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Sordaria brevicollis
is an ascomycete
saprophytic fungus which, like most hyphal
fungi, is haploid. Briefly, though, before meiosis,
the cells become diploid, and ascospores
are formed. These ascospores contain melanin, which helps protect
them from the mutagenic effects of ultraviolet
radiation. These normal wild-type (wt) ascospores
are black. However, mutations can occur in the genes which express
products that regulate the biosynthesis of melanin, resulting in
mutants that are often white or buff-coloured.
The life cycle of Sordaria is well
defined. When two hyphae of different mating types (“sexes”)
meet and fuse to form a dikaryotic cell, conjugate
mitosis occurs (nuclear division with two nuclei to form
a diploid nucleus). The resulting diploid nucleus then divides by
meiosis (the nuclear division process with two divisions, the first
to give two diploid nuclei, and then four haploid nuclei) to give
cells called the ascus initial cells. Subsequently,
these cells divide by mitosis to give eight ascospores.
These ascospores are held tidily together
in a structure called an ascus (Latin for "bag";
and it is best thought of as a bag). The narrow ascus of S.
brevicollis holds the spores in an ordered pattern which reflects
the events of meiosis. This makes it easy to examine the segregation
of mutant genes using a microscope.
If allele segregation occurs at the first
division of meiosis, in a cross between a strain with wild-type
pigmented spores and one with mutant, pale spores, the spores are
arranged so that spores of the same colour are arranged together
in just two groups, eg:
But if segregation occurs at the second division,
the spores are arranged in pairs, eg:
Variations in these patterns, or in the ratio of
spore colours, indicate whether prossesses such as recombination
(i.e. crossing-over of segments
of homologous chromosomes ) or gene conversion
have occurred. Such studies allow the creation of genetic
maps, i.e. establishing where different genes are on the
cromosome, so that we can refer to
genes by their position, much as we would refer to A roads and B
roads on a roadmap.
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