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Help with gene evolution

lhanson lhanson at moose.uvm.edu
Tue Oct 5 14:36:05 EST 1993

To all:

	I've been considering a question of gene evolution and I find
that I need some help.  Let me explain:

	Some fungi have more than 2 different "sexes", we call them
mating-types.  We have found that in the fungus I study (Schizophyllum
commune) that there are four genetic loci that govern mating-type.  One
of these loci has been studied in detail down to the nucleotide sequence
and we have found 2 genes at this locus (complex locus actually).  From
classical studies we know that there are nine different mating-type
specificities that can occupy this locus.  In a haploid individual the genes
might be designated A and B.  We have shown that it is the interaction
of A with B' or A' with B that is the active form in mating.  We have
a naturally occurring mutant that is lacking gene A (it was this strain that
allowed us to show the A/B interaction as opposed to an A/A' or B/B'
interaction).  Using this B only strain we have shown that the A' B haploid
(we created by transformation) can grow, mate and produce viable offspring.

	We have never found (after extensive, world wide searches) an
isolate with the A/B' mating-type and our question is why not?  It is clear
from DNA sequence comparisons among isolates that there is substantial
sequence divergence; they are only about 50% similar.  This low similarity may
account for a failure to synapse and form chiasmata during meiosis, thus
preventing recombination between the two genes (or at least making it a
rare occurrence).

	My question is, Since we know there are nine different specificities
each specificity must have arisen at random resulting in an A/B'
individual and in an A'/B individual (somewhere or sometime later).  At
some point these two individuals mated and produced some offspring with
the recombinant genotypes of A/B and A'/B'.  This was repeated until the nine
known specificities were produced.  Why then don't we find any A/B'
individuals in nature today?

	An obvious answer, but possibly a trivial answer, is that the A/B'
and A'/B genotypes experienced conditions where they were at a competitive
disadvantage, their numbers were reduced to a point (0) where those
genotypes went extinct.

	What about it?  What are some other possibilities?

Lee Hanson
Dept. of Botany
Univ. of Vermont

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