Aberrant segregation patterns and gene mappability in Ascobolus immersus

SummaryCrosses between various types of mutant giving specific patterns of aberrant segregation were performed in the b2 spore colour locus of Ascobolus immersus. The map of 41 mutations showing various patterns of aberrant segregation was established. The frequency of wild-type recombinants and the map additivity, map expansion and map contraction characteristics were shown to be strongly dependent upon the pattern of aberrant segregation of the mutations used. Mutations giving no postmeiotic segregation and an excess of conversion to wild type over conversion to mutant exhibit map expansion in small intervals and a strong map contraction in large intervals. Mutations giving postmeiotic segregations also exhibit map contraction in large intervals. Mutations giving no postmeiotic segregations and an excess of conversion to mutant over conversion to wild type show map additivity and thus provide a simple way for devising gene maps. The relationship between the mapping properties and the pattern of aberrant segregations is accounted for when considering parameters of gene conversion: frequency and distribution of hybrid DNA, frequency and direction of mismatch correction.

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GENETIC RECOMBINATION AT THE BUFF SPORE COLOR LOCUS IN SORDARIA BREVICOLLIS. II. ANALYSIS OF FLANKING MARKER BEHAVIOR IN CROSSES BETWEEN BUFF MUTANTS

Genetics ◽

10.1093/genetics/103.2.161 ◽

1983 ◽

Vol 103 (2) ◽

pp. 161-178

Author(s):

Helen Sang ◽

Harold L K Whitehouse

Keyword(s):

Genetic Recombination ◽

Crossing Over ◽

Wild Type ◽

Equal Frequency ◽

Proximal Site ◽

The Cross ◽

The Relationship ◽

Aberrant Segregation ◽

Distal Site ◽

Hybrid Dna

ABSTRACT Aberrant asci containing one or more wild-type spores were selected from crosses between pairs of alleles of the buff locus in the presence of closely linked flanking markers. Data were obtained relating to the site of aberrant segregation and the position of any associated crossover giving recombination of flanking markers. Aberrant segregation at a proximal site within the buff gene may be associated with a crossover proximal to the site of aberrant segregation or, with equal frequency, with a crossover distal to the site of the second mutant present in the cross. Similarly, segregation at a distal site may be associated with a crossover distal to the site or, with lower frequency, with a crossover proximal to the site of the proximal mutant present in the cross. Crossovers between the alleles were rare. This evidence for the relationship between hybrid DNA and crossing over is discussed in terms of current models for the mechanism of recombination.

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Meiotic mismatch repair quantified on the basis of segregation patterns in Schizosaccharomyces pombe.

Genetics ◽

10.1093/genetics/133.4.815 ◽

1993 ◽

Vol 133 (4) ◽

pp. 815-824 ◽

Cited By ~ 1

Author(s):

P Schär ◽

P Munz ◽

J Kohli

Keyword(s):

Saccharomyces Cerevisiae ◽

Schizosaccharomyces Pombe ◽

Detailed Analysis ◽

Mismatch Repair ◽

Meiotic Recombination ◽

Wild Type ◽

Base Pairs ◽

Ascobolus Immersus ◽

Postmeiotic Segregation ◽

Hybrid Dna

Abstract Hybrid DNA with mismatched base pairs is a central intermediate of meiotic recombination. Mismatch repair leads either to restoration or conversion, while failure of repair results in postmeiotic segregation (PMS). The behavior of three G to C transversions in one-factor crosses with the wild-type alleles is studied in Schizosaccharomyces pombe. They lead to C/C and G/G mismatches and are compared with closely linked mutations yielding other mismatches. A method is presented for the detection of PMS in random spores. The procedure yields accurate PMS frequencies as shown by comparison with tetrad data. A scheme is presented for the calculation of the frequency of hybrid DNA formation and the efficiency of mismatch repair. The efficiency of C/C repair in S. pombe is calculated to be about 70%. Other mismatches are repaired with close to 100% efficiency. These results are compared with data published on mutations in Saccharomyces cerevisiae and Ascobolus immersus. This study forms the basis for the detailed analysis of the marker effects caused by G to C transversions in two-factor crosses.

