NON-FILAMENTOUS AQUATIC FUNGI
89
cannot occur, and the mutant clone therefore continues to proliferate.
An idea frequently encountered in discussions of embryology is that
differentiation involves a change in the nucleus or its genetic material in
such a way that, after a certain period, it loses its totipotency ; thus, the
point is reached where it can only permit the genesis of one type of cell.
It has often been implied that this is a one-way street, and that it is
irreversible. But, to put this concept to the test is another matter. It
would be most difficult if not impossible, today, for the animal embryologist to separate two adjacent cells from one another, put them back
into an external environment similar to the one from which they were
removed, and then induce them to develop along alternate pathways
into—let us say—a liver cell on the one hand and a nerve cell on the
other. With B. emersonii, this can indeed be done. In its life-history,
the kind of irreversibility discussed above simply does not exist;
all the sister nuclei in the fully differentiated organism, whether
this be an OC cell or an RS cell, can go through either process of
ontogeny over and over again via the spores in which they finally end up.
It seems to us, then, that this one aspect of the biology of B. emersonii
deserves a final re-emphasis. Different types of cells can be removed
from a clone, and their development studied under reproducible and
essentially natural conditions; as such, the fungus provides a
£
model
system' with which one may examine how it is that a pair of cells can
sing two entirely different tunes using the same genetic score !
This picture of development in B. emersonii was obtained about
a decade ago. In the intervening years, we have made attempts to
understand its operation in mechanistic terms. Some degree of success
has been achieved as, we trust, this review has shown. The fact that
questions have arisen faster than they could possibly have been answered
reminds us of a remark made recently by Emerson (1955); in paying
tribute to the fungus which he, more than anyone else, helped to put
upon the map, he wrote: 'The versatility of Allomyces has sometimes
angered us, often humiliated us, and always challenged us.' With
Blastocladiella,
we have had the very same experiences. But then,
perhaps, all of this has been its own reward; from a geriatrical point of
view, it has helped to slow down the inevitable hardening of the
concepts.
Acknowledgement
All unpublished work on B. emersonii mentioned in this review was
supported by research grants from the National Science Foundation to
both writers, and from the National Institutes of Health to Edward C.
Cantino.
89
cannot occur, and the mutant clone therefore continues to proliferate.
An idea frequently encountered in discussions of embryology is that
differentiation involves a change in the nucleus or its genetic material in
such a way that, after a certain period, it loses its totipotency ; thus, the
point is reached where it can only permit the genesis of one type of cell.
It has often been implied that this is a one-way street, and that it is
irreversible. But, to put this concept to the test is another matter. It
would be most difficult if not impossible, today, for the animal embryologist to separate two adjacent cells from one another, put them back
into an external environment similar to the one from which they were
removed, and then induce them to develop along alternate pathways
into—let us say—a liver cell on the one hand and a nerve cell on the
other. With B. emersonii, this can indeed be done. In its life-history,
the kind of irreversibility discussed above simply does not exist;
all the sister nuclei in the fully differentiated organism, whether
this be an OC cell or an RS cell, can go through either process of
ontogeny over and over again via the spores in which they finally end up.
It seems to us, then, that this one aspect of the biology of B. emersonii
deserves a final re-emphasis. Different types of cells can be removed
from a clone, and their development studied under reproducible and
essentially natural conditions; as such, the fungus provides a
£
model
system' with which one may examine how it is that a pair of cells can
sing two entirely different tunes using the same genetic score !
This picture of development in B. emersonii was obtained about
a decade ago. In the intervening years, we have made attempts to
understand its operation in mechanistic terms. Some degree of success
has been achieved as, we trust, this review has shown. The fact that
questions have arisen faster than they could possibly have been answered
reminds us of a remark made recently by Emerson (1955); in paying
tribute to the fungus which he, more than anyone else, helped to put
upon the map, he wrote: 'The versatility of Allomyces has sometimes
angered us, often humiliated us, and always challenged us.' With
Blastocladiella,
we have had the very same experiences. But then,
perhaps, all of this has been its own reward; from a geriatrical point of
view, it has helped to slow down the inevitable hardening of the
concepts.
Acknowledgement
All unpublished work on B. emersonii mentioned in this review was
supported by research grants from the National Science Foundation to
both writers, and from the National Institutes of Health to Edward C.
Cantino.