ABSENCE OF INTERFERENCE IN ASSOCIATION WITH GENE CONVERSION IN SORDARIA FIMICOLA, AND PRESENCE OF INTERFERENCE IN ASSOCIATION WITH ORDINARY RECOMBINATION

ABSTRACT From the analysis of large samples of gene conversion asci in the g locus of Sordaria fimicola, it was found that neither the conversion event itself nor conversion-associated recombination of flanking markers cause either chiasma or chromatid interference with crossing over in a neighboring interval. The presence of more than one kind of crossover event, one causing interference the other not, is considered. The existence of two kinds of gene loci, one of single-cistron composition and the other of multiple-cistron composition, is discussed in relation to reciprocal recombination within a locus.

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Inherited Differences in Crossing Over and Gene Conversion Frequencies Between Wild Strains of Sordaria fimicola From “Evolution Canyon”

Genetics ◽

10.1093/genetics/159.4.1573 ◽

2001 ◽

Vol 159 (4) ◽

pp. 1573-1593

Author(s):

Muhammad Saleem ◽

Bernard C Lamb ◽

Eviatar Nevo

Keyword(s):

Genetic Variation ◽

Gene Conversion ◽

Spontaneous Mutation ◽

Crossing Over ◽

Natural Genetic Variation ◽

Evolution Canyon ◽

Wild Strains ◽

Sordaria Fimicola ◽

First And Second Generation ◽

Consistent Direction

Abstract Recombination generates new combinations of existing genetic variation and therefore may be important in adaptation and evolution. We investigated whether there was natural genetic variation for recombination frequencies and whether any such variation was environment related and possibly adaptive. Crossing over and gene conversion frequencies often differed significantly in a consistent direction between wild strains of the fungus Sordaria fimicola isolated from a harsher or a milder microscale environment in “Evolution Canyon,” Israel. First- and second-generation descendants from selfing the original strains from the harsher, more variable, south-facing slope had higher frequencies of crossing over in locus-centromere intervals and of gene conversion than those from the lusher north-facing slopes. There were some significant differences between strains within slopes, but these were less marked than between slopes. Such inherited variation could provide a basis for natural selection for optimum recombination frequencies in each environment. There were no significant differences in meiotic hybrid DNA correction frequencies between strains from the different slopes. The conversion analysis was made using only conversions to wild type, because estimations of conversion to mutant were affected by a high frequency of spontaneous mutation. There was no polarized segregation of chromosomes at meiosis I or of chromatids at meiosis II.

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Homologous recombination involving a large heterology in Escherichia coli.

Genetics ◽

10.1093/genetics/119.4.759 ◽

1988 ◽

Vol 119 (4) ◽

pp. 759-769

Author(s):

K Yamamoto ◽

N Takahashi ◽

H Yoshikura ◽

I Kobayashi

Keyword(s):

Escherichia Coli ◽

Gene Conversion ◽

Kanamycin Resistance ◽

Coli Strain ◽

The Other ◽

Reca Gene ◽

Inverted Repeats ◽

Crossing Over ◽

Wild Type ◽

Parental Allele

Abstract Recombination between two different deletion alleles of a gene (neo) for neomycin and kanamycin resistance was studied in an Escherichia coli sbcA- recB-C- strain. The two homologous regions were in an inverted orientation on the same plasmid molecule. Kanamycin-resistant plasmids were selected and analyzed. The rate of recombination to form kanamycin-resistant plasmids was decreased by mutations in the recE, recF and recJ genes, but was not decreased by a mutation in the recA gene. It was found that these plasmids often possessed one wild-type kanamycin-resistant allele (neo+) while the other neo allele was still in its original (deletion) form. Among kanamycin-resistant plasmids with one wild-type and one parental allele it was often found that the region between the inverted repeats had been flipped (turned around) with respect to sites outside the inverted repeats. These results were interpreted as follows. Gene conversion, analogous to gene conversion in eukaryotic meiosis, is responsible for a unidirectional transfer of information from one neo deletion allele to the other. The flipping of the region between the inverted repeats is interpreted as analogous to the crossing over associated with gene conversion in eukaryotic meiosis. In contrast with a rec+ strain, these products cannot be explained by two rounds of reciprocal crossing over involving a dimeric form as an intermediate. In the accompanying paper we present evidence that gene conversion by double-strand gap repair takes place in the same E. coli strain.

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MAPPING AND GENE CONVERSION STUDIES WITH THE STRUCTURAL GENE FOR ISO-1-CYTOCHROME C IN YEAST

Genetics ◽

10.1093/genetics/81.4.615 ◽

1975 ◽

Vol 81 (4) ◽

pp. 615-629

Author(s):

Christopher W Lawrence ◽

Fred Sherman ◽

Mary Jackson ◽

Richard A Gilmore

Keyword(s):