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Genetic analysis of recombination at the g locus in Sordaria fimicola

Genetics Research ◽

10.1017/s0016672300015275 ◽

1974 ◽

Vol 24 (3) ◽

pp. 251-279 ◽

Cited By ~ 11

Author(s):

H. L. K. Whitehouse

Keyword(s):

Excision Repair ◽

Dna Structure ◽

Repair Process ◽

Wild Type ◽

Mismatch Correction ◽

Co Conversion ◽

Sordaria Fimicola ◽

Comparison Of The Results ◽

Postmeiotic Segregation ◽

Hybrid Dna

SUMMARYInterallelie crosses of mutants at the grey (g) spore colour locus in Sordaria fimicola, heterozygous for flanking markers, give rise to a large number of aberrant ascus genotypes, 45 of which can arise through relatively simple events and have been chosen for study. These genotypes comprise 50–75% of the aberrant asci, depending on the mutants crossed.Comparison of the results from 10 pairwise crosses involving 7 alleles reveals that linked postmeiotic segregation and co-conversion decrease rapidly in frequency with increasing separation of the mutant sites.The data from reciprocally recombinant asci, from asci with normal segregation at one of the two mutant sites, and from flanking marker behaviour in one- and two-point crosses, agree with the Holliday-Sobell formulation, with the following additional features:(1) The nuclease, which nicks homologous polynucleotides and then degrades one of the two nicked chains when a mutant enters the hybrid DNA structure, can show preferential degradation of the mutant (or the wild-type) chain. In addition, a second nuclease is involved in the excision-repair process that introduces an additional preferential (marker specific) bias in the degradation of the mutant (or the wild-type) chain. This could explain why asci with odd-ratio conversion (5:3 and 3:5 ratios) sometimes show a different bias, as first reported by Emerson for Ascobolus, from those with even-ratio conversion (6:2 and 2:6 ratios), since the latter but not the former require, in addition, the action of a mismatch correction enzyme to account for them.(2) The migratory hybrid DNA structure which enters the gene at one end may be of a different size from that which enters from the other end.(3) Mismatch correction at the end of the hybrid DNA structure leads to a non-recombinant outside marker genotype and modifies the 1:1 ratio of parental:recombinant flanking markers that is otherwise found.

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Recombination within the Y locus in Ascobolus immersus

Genetics Research ◽

10.1017/s0016672300010466 ◽

1967 ◽

Vol 9 (2) ◽

pp. 159-177 ◽

Cited By ~ 23

Author(s):

A. Kruszewska ◽

W. Gajewski

Keyword(s):

Opposite Direction ◽

Negative Correlation ◽

Linear Order ◽

Great Majority ◽

Wild Type Allele ◽

Wild Type ◽

Marker Effect ◽

Dna Models ◽

Ascobolus Immersus ◽

Hybrid Dna

Mutants of the Y locus differed appreciably in their basic conversion frequencies (frequencies of conversion in one-point crosses) to wild type. The differences in the basic conversion frequencies in the opposite direction, i.e. from corresponding wild-type allele to mutant, were in general not pronounced. For some alleles frequencies of conversion in both directions were similar, but for the others they differed markedly. No evident correlation between the position of mutants on the map and their basic conversion frequencies was observed.In two-point crosses in repulsion, the great majority of recombinant octads were of conversion type. In these crosses symmetry or asymmetry of conversion depended mainly on similarity or differences in basic conversion frequencies of mutants crossed. In crosses between mutants from different clusters the recombination frequencies were near to the sums of their basic conversion frequencies. Such ‘mutant specificity’ makes it impossible to establish the linear order of mutants on the basis of recombination frequencies in two-point crosses.The results of two-point crosses in repulsion between mutants within clusters pointed to the influence of one allele on the frequency of conversion of another one. This ‘marker effect’ was also evident in some three-point crosses.The frequencies of simultaneous conversions in two-point crosses in coupling did not show negative correlation with the distances between the mutants involved.It seems that many of the data presented here are most easily explained by recently developed hybrid DNA models.

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Reciprocal exchanges instigated by large heterologies in the b2 gene of ascobolus are not associated with long adjacent hybrid DNA stretches.

