Introduction to Fungi, Third Edition

SACCHAROMYCES (SACCHAROMYCETACEAE)
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Fig10.4 Saccharomyces cerevisiae.
Diagrammatic summary of the processes of
meiosis and ascospore delimitation (from
Beckett et al.,1974). (a d) The spindle pole
body replicates and the two new spindle pole
bodies move to opposite poles of the nucleus.
The nuclear membrane remains intact. (e,f)
Further replication of the spindle pole bodies
and rearrangement.The nuclear envelope is
still intact. New membranes, the ascosporedelimiting
membranes, form outside the spindle
pole bodies. (g,h) Envelopment of the
lobes of the dividing nucleus by the ascospore-delimiting
membranes results in the
formation of four haploid uninucleate
ascospores.
10.2.2 Mating in S. cerevisiae
Many strains of S. cerevisiae are heterothallic,
and the ascospores are of two mating types.
Mating type specificity is controlled by a single
genetic locus which exists in two allelic states,
a and a, and segregation at the meiosis preceding
ascospore formation results in two a and two a
ascospores. Fusion normally occurs only between
cells of opposite mating type, and this has
been termed legitimate copulation. Such fusions
result in diploid cells which can, under appropriate
conditions, form asci with viable
ascospores.
The mating type (MAT) alleles are rather small
and structurally similar to each other, but differ
in their central region which comprises about
650 base pairs (bp) in MATa and 750 bp in MATa
(Fig. 10.5). Because of this difference, mating type
alleles are often called idiomorphs. In haploid
a-cells, the MATa locus expresses two genes, both
of which encode regulatory proteins. The a1 gene
product interacts with a constitutively expressed
protein not encoded by the MAT locus, Mcm1p,
to activate several a-specific genes outside
the MAT locus, notably those encoding the
a-pheromone which is secreted by a-cells, and