NON-FILAMENTOUS AQUATIC FUNGI
73
now be both possible and profitable to establish the extent and significance of turnover during morphogenetic reversal in B. emersonii using
tracers in combination with amino-acid and base analogues (Halvorson
and Spiegelman, 1952; Wecker and Schonne, 1961; Aronson, 1961;
Young and Fitz-James, 1959).
Let us return, then, to the significance of bicarbonate-induced changes
which occur before the point of no return. We are faced once again, as
we were on the question of reversibility, with the need to establish valid
criteria for distinguishing between causal and dependent factors
associated with a morphogenetic event. A set of possible standards for
measuring the importance of a biochemical process in differentiation has
been suggested by White and Sussman (1961), as follows : "(a) that it can
be shown to initiate, regulate, or be the consequence of a particular
morphogenetic act; (b) that it be chronologically co-ordinated with a
specific morphogenetic act; (c) that it be altered, inhibited, or be
completely absent under conditions which prevent the consummation of
some part of the normal morphogenetic sequence.' Although these
criteria are not entirely adequate for our purpose, they do provide a
useful basis for discussion. For the sake of simplicity, we will consider
the formation of a septum and the morphological irreversibility in B.
emersonii as equivalent, since both events occur at the same time during
development. It is realized, of course, that they may ultimately be
separable by experimental manipulation ; however, no serious attempt
has been made so far to do this.
The first standard of White and Sussman has limited utility at the
moment. Although there is considerable circumstantial evidence for
direct involvement of the tricarboxylic acid cycle, no one specific
biochemical event has been shown, without question, to initiate RS
morphogenesis at the critical stage of irreversibility. On the other hand,
continued synthesis of chitin, lipid, melanin, polysaccharide, and RNA
can most certainly be considered a consequence of the morphogenetic act
of septation; i.e., on the OC pathway of development, continued
synthesis of these components does not occur after completion of the
active growth phase. However, the uninterrupted accumulation of these
materials following formation of the cross-wall, although no doubt
important, does not tell us much concerning their role in the initiation
of this phenomenon. A possible control mechanism linked to chitin
synthesis was once proposed (Cantino, 1952); it was thought that the
gradually-increasing thickness of the wall caused a decrease in permeability to the CO a produced endogenously, thereby retaining a high
internal level of the bicarbonate inducer which would gradually prevent
morphological reversal. Additional experimental verification was never
provided; furthermore, it now appears unlikely that this notion, by
73
now be both possible and profitable to establish the extent and significance of turnover during morphogenetic reversal in B. emersonii using
tracers in combination with amino-acid and base analogues (Halvorson
and Spiegelman, 1952; Wecker and Schonne, 1961; Aronson, 1961;
Young and Fitz-James, 1959).
Let us return, then, to the significance of bicarbonate-induced changes
which occur before the point of no return. We are faced once again, as
we were on the question of reversibility, with the need to establish valid
criteria for distinguishing between causal and dependent factors
associated with a morphogenetic event. A set of possible standards for
measuring the importance of a biochemical process in differentiation has
been suggested by White and Sussman (1961), as follows : "(a) that it can
be shown to initiate, regulate, or be the consequence of a particular
morphogenetic act; (b) that it be chronologically co-ordinated with a
specific morphogenetic act; (c) that it be altered, inhibited, or be
completely absent under conditions which prevent the consummation of
some part of the normal morphogenetic sequence.' Although these
criteria are not entirely adequate for our purpose, they do provide a
useful basis for discussion. For the sake of simplicity, we will consider
the formation of a septum and the morphological irreversibility in B.
emersonii as equivalent, since both events occur at the same time during
development. It is realized, of course, that they may ultimately be
separable by experimental manipulation ; however, no serious attempt
has been made so far to do this.
The first standard of White and Sussman has limited utility at the
moment. Although there is considerable circumstantial evidence for
direct involvement of the tricarboxylic acid cycle, no one specific
biochemical event has been shown, without question, to initiate RS
morphogenesis at the critical stage of irreversibility. On the other hand,
continued synthesis of chitin, lipid, melanin, polysaccharide, and RNA
can most certainly be considered a consequence of the morphogenetic act
of septation; i.e., on the OC pathway of development, continued
synthesis of these components does not occur after completion of the
active growth phase. However, the uninterrupted accumulation of these
materials following formation of the cross-wall, although no doubt
important, does not tell us much concerning their role in the initiation
of this phenomenon. A possible control mechanism linked to chitin
synthesis was once proposed (Cantino, 1952); it was thought that the
gradually-increasing thickness of the wall caused a decrease in permeability to the CO a produced endogenously, thereby retaining a high
internal level of the bicarbonate inducer which would gradually prevent
morphological reversal. Additional experimental verification was never
provided; furthermore, it now appears unlikely that this notion, by