Genetics ◽

10.1093/genetics/119.2.329 ◽

1988 ◽

Vol 119 (2) ◽

pp. 329-336

Author(s):

T Langin ◽

H Hamza ◽

V Haedens ◽

J L Rossignol

Keyword(s):

Gene Conversion ◽

Dna Sequence ◽

Sequence Homology ◽

Holliday Junctions ◽

Holliday Junction ◽

Mendelian Segregation ◽

Ascobolus Immersus ◽

Postmeiotic Segregation ◽

Single Branch ◽

Hybrid Dna

Abstract In the gene b2 of Ascobolus immersus, large heterologies increase the frequencies of reciprocal exchanges on their upstream border (corresponding to the high non-Mendelian segregation side). Tests were made to determine whether these reciprocal exchanges, instigated by large heterologies, resulted from the blockage of a Holliday junction bordering a hybrid DNA tract extending from the end of the gene to the heterology. Three types of experiments were performed to answer this question. In all cases, results did not correlate the presence of reciprocal exchanges instigated by large heterologies with the presence of adjacent hybrid DNA tracts. These reciprocal exchanges were rarely associated with postmeiotic segregation at upstream markers, they were not associated with gene conversion of a marker within the interval and their frequency was not decreased by decreasing the frequency of hybrid DNA formation in the gene. These results led to the proposal of the existence of a precursor to reciprocal exchange different from a single branch-migrating Holliday junction. This precursor migrates rightward and its migration is dependent on the DNA sequence homology. The existence of this precursor does not exclude that reciprocal exchanges resulting from the maturation of single Holliday junctions bordering adjacent hybrid DNA tracts could also occur.

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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

Genetics Research ◽

10.1017/s001667230001288x ◽

1973 ◽

Vol 22 (1) ◽

pp. 113-124 ◽

Cited By ~ 21

Author(s):

B. C. Lamb ◽

M. R. T. Wickramaratne

Keyword(s):

Complex Structure ◽

Wild Type ◽

Synaptinemal Complex ◽

Sister Chromatids ◽

Ascobolus Immersus ◽

Type Mutant ◽

Hybrid Dna

SUMMARY‘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.

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THE INTERGRADATION, GENETIC INTERCHANGEABILITY AND INTERPRETATION OF GENE CONVERSION SPECTRUM TYPES

Genetics ◽

10.1093/genetics/92.1.49 ◽

1979 ◽

Vol 92 (1) ◽

pp. 49-65

Author(s):

Bernard C Lamb ◽

Aglaia Ghikas

Keyword(s):

Gene Conversion ◽

Base Substitution ◽

Meiotic Segregation ◽

Wild Type ◽

Conversion Frequency ◽

Ascobolus Immersus ◽

Asymmetric Hybrid ◽

Type C ◽

Conversion Spectrum ◽

Type Strains

ABSTRACT In the Pasadena strains of Ascobolus immersus, the gene conversion propperties of 29 induced (nine UV, nine NG, and 11 ICR-170) and nine spontaneous white-ascospore mutations have been studied. Each mutant was cyossed to three types of derived wild-type strains; single mutants often gave very different conversion results in the three types of crosses, with any or all of the following changes in: percentage with post-meiotic segregation among aberrant-ratio asci; percentage with conversion to wild type among aberrant-ratio asci; and in total conversion frequency. — These results are compared with those of LEBLON (1972 a, b) from Ascobolus immersus and Yu-SUN, WICRRAMARATNE and WHITEHOUSE (1977) from Sordaria breuicollis. It is shown that conversion spectrum types are not necessarily distinct, but can completely intergrade, on the criteria of both post-meiotic segregation frequency and direction of correction. Genetic differences between strains in the present work resulted in much interchangeability of spectrum types for the same mutation in different crosses; e.g., from type C in one cross to type B/D type in another cross, although the mutation is presumably of the same molecular type (addition or deletion frame shift, or base substitution) in each cross. These changes of conversion properties for a given mutation in different crosses mean that previous interpretations of spectrum types in terms of specific conversion properties for various molecular types of mutation are inapplicable, or inadequate on their own, to explain the present data. Other factors, such as heterozygous cryptic mutations or conversion control genes, are probably involved. Because of asymmetric hybrid DNA formation, correction properties may differ from observed conversion properties.

