NON-FILAMENTOUS AQUATIC FUNGI
71
removal of inducer, do such characteristics also revert to their original
states ; and (3) if they do, are such changes necessary for the shift in the
morphogenetic pathway, or do they merely result from it—that is, are
they causes or effects?
The first question will be discussed in a later section. Attempts have
been made to answer the second question. Synchronized RS cultures
were analysed before and after morphological reversal. When 28-32
hour thalli are removed from bicarbonate media and resuspended in
water, pigmentation and wall-thickening cease. Then, after approximately 3 more hours, they form papillae and discharge spores in the
manner of OC cells. This does not occur, however, when RS thalli have
gone beyond 36 hours of growth. During such a morphogenetic reversal,
total protein, cytochrome oxidase activity, and initial stages of the
glucose-monophosphate shunt remain essentially unaltered; however,
the activity of a-ketoglutaric dehydrogenase increases abruptly while
that of isocitric dehydrogenase decreases (Lovett and Cantino, 1961).
Simultaneously, the soluble-protein pattern immediately reverts to one
that is more characteristic of OC cells (Cantino and Goldstein, 1962), as
does part of the nucleic acid pool (Cantino, 1961b). In particular, these
differential changes in two enzymes of the citric acid cycle, under
conditions which prevent consummation of normal RS development,
are significant and directly in agreement with their suggested role in
morphogenesis.
So far, these investigations have been limited to a few enzymes; we
do not know, as yet, the full extent of the enzymatic alterations which
are involved. In this connection, it is important to emphasize that a
visible manifestation of cell activity was selected as the primary criterion
for reversibility; i.e., prompt discharge of spores from a thin-walled
sporangium, a combination characteristic of OC cells. This does not
mean, of course, that all parameters associated with an RS cell before
reversal will 'de-differentiate'. Some, or even most, of them may remain
unchanged. For example, there is no evidence that the small amounts of
melanin accumulated in the cell by this time, nor the chitin built up into
the thallus wall, are degraded during this period of morphogenetic
reversal. In fact, while the quantitative relationship between some
soluble protein fractions reverts to levels found in OC cells, that for
other protein components does not do so (Cantino and Goldstein, 1962).
Continued formation of the cross-wall until it is complete, whereby the
basal rhizoidal cell is separated from the terminal sporangium—albeit a
thin-walled onef—occurs in OC cells derived from RS cells by morphogenetic reversal; this clearly shows that at least some of the gross
morphological structures of the cell are not easily modified.
f See'footnote on opposite page.
71
removal of inducer, do such characteristics also revert to their original
states ; and (3) if they do, are such changes necessary for the shift in the
morphogenetic pathway, or do they merely result from it—that is, are
they causes or effects?
The first question will be discussed in a later section. Attempts have
been made to answer the second question. Synchronized RS cultures
were analysed before and after morphological reversal. When 28-32
hour thalli are removed from bicarbonate media and resuspended in
water, pigmentation and wall-thickening cease. Then, after approximately 3 more hours, they form papillae and discharge spores in the
manner of OC cells. This does not occur, however, when RS thalli have
gone beyond 36 hours of growth. During such a morphogenetic reversal,
total protein, cytochrome oxidase activity, and initial stages of the
glucose-monophosphate shunt remain essentially unaltered; however,
the activity of a-ketoglutaric dehydrogenase increases abruptly while
that of isocitric dehydrogenase decreases (Lovett and Cantino, 1961).
Simultaneously, the soluble-protein pattern immediately reverts to one
that is more characteristic of OC cells (Cantino and Goldstein, 1962), as
does part of the nucleic acid pool (Cantino, 1961b). In particular, these
differential changes in two enzymes of the citric acid cycle, under
conditions which prevent consummation of normal RS development,
are significant and directly in agreement with their suggested role in
morphogenesis.
So far, these investigations have been limited to a few enzymes; we
do not know, as yet, the full extent of the enzymatic alterations which
are involved. In this connection, it is important to emphasize that a
visible manifestation of cell activity was selected as the primary criterion
for reversibility; i.e., prompt discharge of spores from a thin-walled
sporangium, a combination characteristic of OC cells. This does not
mean, of course, that all parameters associated with an RS cell before
reversal will 'de-differentiate'. Some, or even most, of them may remain
unchanged. For example, there is no evidence that the small amounts of
melanin accumulated in the cell by this time, nor the chitin built up into
the thallus wall, are degraded during this period of morphogenetic
reversal. In fact, while the quantitative relationship between some
soluble protein fractions reverts to levels found in OC cells, that for
other protein components does not do so (Cantino and Goldstein, 1962).
Continued formation of the cross-wall until it is complete, whereby the
basal rhizoidal cell is separated from the terminal sporangium—albeit a
thin-walled onef—occurs in OC cells derived from RS cells by morphogenetic reversal; this clearly shows that at least some of the gross
morphological structures of the cell are not easily modified.
f See'footnote on opposite page.