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DIFFERENTIATION AND DEVELOPMENT 105

the prohormones are produced by each strain, but
trisporic acids cause gene derepression so that increas-
ing levels of prohormones are produced. Trisporic
acids produced by this mutual escalation cause the
production of zygophores(aerial sexual branches)
which grow towards one another and lead to sexual
fusion (see Fig. 2.8).

Ascomycota

The Ascomycota usually have two mating types,
termed Aand a, but aand ααin Saccharomyces. These

mating-type genes are regulatory elements, controlling

the activities of other genes, but only the system in

yeasts has been well characterized. One of the chromo-

somes (III) of the yeast Saccharomyces cerevisiaehas a

MATgene locus, which is flanked by two other loci

termed MATaand MATα. Haploid cells will normally

behave as ααmating type, but can change to amating

type when the information at the MATa locus is

copied and transferred into the MAT locus (the expres-

sion of the MATα. locus is then shut down). This is a

natural transposition event, which causes the switch-

ing of mating type every time a cell produces a bud.

β-Carotene

+ Mating type

prohormone

• Mating type
  prohormone

Converted

via – mating type

Converted

via + mating type

+ strain – strain

OH

O

CH 2 OH

Trisporic acid

Sporangia Zygophores Sporangia

O

O

COOH

O

O

CH 2 OH

OH

O

COOH

Fig. 5.23The trisporic acid hormonal system
of Zygomycota. See text for details.