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

Genetics ◽  
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.

New equations and a method for finding nine parameter values for two alleles at one locus to study gene conversion using Ascobolus immersus

Genome ◽  
1992 ◽  
Vol 35 (3) ◽  
pp. 421-427 ◽  
Author(s):  
B. C. Lamb ◽  
S. A. Zwolinski
Keyword(s):  
Gene Conversion ◽  
Control Factors ◽  
Dna Models ◽  
Ascobolus Immersus ◽  
Asymmetric Hybrid ◽  
Quantitative Analyses ◽  
Allele Segregation ◽  
Parameter Values ◽  
Quantitative Treatment ◽  
Hybrid Dna

A quantitative treatment is given for meiotic gene conversion with its parameters and equations for their interactions to determine allele segregation class frequencies from heterozygotes. The possible pairing of both pairs of nonsister chromatids in a bivalent at exactly the same point is included. Using sets of data from Ascobolus immersus, it is shown that values for all nine parameters for hybrid DNA models of recombination can be obtained using an iterative computer program. The accuracy of the values is estimated and the double-strand gap repair model is considered. The parameter values obtained invalidate most of the simplifications used in previous quantitative analyses of gene conversion data. They showed total bias in strand preference in asymmetric hybrid DNA formation and some bias in which type of chromatid is the invading one. There were slight differences in repair frequency between the two types of mispair and very large differences in the direction of repair. Conversion control factors had major effects on hybrid DNA formation and repair of mispairs.Key words: Ascobolus, gene conversion, quantitative analysis, recombination mechanisms.

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

Genetics ◽  
1993 ◽  
Vol 133 (4) ◽  
pp. 815-824 ◽  
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.

DETECTION OF ABERRANT 4:4 ASCI IN ASCOBOLUS IMMERSUS

1978 ◽  
Vol 20 (1) ◽  
pp. 9-17 ◽  
Author(s):  
N. Paquette
Keyword(s):  
Gene Conversion ◽  
High Frequencies ◽  
Ascobolus Immersus ◽  
Very High Frequencies ◽  
Very High ◽  
Hybrid Dna

Ascopore and mycelium markers were used to detect the aberrant 4:4 asci of the mutation b2A4 of the b2 locus of Ascobolus immersus. Previous studies have shown that this mutation produces gene conversion events with many postmeiotic (30+:5 m and 5+:3 m) in addition to meiotic (2+:6 m and 6+:2 m) segregations. But the aberrant 4:4 asci could not be scored because the octads of this fungus are unordered. The asci were screened using round-spored and granular-spored mutants and confirmed with two mycelium markers, a brown mutant and a wave mutant. Very high frequencies of aberrant 4:4 asci were observed in each cross performed (more than 40% of the postmeiotic segregations). This suggested that both chromatids are very often implicated in the process of hybrid DNA formation at this site of the locus.

scholarly journals Fission Yeast Mus81·Eme1 Holliday Junction Resolvase Is Required for Meiotic Crossing Over but Not for Gene Conversion

Genetics ◽  
2003 ◽  
Vol 165 (4) ◽  
pp. 2289-2293 ◽  
Author(s):  
Gerald R Smith ◽  
Michael N Boddy ◽  
Paul Shanahan ◽  
Paul Russell
Keyword(s):  
Homologous Recombination ◽  
Fission Yeast ◽  
Gene Conversion ◽  
Protein Complex ◽  
Holliday Junctions ◽  
Holliday Junction ◽  
Crossing Over ◽  
Dna Structures ◽  
Branched Dna ◽  
Holliday Junction Resolvase

Abstract Most models of homologous recombination invoke cleavage of Holliday junctions to explain crossing over. The Mus81·Eme1 endonuclease from fission yeast and humans cleaves Holliday junctions and other branched DNA structures, leaving its physiological substrate uncertain. We report here that Schizosaccharomyces pombe mus81 mutants have normal or elevated frequencies of gene conversion but 20- to 100-fold reduced frequencies of crossing over. Thus, gene conversion and crossing over can be genetically separated, and Mus81 is required for crossing over, supporting the hypothesis that the fission yeast Mus81·Eme1 protein complex resolves Holliday junctions in meiotic cells.

