nmm sP

3036^
BIOLOGY 25
in others. Dickinson (1927), working with the covered smuts of oats and barley,
found that dicaryophytic mycelium, produced on ag*r by the fusion of appropriate
haplonts, failed to form a stable growth but reverted to the haploid
condition. Such a culture would be, at least for a time, a mixture of two biotypes,
but the suppression of one of them or its loss in transfer might reduce it to
the state of a monosporidial culture. From other evidence it seems that the
haplonts derived from a single chlamydospore may produce, in culture, a composite
growth effect with distinctive characteristics, which wiU persist through
a number of sub-cultures. A monospore culture usuttUy differs from the component
monosporidial cultures, because chlamydospores are often heterozygous
for cultural characters, but one collection (Lll) of Ustilago avenae showed no
such segregation, the four haplonts derived from a siiigle spore were uniform in
appearance and closely resembled all monospore cultiires of this race (Sampson
& Western, 1938).
Dicaryophytic mycelium is not always unstable cm culture media. Thren
(1937) succeeded, by the use of a low temperature, in separating the haplonts of
U. ntida. He grew them singly or in pairs and compared their growth with that
derived from a single chlamydospore. Dicaryophytic hyphae were wider, their
growth was stronger, and the resulting colony of a dicaryont had a smooth,
homogeneous appearance and lacked the radial corrugations which characterized
both plus and minus haplonts. In exceptional cases dicaryont colonies
developed sectors of monocaryotic mycehum. In flU examples tested these
sectors represented the minus haplonts, which could be recognized by their
weaker growth and by their long radial folds. * \
Normally in V. nuda stable dicaryophytic growth is maiatained in culture by
a regular method of cell division followed by the fusion of haploid cells (diagram
in Thren, 1941, p. 482). The forms of this smut on wheat and barley are not
identical in their mode of growth (Thren, 1941).
Stable dicaryophjrtic growth can also be obtained in species of Entyloma,
which produce binucleate, half-moon-shaped sporidia on the host (p. 22). These
germinate to give clamp mycelium, which spreads rather quickly over the agar,
produces abundant sporidia above the surface, and a niass of submerged chlamydospores.
Stempell (1935) found that some of these chlamydospores from
cultures of E. calendulae germinated normally, forming a promycelium with
terminal sporidia. Others, of later origin, developed a germ-tube which terminated
in another chlamydospore'or directly gave rise to clamp mycelium.
Cytological evidence indicated that caryogamy had failed in the abnormal
spores. Cultures of E. jicariae and E. calendulae, originating from the smaller
uninucleate sporidia, consisted of thinner mycelium without clamps. They
developed chlamydospores, but these only formed ordinary mycelium on germination
and were assumed to be haploid. Only a study of their origin, cytology,
and mode of germination can decide whether artificially produced chlamydospores
are haploid or diploid. In outward appearance, except possibly in degree
of pigmentation, they resemble those found on the host. Sartoris (1924)
obtamed, in a culture of Ustilago heuffleri from the dogtooth violet, uninucleate
chlamydospores which had their origin in a binucleate cell. They germinated to
give a four-celled promycelium with lateral sporidia and appeared to correspond
in every way with natural spores formed on the living host. D. T. Wang (1984)