Growth, Differentiation and Sexuality

76 J.P. Latgé and R. Calderone
Fig. 5.3. Emergence of polysaccharides in the fungal cell wall
based upon taxonomical clustering and fungal evolution fitted
to the geological time scale. Branch lengths are propor-
ancestors, contains both glucan and chitin, the
later being present in the highest amount among
all fungi. These two polysaccharides represent
the first in the neosynthesis of a cell wall in the
fungal kingdom, as seen especially in this class
of fungi which exists mostly without a wall. Cell
wall composition evolves from this central core
either positively or negatively. For example, an
evolutionary dead end seen in one branch of the
Zygomycetes, the Mucorales, is the loss of the β1,3
tional to the average nucleotide substitution of nuclear SSU
rDNA sequences (adapted from and with J. Taylor; arrow ↓
polysaccharide emerging, arrow ↑ polysaccharide lost)
glucan and the occurrence of deacetylated chitin
and polymers of uronic acid. Modifications in
other fungal orders consisted mainly of a more or
less complex decoration of this β glucan–chitin
skeleton. Some essential steps in the evolution
are the appearance of α1,3 glucans both in the
Ascomycetes and Basidiomycetes. Pentoses are
the hallmark of Basidiomycetes. Galactofuranose
seems to be present only amongst the most
recently evolved fungi, the higher Ascomycetes.