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Corresponding-site interference, synaptinemal complex structure, and 8+:0m and 7+:1m octads from wild-type × mutant crosses of Ascobolus immersus

Published online by Cambridge University Press:  14 April 2009

B. C. Lamb  and
M. R. T. Wickramaratne
B. C. Lamb
Affiliation:
Department of Botany, Imperial College, SW7 2BB
M. R. T. Wickramaratne
Affiliation:
Department of Botany, Imperial College, SW7 2BB

Summary

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‘Wider ratio’ octads (8:0, 0:8, 7:1 and 1:7) regularly occurred in wild-type(+) × white ascospore(w) crosses of the Pasadena strains of Ascobolus. Control crosses showed that phenocopies and false octad clusters were absent or rare; no reversion from w to + occurred, but mutation from + to w was found at a number of loci, with nearly all 0+:8w and many 2+:6w octads in + × w crosses arising from mutation, not conversion. Nearly all 8+:0w, 7+:1w and 6+:2w octads appeared to arise by conversion.

The finding of genuine wider ratio octads implies hybrid-DNA formation at corresponding sites in both pairs of non-sister chromatids in the same bivalent, which conflicts with models of the synaptinemal complex requiring that only two of the four chromatids pair intimately at any point. Octad types arising from hybrid-DNA formation at corresponding sites in both pairs of non-sister chromatids were described and formulae were derived for their frequencies. The lack of genuine wider ratio octads in several other Ascobolus studies was shown to be explicable quantitatively in terms of their conversion frequencies.

‘Corresponding-site interference’ is defined as interference between the two pairs of non-sister chromatids of a bivalent in hybrid-DNA formation at exactly corresponding sites. Formulae based on observed octad frequencies were derived for calculating coincidence values for this kind of interference. Corresponding-site interference was found to be weak, with coincidence values differing between crosses with high and with low conversion frequencies.

Type
Research Article
Information
Genetics Research , Volume 22 , Issue 1 , August 1973 , pp. 113 - 124
Copyright
Copyright © Cambridge University Press 1973

References

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