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

1984 ◽  
Vol 43 (2) ◽  
pp. 107-121 ◽  
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.

scholarly journals Non-locus-specific polygenes giving responses to selection for gene conversion frequencies in Ascobolus immersus.

Genetics ◽  
1995 ◽  
Vol 140 (4) ◽  
pp. 1277-1287
Author(s):  
S A Zwolinski ◽  
B C Lamb
Keyword(s):  
Gene Conversion ◽  
Major Effect ◽  
Control Factor ◽  
Minor Effect ◽  
Control Factors ◽  
Conversion Frequency ◽  
Ascobolus Immersus ◽  
Selection For ◽  
Postmeiotic Segregation ◽  
Or Genes

Abstract Selection for higher and lower meiotic conversion frequencies was investigated in the fungus Ascobolus immersus. Strains carrying the same known gene conversion control factors, which have major effects on conversion frequencies at their specific target locus, sometimes gave significant differences in conversion frequency. Selection for high or low conversion frequencies at the w1-78 site was practiced for five generations, giving significant responses in both directions. These responses were due to polygenes, or genes of minor effect, not to new conversion control factors of major effect. Crosses of selected strains to strains with other mutations showed that the genes' effects were not specific to w1-78, but could affect conversion frequencies of another mutation, w1-3C1, at that locus and of two other loci, w-BHj and w9, which are unlinked to w1 or to each other. The proportional changes in gene conversion frequency due to selection varied according to the locus and site involved and according to the conversion control factor alleles present. There were differences of > or = 277% in conversion frequency between "high" and "low" strains. Selection for conversion frequency had little effect on other features of conversion, such as the frequency of postmeiotic segregation or the relative frequencies of conversion to mutant or wild type.

scholarly journals A new type of genetic control of gene conversion, from Ascobolus immersus

1978 ◽  
Vol 32 (1) ◽  
pp. 67-78 ◽  
Author(s):  
B. C. Lamb ◽  
S. Helmi
Keyword(s):  
Gene Conversion ◽  
Genetic Control ◽  
Total Conversion ◽  
Position Effects ◽  
Homologous Chromosomes ◽  
Control Factors ◽  
Ascobolus Immersus ◽  
New Type ◽  
Asymmetric Hybrid ◽  
Hybrid Dna

SUMMARYA new type of genetic control of gene conversion is described from the Pasadena strains of the fungus Ascobolus immersus. It is characterized by cis/trans position effects and incomplete dominance. The P, K and 91 factors segregated from each other like Mendelian alleles and controlled the conversion frequencies and patterns of four nearby, closely-linked white ascospore colour mutations, although they did not usually coconvert with these w sites. Mutations of different origin responded similarly to the same control factors.These control factors greatly affected the total conversion frequencies, the relative frequencies of the different detected conversion classes and various other conversion parameters. The detailed results are consistent either with P, K and 91 affecting both the frequency of hybrid DNA formation and the correction processes for removing mispaired bases, or if they do not affect the correction processes directly, then they must have large effects on the frequency of asymmetrical hybrid DNA formation, which must usually be much more common than symmetric hybrid DNA, and there must be both an inequality in the frequency with which the two homologous chromosomes (in these crosses, + bearing and w bearing) invade each other, and in the frequency with which the two strands of each chromatid invade the homologue in asymmetric hybrid DNA formation.

scholarly journals Aberrant segregation patterns and gene mappability in Ascobolus immersus

1982 ◽  
Vol 39 (2) ◽  
pp. 121-138 ◽  
Author(s):  
G. Leblon ◽  
V. Haedens ◽  
A. Kalogeropoulos ◽  
N. Paquette ◽  
J.-L. Rossignol
Keyword(s):  
Wild Type ◽  
Mismatch Correction ◽  
Conversion Frequency ◽  
Ascobolus Immersus ◽  
Strong Map ◽  
Postmeiotic Segregation ◽  
The Relationship ◽  
Gene Maps ◽  
Aberrant Segregation ◽  
Hybrid Dna

