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Positional regulation and evolution of mating type genes in heterothallic and homothallic species of neurospora

University of British Columbia

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.

Thesis/Dissertation

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2009-06-25

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http://hdl.handle.net/2429/9604

Botany

University of British Columbia

UBCV

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