Introduction to Fungi, Third Edition

INTRODUCTION
133
Fig 6.5 Sexual reproduction in Zygorhizidium planktonicum. (a) Empty donor gametangium to the left connected by a
conjugation tube to a mature resting spore. (b) Near-median section of a fully formed meiosporangium which has developed
from a germinating resting spore.The donor gametangium is on the right. Scale bar ¼ 4 mm. After Doggett and Porter (1996).
been reported in species of Rhizophydium (Karling,
1977); this involves copulation between the
gametangium of a rhizoid-forming thallus and
a motile gamete that encysts directly on the
gametangium.
Generally the product of sexual reproduction
is a resting spore or resting sporangium with
thick walls, but it is known that thick-walled
sporangia may also develop asexually and in
many chytrids sexual reproduction has not been
described and possibly does not occur. Resting
sporangia of some chytrids may remain viable
for many years.
6.1.4 Classification and evolution
Fossil chytrids have been reported from the
400 million-year-old Rhynie chert, a site known
for the discovery of fossil remains of the
earliest known vascular land plants. Clusters
of holocarpic, endobiotic thalli resembling the
present day Olpidium have been found inside
cells of a coenobial alga preserved within the
hollow axes of a vascular plant, and epibiotic
sporangia with endobiotic rhizoids have been
seen attached to meiospores of a vascular plant,
much like those of extant chytrids like
Rhizophydium which grow on pollen grains
(Taylor et al., 1992). Chytrid-like fossils have also
been found in strata of the 340 million-year-old
Pennsylvanian (Carboniferous) era (Millay &
Taylor, 1978) and from the more recent Eocene
strata (Bradley, 1967).
Formerly thought to have an affinity for
the Oomycota, Hyphochytriomycota or protists,
the Chytridiomycota are now accepted as
members of the true fungi, the Eumycota. They
are probably ancestral to other groups of true
fungi, especially the Zygomycota (Cavalier-Smith,
1987, 2001; D. J. S. Barr, 2001). The inclusion of the
chytrids in the Eumycota is supported by several
DNA-based phylogenetic analyses (e.g. Bowman
et al., 1992; James et al., 2000), but the delimitation
of orders within the Chytridiomycota is still
problematic. Particularly puzzling is the grouping
of the Blastocladiales with the Zygomycota on
the basis of 18S ribosomal DNA sequences
(see Fig. 1.26).
D. J. S. Barr (2001) and Kirk et al. (2001) have
classified the Chytridiomycota into five orders
(Table 6.1) but the details of their distinguishing
features need not concern us here. We shall
study examples from each order.