88
EDWARD C. CANTINO AND JAMES S. LOVETT
clues which are not as easily obtainable from a blastoderm or the
embryo of a seed.
When the spore of Blastocladiella
emersonii settles down and germinates, it gives rise to a cell with a determinate system of growth. If
provided with exogenous bicarbonate, it differentiates directly into a
deep-brown, thick-walled RS cell which proliferates no further in this
habitat.
But, if the spore develops in the absence of exogenous bicarbonate,
the picture which unfolds is vastly different A thin-walled OC cell is
formed, and it proliferates in situ by liberating another crop of spores ;
the clone of second-generation cells produced therefrom, if left in place,
now displays its totipotency.
If individual, thin-walled growing cells are removed at random from
this clone, and if they are then placed in a fresh environment wellisolated from one another, they all develop into OC cells once more.
These cells, in turn, proliferate again by producing spores, and this
cycle can be repeated ad infinitum. However, if the cells are left in place
within the clone, some develop into colourless, thin-walled OC cells but
others differentiate into thick-walled, non-proliferating RS cells. Some
of the OC cells may then cease growth prematurely ; but others may in
turn reproduce again, and in this way add a third generation of progeny
to the clone. Thus, when this heterogeneous multicellular colony finally
ceases to proliferate, it consists mainly of the brown, thick-walled
differentiated cells, along with some colourless thin-walled cells the
growth of which was interrupted at different stages in their ontogeny.
Cessation of clonal growth is due, at least in part, to the accumulation of
metabolic bicarbonate and/or C0 2 within this dense package of cells,
whereby the great majority of the members of the clone are induced to
differentiate into RS cells. Thus, the growth of this cell colony is selflimited) it is terminated because of the metabolic formation of the
morphogenetic inducer, bicarbonate, which brings about differentiation
into non-proliferating cells.
The cells of the mutant strains of B. emersonii lack the capacity to
produce differentiated RS cells, even if bicarbonate is provided;
similarly, clones of these mutants do not behave as described above for
the wild type. All of the cells in a clone are thin-walled. Thus, although
the individual cells display the usual determinate system of growth, the
colonies do not. Peripheral spread of a mutant clone does not cease
after two or three generations; instead, it continues to
proliferate
indefinitely so long as nutrients are available for new cell generations. The
mutants of B. emersonii have lost the enzyme, ketoglutaric dehydrogenase, which is involved in the response to accumulation of metabolic bicarbonate ; limitation of growth by induction of differentiation
EDWARD C. CANTINO AND JAMES S. LOVETT
clues which are not as easily obtainable from a blastoderm or the
embryo of a seed.
When the spore of Blastocladiella
emersonii settles down and germinates, it gives rise to a cell with a determinate system of growth. If
provided with exogenous bicarbonate, it differentiates directly into a
deep-brown, thick-walled RS cell which proliferates no further in this
habitat.
But, if the spore develops in the absence of exogenous bicarbonate,
the picture which unfolds is vastly different A thin-walled OC cell is
formed, and it proliferates in situ by liberating another crop of spores ;
the clone of second-generation cells produced therefrom, if left in place,
now displays its totipotency.
If individual, thin-walled growing cells are removed at random from
this clone, and if they are then placed in a fresh environment wellisolated from one another, they all develop into OC cells once more.
These cells, in turn, proliferate again by producing spores, and this
cycle can be repeated ad infinitum. However, if the cells are left in place
within the clone, some develop into colourless, thin-walled OC cells but
others differentiate into thick-walled, non-proliferating RS cells. Some
of the OC cells may then cease growth prematurely ; but others may in
turn reproduce again, and in this way add a third generation of progeny
to the clone. Thus, when this heterogeneous multicellular colony finally
ceases to proliferate, it consists mainly of the brown, thick-walled
differentiated cells, along with some colourless thin-walled cells the
growth of which was interrupted at different stages in their ontogeny.
Cessation of clonal growth is due, at least in part, to the accumulation of
metabolic bicarbonate and/or C0 2 within this dense package of cells,
whereby the great majority of the members of the clone are induced to
differentiate into RS cells. Thus, the growth of this cell colony is selflimited) it is terminated because of the metabolic formation of the
morphogenetic inducer, bicarbonate, which brings about differentiation
into non-proliferating cells.
The cells of the mutant strains of B. emersonii lack the capacity to
produce differentiated RS cells, even if bicarbonate is provided;
similarly, clones of these mutants do not behave as described above for
the wild type. All of the cells in a clone are thin-walled. Thus, although
the individual cells display the usual determinate system of growth, the
colonies do not. Peripheral spread of a mutant clone does not cease
after two or three generations; instead, it continues to
proliferate
indefinitely so long as nutrients are available for new cell generations. The
mutants of B. emersonii have lost the enzyme, ketoglutaric dehydrogenase, which is involved in the response to accumulation of metabolic bicarbonate ; limitation of growth by induction of differentiation