42
EDWARD C. CANTINO AND JAMES S. LOVETT
enzymes and very few species—with the latter usually employed in the
single physiological state of logarithmic growth. Clearly, though they
cannot be handled quite as easily as bacteria, nor grown quite as fast, it
is time the water-moulds found their way into the biochemist's lair. To
make a case for them (if not to prove the point), we have selected
Blastocladiella emersonii as our model system and the focal point for our
discussion.
Some aspects of the biology of this organism, and in particular the
relationship between biochemical and morphological differentiation in
it, have been reviewed and summarized in recent publications (Cantino,
1956, 1961a; Cantino and Turian, 1959; Emerson, 1955). Thus, we shall
avoid as much as possible, reshuffling outworn facts and figures,
sprinkling in some new material, and presenting all of it once again. We
shall take this opportunity, instead, to emphasize the inherent remarkable utility of B. emersonii as a model system for experimental studies of
cell differentiation ; a system in which all the stages of differentiation of
a structural and functional complex occur, from start to finish, within a
single cell. We propose to do this by disconnecting, in sequential fashion,
the organism's various developmental stages from one another. We hope
to emphasize, thereby, not only some of the things that have been done
with them but, perhaps above all, what can be done with them.
Let us consider, first, the basic pattern of development in Blastocladiella emersonii (Fig. 2). Upon retracting its flagellum, the spore sends
out a germ tube which becomes the rhizoidal system of the plant.
Simultaneously, the body of the cell begins to enlarge and enters a phase
of exponential growth. When increase in size has ceased, and the
subterminal rhizoidal cell has been delimited by a septum, the latter
compartment is seen to be devoid of contents while the terminal,
multinucleate cell goes on to differentiate into a sac of spores (a
sporangium). Disregarding two minor developmental paths that can be
taken by a spore (which lead to 'orange' and 'late colourless' cells,
respectively; Cantino and Hyatt, 1953a), there remain two major routes
along which the spores develop (Fig. 2). One of these gives rise to an
Ordinary colourless' sporangium (the OC cell), the other to a resistant
sporangium (the RS cell). At maturity, the OC cell liberates its spores,
whereas the RS cell remains a dormant, so-called 'resting' structure
unless it is induced to discharge spores. In either case, such spores begin
the cycle once again. Finally, the morphogenetic course taken by the
spore can be controlled at will by simply providing or withholding
exogenous bicarbonate ; in its presence, RS cells are formed while in its
absence, OC cells appear instead. Later on, we shall have much more to
say about this bicarbonate trigger mechanism; let us examine, now,
these ontogenetic events in greater detail.
EDWARD C. CANTINO AND JAMES S. LOVETT
enzymes and very few species—with the latter usually employed in the
single physiological state of logarithmic growth. Clearly, though they
cannot be handled quite as easily as bacteria, nor grown quite as fast, it
is time the water-moulds found their way into the biochemist's lair. To
make a case for them (if not to prove the point), we have selected
Blastocladiella emersonii as our model system and the focal point for our
discussion.
Some aspects of the biology of this organism, and in particular the
relationship between biochemical and morphological differentiation in
it, have been reviewed and summarized in recent publications (Cantino,
1956, 1961a; Cantino and Turian, 1959; Emerson, 1955). Thus, we shall
avoid as much as possible, reshuffling outworn facts and figures,
sprinkling in some new material, and presenting all of it once again. We
shall take this opportunity, instead, to emphasize the inherent remarkable utility of B. emersonii as a model system for experimental studies of
cell differentiation ; a system in which all the stages of differentiation of
a structural and functional complex occur, from start to finish, within a
single cell. We propose to do this by disconnecting, in sequential fashion,
the organism's various developmental stages from one another. We hope
to emphasize, thereby, not only some of the things that have been done
with them but, perhaps above all, what can be done with them.
Let us consider, first, the basic pattern of development in Blastocladiella emersonii (Fig. 2). Upon retracting its flagellum, the spore sends
out a germ tube which becomes the rhizoidal system of the plant.
Simultaneously, the body of the cell begins to enlarge and enters a phase
of exponential growth. When increase in size has ceased, and the
subterminal rhizoidal cell has been delimited by a septum, the latter
compartment is seen to be devoid of contents while the terminal,
multinucleate cell goes on to differentiate into a sac of spores (a
sporangium). Disregarding two minor developmental paths that can be
taken by a spore (which lead to 'orange' and 'late colourless' cells,
respectively; Cantino and Hyatt, 1953a), there remain two major routes
along which the spores develop (Fig. 2). One of these gives rise to an
Ordinary colourless' sporangium (the OC cell), the other to a resistant
sporangium (the RS cell). At maturity, the OC cell liberates its spores,
whereas the RS cell remains a dormant, so-called 'resting' structure
unless it is induced to discharge spores. In either case, such spores begin
the cycle once again. Finally, the morphogenetic course taken by the
spore can be controlled at will by simply providing or withholding
exogenous bicarbonate ; in its presence, RS cells are formed while in its
absence, OC cells appear instead. Later on, we shall have much more to
say about this bicarbonate trigger mechanism; let us examine, now,
these ontogenetic events in greater detail.