling, with reorganization
of its internal architecture in a state of flux, and dependent for survival
upon exogenous nutrition and profoundly subject to environmental
variations.
The developmental path taken by the germling can be induced and
maintained by either providing or withholding exogenous bicarbonate,
whereby either RS or OC cells, respectively, are formed (Fig. 2)f. Let us
consider, therefore, the exponential phase of growth along both pathways.
t Whether or not the immediate causal factor for this effect is solely bicarbonate,
carbonate, C0 2 , or a combination thereof, and to what extent other factors are involved
became a focal point for some criticism in the past (cf. Cantino, 1956; Cantino and
Turian, 1959). However, the agitated, synchronized, single-generation cultures of
Blastocladiella used in recent years have been grown at constant pH and with heavy,
continuous aeration (thus insuring a constant partial-pressure of C0 2 ). Under these
conditions, a decrease in the optimum concentration of exogenous bicarbonate needed for
RS formation results in genesis of only OC plants (Lovett and Cantino, 1960b), while
an increase in bicarbonate so accentuates formation of RS cells—and 'overdrives' the
mechanism, so to speak—that RS cells with abnormally thickened walls are formed
(Lovett and Cantino, 1960c). Thus, it is clearly not the partial pressure of C0 2 that is
involved. It would be important to know precisely, of course, the relative contribution of
each of these molecular species, and there is no reason why experiments cannot be done
to establish this. But, for the purposes of our discussion in this paper, the question is
essentially an academic one.