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Sexual reproduction can also occur in fungi. This occurs when hyphae of opposite mating types grow
together and fuse. The haploid hyphal nuclei grow and undergo subsequent division, but they remain
in pairs, one nucleus from each of the two parental hyphae. Such hyphae are called dikaryotic. Nuclear
fusion occurs in time, forming diploid zygotes. The zygote then immediately undergoes meiosis to
form haploid spores that are distributed in the same manner as the vegetative spores.
Sexual reproduction modes and structures form the basis for classifying the major groups of fungi.
There are four phyla of fungi.
1. Zygomycota. These are the "algae-like" fungi. There are no septa in the hyphae, though the
reproductive structures are separated from the rest of the mycelium. Genetic material is
exchanged in thick-walled zygospores, which are formed by the conjugation of opposite mating
types, and then the haploid spores are released from the zygospores. Asexual reproduction is by
conidia, resistant spores that develop within the sporangium. Some examples are common bread
molds and parasites of protists, nematodes, insects, and small animals (Figure 2).
Figure 2
Sporangium
Columella
Sporangiophore
Mycellium
Rhizoid
2. Ascomycota. The mycelium of Ascomycetes form a cottony mass of multi-branched hyphae.
This phylum is characterized by sac-like reproductive structures called asci, which result from the
conjugation of two compatible mating strains. Each ascus typically contains eight ascospores,
which form when the short-lived diploid nuclei undergo meiosis. Many serious plant pathogens
are in this phylum, including Dutch Elm Disease and apple scab. The group also includes beneficial
yeasts (vital to baking and brewing industries) and highly prized edible fungi, including morels
and truffles (Figure 3).
Figure 3
A
B
C
D
E
A. Haploid yeast cells budding
B. Haploid cells forming smoos and zygotes
C. Zygote budding off diploid
D. Diploid budding
E. Diploid forming asci with ascospores,
freed spores with are haploid
