Blakeslee: Mutations in Mucors 
it seems to have lost its distinctive 
characteristics. 
The history of the mutant “X”’ is 
given in the Table on page 284. In 
a series of isolation cultures in 1914 the 
mutant bred practically true with only 
three possible reversions to normal out 
of nearly 1500 colonies. In the 17th 
generation in 1916 the mutant seemed 
to have entirely reverted. It was 
regained, however, from a culture of an 
earlier generation and by a series of 
isolations its ability to throw offspring 
like itself was again increased. In 1917, 
after a few generations grown in test 
tube cultures, the mutant again ap- 
peared to have entirely reverted to the 
normal parental type and could not be 
regained. 
At two other times in this species 
have mutations been observed which 
form lines of zygospores with the nor- 
nal stock: once in an isolation culture 
in 1916 and once ten years earlier at 
the germination of the zygospores. 
Other mutants tested have shown 
a minus tendency like the parent stock. 
A STRIKING MUTANT FORM 
One of the most striking mutant 
forms appeared as a small warty colony 
in an isolation culture of 949 colonies. 
A microscopic examination showed that 
the colony was composed exclusively 
of a mass of yeast-like cells somewhat 
similar to those that are formed when 
the normal mycelium of this species is 
submerged in a sugar solution and 
takes part in alcoholic fermentation. 
An isolation made from this original 
colony gave predominatingly yeast- 
like colonies with only a few normal 
colonies. At first no filaments were 
found and the accumulation of yeast- 
like cells formed a warty mound above 
the surface of the agar. Often the 
drops of water exuding from the agar 
in running down the inside of the tube 
would carry with them the yeast-like 
cells of the mutant and form streaks 
of secondary ‘“‘yeast”’ colonies. Later 
each colony gave rise to a few normal 
filaments, the further rapid growth of 
which filled the culture and covered 
over the warty mutants. During 
283 
April, 1913, a series of four isolation 
cultures were made of the “yeast” 
mutant, resulting in 721 ‘‘yeast’’ colo- 
nies to 423 early reverting colonies. 
The records were taken on the fourth 
or fifth day. Eventually even the 
typical ‘‘yeast”’ colonies reverted. Dur- 
ing August and September 1914, an 
attempt was made to regain the 
“yeast”? condition from five test tube 
cultures which had originally contained 
“veasts.”” A total of 5,995 colonies 
were examined from these tubes, but 
the ‘“‘yeast’’ mutant could not be 
recovered. Reversion in this mutant 
takes place regularly in the mycelium. 
In other mutants reversion is appar- 
ently more common at the formation of 
spores. 
The ‘“‘X”’ and the “A” mutants are 
of especial interest from the stand- 
point of sexual differentiation. On ac- 
count of its freedom from zygospores 
and its relatively strong reaction with 
plus test races, mutant “‘A,” if found 
alone, would appear to be an unmated 
minus race of some dioecious species. 
Its very weak reaction with certain 
minus races might easily be missed. 
If mutant ‘‘X,’’ which is a mutant in 
the plus direction, had been likewise 
devoid of zygospores and found to 
conjugate with mutant “‘A,” as it 
actually did, one would have felt 
justified in considering ‘““X”’ and “A” 
as the mated plus and minus races 
of a dioecious species. It is possible 
that in nature dioecious races may 
have arisen from hermaphrodites 
through mutations which have carried 
the sexual differentiation farther than 
was observed in our two mutants. 
Burgeff has obtained mutations in 
the dioecious mucor genus Phycomy- 
ces. The mucors are multinucleate, 
normally without cross walls in the 
vegetative mycelium. Mutations, he 
considers, affect only a part of the 
nuclei. The more rapid division of the 
normal nuclei in these mixo-chimeras, 
as he calls the variants, would account 
for the reversions which almost always 
take place. It is possible that our re- 
verting mutants in the hermaphro- 
ditic Mucor genevensis are in fact 