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THE INTERACTIONS OF THREE WIDELY SEPARATED LOCI CONTROLLING CONVERSION PROPERTIES OF w LOCUS I IN ASCOBOLUS IMMERSUS

Genetics ◽

10.1093/genetics/104.1.23 ◽

1983 ◽

Vol 104 (1) ◽

pp. 23-40

Author(s):

Shaker Helmi ◽

Bernard C Lamb

Keyword(s):

Linkage Group ◽

Trans Position ◽

Group Viii ◽

Position Effects ◽

Control Factors ◽

Complex Interactions ◽

Group I ◽

Conversion Frequency ◽

Ascobolus Immersus ◽

Hybrid Dna

ABSTRACT Gene conversion properties of white (w) ascospore locus I in the Pasadena strains of Ascobolus immersus are controlled by complex interactions between three separate conversion control factors (ccfs), which can give conversion frequencies at wI ranging from less than 1% up to 33%. ccf-2, which has three alleles, is very closely linked to wI but does not usually co-convert with it. ccf-2(K) and ccf-2(91) give lower conversion frequencies than ccf-2(P) and are incompletely dominant to ccf-2(P), with cis/trans position effects on conversion of wI. The "super" factor (Helmi and Lamb 1979) has two interacting but unlinked components, ccf-3E and ccf-4r, which approximately double the conversion frequency at wI. ccf-2 (linkage group VIII), ccf-3 (linkage group I) and ccf-4 are probably all unlinked but interact and specifically control conversion at wI. ccf-3E could code for a diffusible product that affects the action of different ccf-2 alleles, which probably act by controlling the frequency of initiation of hybrid-DNA, which spreads into the adjacent wI locus. ccf-4R could code for a diffusible inhibitor of ccf-3E product, or be an alternative binding site for ccf-3E product. The dominance of ccf-4R depends on which ccf-2 alleles are present in the cross.

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Postmeiotic segregation in locus ‘46’ of Ascobolus immersus

Genetics Research ◽

10.1017/s0016672300011496 ◽

1968 ◽

Vol 11 (3) ◽

pp. 311-318 ◽

Cited By ~ 8

Author(s):

W. Gajewski ◽

A. Paszewski ◽

A. Dawidowicz ◽

B. Dudzińska

Keyword(s):

Type Strain ◽

Wild Type Strain ◽

Wild Type ◽

Type Conversion ◽

Satisfactory Explanation ◽

Ascobolus Immersus ◽

Postmeiotic Segregation

White-spored mutants of Ascobolus immersus were used to study postmeiotic segregation within a gene. It was found that in two-point crosses between mutants showing postmeiotic segregation, polarization in recombinant asci resulting from this type of segregation resembled that in normal (even-type) conversion. However, the ratio of recombinant asci with even segregation (6:2 asci) to those with odd segregation (7:1 asci) was much higher than might have been expected on the basis of the frequencies of various recombinant asci found in crosses between the same mutants and the wild-type strain. It seems that none of the current models of recombination gives a satisfactory explanation of the results obtained.

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Two locally acting genetic controls of gene conversion, ccf-5 and ccf-6, in Ascobolus immersus

Genetics Research ◽

10.1017/s0016672300025830 ◽

1984 ◽

Vol 43 (2) ◽

pp. 107-121 ◽

Cited By ~ 6

Author(s):

W. M. Howell ◽

B. C. Lamb

Keyword(s):

Gene Conversion ◽

Major Effect ◽

Control Factor ◽

Homozygous State ◽

Initiation Point ◽

Control Factors ◽

Conversion Frequency ◽

Ascobolus Immersus ◽

Higher Eukaryotes ◽

Hybrid Dna

SUMMARYTwo new conversion control factors (ccfs), ccf-5 and ccf-6, have been characterized in the Pasadena strains of Ascobolus immersus. Both are monogenic, with two known allelic forms (called A and B) of each factor, and affect the frequency of meiotic gene conversion at a white (w) ascospore locus closely linked to it, ccf-5 affecting w-9 and ccf-6 affecting w-BHj. The ccfs appear to be specific to their own target site, with no effect on at least nine unlinked w mutations. Conversion of the w locus affected was studied in + × w crosses with all four possible ccf arrangements: for example, for + × w-9, with ccf-5(A) in both parents, with ccf-5(B) in both parents, with ccf-5(A) in +, B in w-9, and with ccf-5(B) in +, A in w-9. For both ccfs, there were slight differences between crosses homozygous for A and those homozygous for B, and also slight differences between the two forms of heterozygous cross, A/B and B/A, but the major effect was for heterozygosity for the control factor to depress conversion frequency of the w locus, compared with either homozygous state. These two ccfs are compared with other sites affecting recombination in fungi and higher eukaryotes. Two possible modes of action of ccfs 5 and 6 are (i) on pairing closeness before hybrid DNA initiation, and (ii) on later stages such as the spread of hybrid DNA from an initiation point.

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