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Spontaneous mutation during the sexual cycle of Neurospora crassa
1Department of Genetics, University of Washington, Seattle 98195.
Abstract:
The DNA sequences of 42 spontaneous mutations of the mtr gene in Neurospora crassa have been determined. The mutants were selected among sexual spores to represent mutations arising in the sexual cycle. Three sexual-cycle-specific mutational classes are described: hotspot mutants, spontaneous repeat-induced point mutation (RIPs) and mutations occurring during a mutagenic phase of the sexual cycle. Together, these three sexual-cycle-specific mutational classes account for 50% of the mutations in the sexual-cycle mutational spectrum. One third of all mutations occurred at one of two mutational hotspots that predominantly produced tandem duplications of varying lengths with short repeats at their end-points. Neither of the two hotspots are present in the vegetative spectrum, suggesting that sexual-cycle-specific mutational pathways are responsible for their presence in the spectrum. One mutant was observed that appeared to have been RIPed precociously. The usual prerequisite for RIP, a duplication of the affected region, was not present in the parent stocks and was not detected in this mutant. Finally, there is a phase early in the premeiotic sexual cycle that is overrepresented in the generation of mutations. This "peak" appears to represent a phase during which the mutation rate rises significantly. This phase produces a disproportionally high fraction of frame shift mutations (3/6). In divisions subsequent to this, the mutation rate appears to be constant.
Insights
This study details DNA mutations in Neurospora crassa's sexual cycle, identifying unique mutation types like hotspot mutants and repeat-induced mutations (RIPs). These findings reveal distinct mutational pathways specific to sexual reproduction.
Area of Science:
- Molecular Biology
- Genetics
- Mycology
Background:
- Understanding spontaneous mutations is crucial for comprehending genome stability.
- The sexual cycle in eukaryotes involves complex genetic processes that can influence mutation rates and types.
- Neurospora crassa serves as a model organism for studying fungal genetics and mutation mechanisms.
Purpose of the Study:
- To determine the DNA sequences of spontaneous mutations in the mtr gene of Neurospora crassa arising during the sexual cycle.
- To identify and characterize distinct classes of mutations specific to the sexual cycle.
- To investigate the role of mutational hotspots and a mutagenic phase within the sexual cycle.
Main Methods:
- Selection of 42 spontaneous mtr mutants from sexual spores of Neurospora crassa.
- DNA sequencing to determine the precise nature and location of mutations.
- Comparative analysis of mutation spectra between sexual and vegetative cycles.
Main Results:
- Three sexual-cycle-specific mutational classes were identified: hotspot mutants, repeat-induced point mutations (RIPs), and mutations during a mutagenic phase, accounting for 50% of sexual cycle mutations.
- Two mutational hotspots, absent in the vegetative spectrum, predominantly generated tandem duplications.
- A mutagenic phase early in the premeiotic sexual cycle showed a significantly elevated mutation rate, particularly for frameshift mutations.
Conclusions:
- The sexual cycle in Neurospora crassa harbors unique mutational pathways distinct from vegetative processes.
- Mutational hotspots and a transient mutagenic phase contribute significantly to the sexual cycle's mutation spectrum.
- These findings enhance our understanding of genome evolution and the mechanisms driving genetic variation during sexual reproduction.