Introduction to Fungi, Third Edition

SORDARIALES
319
vitamins by the stimulating fungi (Watanabe,
1997).
Mating systems of Sordaria
Although S. fimicola is homothallic, it has the
ability to hybridize. Wild-type strains have black
ascospores, but mutants are known with pale or
colourless spores. If a wild-type strain and a
white-spored mutant strain are inoculated on
opposite sides of a Petri dish, hybrid perithecia
develop from heterokaryotic parts of the mycelium.
The asci from hybrid perithecia usually
have four black and four white ascospores. Six
different arrangements of the ascospores are
found in such hybrid asci (Fig. 12.2). Asci with
four black or four white ascospores at the tip of
the ascus are those in which the gene for spore
colour segregated at the first meiotic nuclear
division separating the paired chromosomes. In
those with two black or two white ascospores at
the tip of the ascus, segregation of the gene for
spore colour occurred at the second meiotic
division separating the two sister chromatids of
each chromosome. First-division segregation
results from the absence of a cross-over between
the gene for spore colour and the centromere of
the chromosome, whilst second-division segregation
results from a single cross-over between
gene and centromere. Since the likelihood of
crossing-over depends on the distance between
gene and centromere, the frequency of the two
kinds of segregation pattern can be used for
determining the distance of the gene for spore
colour relative to the centromere.
A low proportion of hybrid asci shows 5 : 3,
6 : 2 or (very rarely) 7 : 1 colour segregation
patterns. These findings are explained in terms
of gene conversion, a non-reciprocal process
where one allele of a gene converts another
allele at the same locus to its own type (Lamb,
1996). Similar patterns have been reported
in other ascomycetes, e.g. Ascobolus immersus
(p. 423). A model to account for the molecular
basis of gene conversion was developed with the
smut fungus Ustilago maydis (see p. 652). In this
so-called ‘Holliday model’ DNA strands are
exchanged at ‘Holliday junctions’ between two
paired DNA double helices, which can result in
the generation of hybrid or heteroduplex DNA.
Following separation of the two double helices
from each other, non-matching DNA will be
excised and the undamaged strand will be used
as template to synthesize the second, damaged
strand (Holliday, 1964; Lewin, 2000). This can
then give rise to the phenomenon of gene
conversion which, if it happens in the ascus,
is manifested as deviations from the 4 : 4 gene
segregation patterns.
Fig12.2 Sordaria fimicola.Squash
preparation from a hybrid perithecium.
Most ripe asci contain four black and
four white ascospores.