scholarly journals The extent to which gene conversion can change allele frequencies in populations

1982 ◽  
Vol 39 (2) ◽  
pp. 199-217 ◽  
Author(s):  
B. C. Lamb ◽  
S. Helmi
Keyword(s):  
Gene Conversion ◽  
Allele Frequencies ◽  
Numerical Examples ◽  
Critical Comparison ◽  
Control Factors ◽  
Ascobolus Immersus ◽  
Genetic Conversion

SummaryThe gene conversion parameters which affect allele frequencies in populations are defined, and their ranges and typical values are given for several genera of fungi, where meiotic octads and tetrads provide the best information on conversion. Both gene conversion and disparity in direction of conversion are common. Data from Ascobolus immersus show that conversion properties are largely stable with time, but can be changed environmentally and by genetic conversion control factors. Equations are given for the interactions of selection, mutation and gene conversion in determining equilibrium frequencies. Numerical examples, using typical values of conversion parameters from the fungal data, show that for alleles which are selectively neutral or have very low selection coefficients, conversion will often have very large effects on their equilibrium frequencies and may lead to fixation. Where selection coefficients are higher, conversion has major effects on the frequencies of deleterious recessive alleles, but lesser effects on deleterious dominant alleles: a critical comparison is that of s with 2y. The available estimates for conversion parameters (at least in fungi) are of a magnitude to make gene conversion an important factor in evolution.

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 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 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.

Mechanism of gene conversion in Ascobolus immersus

1973 ◽  
Vol 122 (2) ◽  
pp. 165-182 ◽  
Author(s):  
G. Leblon ◽  
J. -L. Rossignol
Keyword(s):  
Gene Conversion ◽  
Ascobolus Immersus

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

Genetics ◽  
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.

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

Genetics ◽  
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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