Re: Why are ascomycete fungi particularly suitable for studying meiosis?

Hi Deborah,

	Ascomycetes are a class of fungi which produce a sac-like structure known as
an ascus at the end of meiosis, containing four or eight gametes, also
known as a tetrad.  This class includes several major model organisms,
including Saccharomyces cerevisiae (baker?s yeast) and Neurospora crassa
(red bread mold). The ascus conveniently keeps the meiotic products
together, facilitating studies of recombination and meiosis.  If two
haploid strains of fungus ab and AB are crossed, there are three different
types of unordered tetrad possible:

Parental ditype (PD): ab,ab,AB,AB
Non-Parental Ditype (NPD): aB,aB,Ab,Ab
Tetratype (T): AB,Ab,aB,ab

	The frequency of each type of tetrad will reflect the frequency of
recombination between loci.  Put simply, the recombination frequency (RF)
may be calculated from the frequency of NPD and T using the following
formula: RF = 1/2*T + NPD.  Thus, tetrad analysis was one of the first easy
ways to perform linkage mapping.

	Neurospora crassa is even more interesting: not only does the ascus keep the
meiotic products together, but the tetrad is kept in order.  Starting with
a diploid meiocyte, after the first meiotic division there are two 2n
nuclei (A and B).  After the second meiotic division there are four 1n
nuclei (1, 2, 3, and 4).  1 and 2 are derived from A, and 3 and 4 are
derived from B.  The four 1n nuclei then go through a single round of
mitotic division, producing an octad containing eight spores (1, 1, 2, 2,
3, 3, 4, and 4).  Again, by analyzing the patterns of spores within linear
octads it is possible to estimate recombination fraction.

	Anyhow, the short answer to your question is that Ascomycetes are useful in
studies of meiosis because they produce tetrads of gametes.  I?m not sure
why Sordaria in particular would be useful, but suspect that it may simply
represent a species with many spore color variants, therefore quite useful
in segregation analysis.

	Chris