> 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
1====>>> Is it possible that A/B' is lethal? ( Prade at BSCR.UGA.EDU)