Introduction to Fungi, Third Edition

638 USTILAGINOMYCETES: SMUT FUNGI AND THEIR ALLIES
Fig 23.1 The life cycle of Ustilago maydis, diagrammatic and not to scale.Teliospores are formed in galls developing from dikaryotic
hyphae. Karyogamy (K) occurs in the teliospore, followed by meiosis (M) and germination to form a three-septate promycelium.
Each segment buds off numerous haploid sporidia. Sporidia bud in a yeast-like manner and are capable of saprotrophic growth but
unable to infect maize plants.When yeast cells or sporidia of compatible mating type meet, conjugation tubes are formed, and their
fusion (plasmogamy, P) gives rise to a dikaryotic hypha which is infectious on maize.Open and closed circles represent haploid nuclei
of opposite mating type; the diploid nucleus is drawn larger and half-filled.
compatible cells. The multiallelic b locus controls
growth of the dikaryon, as well as its ability to
infect the host and to complete sexual development
(see p. 643). In the bipolar species U. hordei,
the genetic control of fusion and dikaryon growth
is very similar to that in U. maydis, with the
important differences that there are only
two instead of multiple b alleles and that the
a and b loci are tightly linked on the same
chromosome (Lee et al., 1999). Genetic recombination
during meiosis is suppressed in the region
containing the a and b loci. This region is
substantially larger than the mating type genes
themselves, making up about one-sixth of the
chromosome on which it resides. Hence, this
chromosome is regarded as a primitive form of
the sex chromosome as found, for example,
in mammalian organisms (Fraser & Heitman,
2004). Mammalian sex chromosomes are recognizably
different when they are condensed at
mitosis, the X chromosome being very much
larger than the Y chromosome. Intriguingly,
measurable size differences have also been
found between the two sex chromosomes of the
urediniomycetous smut, Microbotryum violaceum
(Hood, 2002).
The smut fungi are dimorphic, producing
both yeast cells and true hyphae. This feature has
aroused the interest of cell biologists, and
especially Ustilago maydis is being used extensively
as a tool to examine fundamental aspects
of eukaryotic biology (see Fig. 23.8).