| Summary: | The mating type genes of Neurospora crassa were shown to function abnormally when
located at ectopic chromosomal positions. Crosses involving strains with ectopic mating type
genes produce defective perithecia. Ascus number is reduced. The hypothesis that perithecial
development requires physical proximity of opposite mating type homologs during meiosis was
tested. The sterility of the crosses made between strains with both mating type regions relocated
to the same ectopic position failed to support the hypothesis. To test the hypothesis that normal
expression levels of the mating type genes require distant cw-acting sequences not present on
ectopic fragments, autoradiograms of mRNA from wild type and ectopic-w^ strains were
compared. Differences between expression levels in ectopic-/rtf and wild type cells were
observed, but their significance cannot be assessed without additional studies. The hypothesis
that nuclear identity is disrupted in ectopic-m/ strains was tested. Strains with disturbed nuclear
identity (dual mating type) were crossed to wild type. The appearance of the reduced ascus
number phenotype suggested that the affected function in ectopic-m/ strains is nuclear identity.
The homothallic species N. terricola contains mt A- and mt a-like sequences. The genes
were cloned and sequenced to determine whether or not they were functional. The genes
specifying identity, mt A-l and mt a-l, are more than 95% similar at the amino acid level to the
N. crassa homologs, but the putative MT A-2 polypeptide is truncated. N. terricola ml A-l and
mt a-l genes induced mating and vegetative incompatibility in N. crassa mating type mutants.
Expression in N. terricola of mt A-l and mt a-l was detected by reverse transcriptase PCR,
upholding the hypothesis that the genes are functional.
To determine the pattern of evolution of homothallism, a phylogeny of Neurospora was
reconstructed from mt A-l DNA and amino acid sequences. Homothallic Neurospora species
that carry both mating type genes are more closely related to heterothallic species than they are
to the ^4-only homothallic species, suggesting that either heterothallic species are derived from a
homothallic ancestor or that homothallism arose twice. === Science, Faculty of === Botany, Department of === Graduate
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