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.

scholarly journals GENE CONVERSION AT THE GRAY LOCUS OF SORDARIA FIMICOLA: FIT OF THE EXPERIMENTAL DATA TO A HYBRID DNA MODEL OF RECOMBINATION

Genetics ◽  
1985 ◽  
Vol 109 (3) ◽  
pp. 599-610
Author(s):  
Angelos Kalogeropoulos ◽  
Pierre Thuriaux
Keyword(s):  
Experimental Data ◽  
Gene Conversion ◽  
Sampling Error ◽  
Linear Equations ◽  
Initiation Site ◽  
Crossing Over ◽  
Dna Strands ◽  
Sordaria Fimicola ◽  
Postmeiotic Segregation ◽  
Hybrid Dna

ABSTRACT A hybrid DNA (hDNA) model of recombination has been algebraically formulated, which allows the prediction of frequencies of postmeiotic segregation and conversion of a given allele and their probability of being associated with a crossing over. The model considered is essentially the "Aviemore model." In contrast to some other interpretations of recombination, it states that gene conversion can only result from the repair of heteroduplex hDNA, with postmeiotic segregation resulting from unrepaired heteroduplexes. The model also postulates that crossing over always occurs distally to the initiation site of the hDNA. Eleven types of conversion and postmeiotic segregation with or without associated crossover were considered. Their theoretical frequencies are given by 11 linear equations with ten variables, four describing heteroduplex repair, four giving the probability of hDNA formation and its topological properties and two giving the probability that crossing over occurs at the left or right of the converting allele.—Using the experimental data of Kitani and coworkers on conversion at the six best studied gray alleles of Sordaria fimicola, we found that the model considered fit the data at a P level above or very close (allele h4) to the 5% level of sampling error provided that the hDNA is partly asymmetric. The best fitting solutions are such that the hDNA has an equal probability of being formed on either chromatid or, alternatively, that both DNA strands have the same probability of acting as the invading strand during hDNA formation. The two mismatches corresponding to a given allele are repaired with different efficiencies. Optimal solutions are found if one allows for repair to be more efficient on the asymmetric hDNA than on the symmetric one. In the case of allele g1, our data imply that the direction of repair is nonrandom with respect to the strand on which it occurs.

scholarly journals Parameters in gene conversion: An algebraic analysis of the hybrid DNA model at the gray locus of Sordaria fimicola.

1982 ◽  
Vol 40 (1) ◽  
pp. 1-18 ◽  
Author(s):  
Angelos Kalogeropoulos ◽  
Pierre Thuriaux
Keyword(s):  
Experimental Data ◽  
Gene Conversion ◽  
Mismatch Repair ◽  
Complex Situation ◽  
Algebraic Analysis ◽  
Sordaria Fimicola ◽  
Postmeiotic Segregation ◽  
Aberrant Segregation ◽  
Three Factor ◽  
Hybrid Dna

SUMMARYWe have extended previous algebraic analyses of aberrant segregation at the gray locus of Sordaria fimicola (Whitehouse, 1965; Emerson, 1966; Fincham, Hill & Reeve, 1980) to the more complex situation where aberrant segregations are detected in three factor crosses involving two flanking markers. This algebra has been applied to seven gray alleles which have been extensively characterized for their pattern of gene conversion and postmeiotic segregation by Kitani & Olive (1967). It is based on seven major types of aberrant segregation which can be distinguished in the presence of flanking markers spanning the converting site, and allows us to use up to six parameters to describe hDNA formation and mismatch repair. We present solutions which predict a spectrum of aberrant segregation fitting the experimental data at the P > 0·05 level for six of the seven alleles tested. They are consistent with the following properties of hDNA at the gray locus: (1) the single stranded DNA transferred during hDNA formation has always the same chemical polarity. (2) hDNA is mostly, if not entirely, symmetric, and its probability of formation is constant over the whole gene. (3) Disparity in aberrant segregation is mostly, if not entirely due to disparity in mismatch repair.

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