Saccharomyces Cerevisiae ◽

Gene Conversion ◽

The Other ◽

Crossing Over ◽

Wild Type ◽

Chromosome X ◽

Tetrad Analysis ◽

Yeast Saccharomyces Cerevisiae ◽

Distal Marker ◽

The Right

ABSTRACT We have investigated the order of the four genes cyc1, rad7, SUP4, and cdc8 which form a tightly linked cluster on the right arm of chromosome X in the yeast Saccharomyces cerevisiae. Crossing over and coconversion data from tetrad analysis established the gene order to be centromere–cyc1–rad7–SUP4. Also cdc8 appeared to be distal to SUP4 on the basis of crossovers that were associated with conversion of SUP4. The frequencies of recombination and the occurrence of coconversions suggest that these four genes are contiguous or at least nearly so. Gene-conversion frequencies for several cyc1 alleles were studied, including cyc1–1, a deletion of the whole gene that extends into the rad7 locus. The cyc1–1 deletion was found to be capable of conversion, though at a frequency some fivefold less than the other alleles studied, and both 3:1 and 1:3 events were detected. In general 1:3 and 3:1 conversion events were equally frequent at all loci studied, and approximately 50% of conversions were accompanied by reciprocal recombination for flanking markers. The orientation of the cyc1 gene could not be clearly deduced from the behavior of the distal marker SUP4 in wild-type recombinants that arose from diploids heteroallelic for cyc1 mutations.

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THE SUPPRESSION OF GENE CONVERSION AND INTRAGENIC CROSSING OVER IN ASCOBOLUS IMMERSUS: EVIDENCE FOR MODIFIERS ACTING IN THE HETEROZYGOUS STATE

Genetics ◽

10.1093/genetics/76.2.221 ◽

1974 ◽

Vol 76 (2) ◽

pp. 221-243

Author(s):

J Girard ◽

J L Rossignol

Keyword(s):

Genetic Distance ◽

Gene Conversion ◽

The Other ◽

Intragenic Recombination ◽

Crossing Over ◽

Heterozygous State ◽

Heterozygous Condition ◽

Pigmentation Gene ◽

Ascobolus Immersus ◽

Intergenic Recombination

ABSTRACT Four distinct systems cv1, cv2, cv4 and cv6 behaving as monogenic factors have been found in the stock 28 of Ascobolus immersus. They each specifically suppress gene conversion in a spore pigmentation gene (respectively, b1, b2, b4 and b6). In the gene b2, where this phenomenon was studied in most detail, the effect of cv2 on the suppression of gene conversion is almost total; intragenic recombination is very strongly diminished and crossing over is also affected. In the genes b1, b2 and b4 the suppression of conversion is observed only when the corresponding modifier (cv1, cv2 and cv4) is in the heterozygous condition. It is likely that cv6 acts in the same way on b6. cv1 and b1 on one hand, cv2 and b2 on the other, are very closely linked; no recombination was observed between these modifiers and the corresponding spore pigmentation genes. The same genetic distance is observed between the linked genes b4 and b6 and between cv4 and cv6. This could indicate that these two modifiers are also closely linked to the corresponding genes. A possible effect of cv4 on intergenic recombination between b4 and b6 was shown: here again it acts by diminishing the frequency of recombination when cv4 is in the heterozygous state. The problem of the nature of the cv modifiers is discussed.

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

Genetics ◽

10.1093/genetics/109.3.599 ◽

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.

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Mechanisms of gene conversion in Saccharomyces cerevisiae.

Genetics ◽

10.1093/genetics/124.1.7 ◽

1990 ◽

Vol 124 (1) ◽

pp. 7-25

Author(s):

H Roman ◽

M M Ruzinski

Keyword(s):

Saccharomyces Cerevisiae ◽

Stationary Phase ◽

Gene Conversion ◽

Close Contact ◽

Light Dose ◽

The Other ◽

Crossing Over ◽

X Rays ◽

Primary Event ◽

Mitotic Cells

Abstract In red-white sectored colonies of Saccharomyces cerevisiae, derived from mitotic cells grown to stationary phase and irradiated with a light dose of x-rays, all of the segregational products of gene conversion and crossing over can be ascertained. Approximately 80% of convertants are induced in G1, the remaining 20% in G2. Crossing over, in the amount of 20%, is found among G1 convertants but most of the crossovers are delayed until G2. About 20% of all sectored colonies had more than one genotype in one or the other sector, thus confirming the hypothesis that conversion also occurs in G2. The principal primary event in G2 conversion is a single DNA heteroduplex. It is suggested that the close contact that this implies carries over to G2 when crossing over and a second round of conversion occurs.

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INDUCTION OF INTRACHROMOSOMAL RECOMBINATION IN YEAST BY INHIBITION OF THYMIDYLATE BIOSYNTHESIS

Genetics ◽

10.1093/genetics/114.2.375 ◽

1986 ◽

Vol 114 (2) ◽

pp. 375-392

Author(s):

B A Kunz ◽

G R Taylor ◽

R H Haynes

Keyword(s):

Gene Conversion ◽

Dna Sequences ◽

Dna Hybridization ◽

Crossing Over ◽

Repeated Dna ◽

Yeast Saccharomyces Cerevisiae ◽

Haploid Strain ◽

Reciprocal Exchange ◽

Coli Plasmid ◽

Antifolate Drugs

ABSTRACT The biosynthesis of thymidylate in the yeast Saccharomyces cerevisiae can be inhibited by antifolate drugs. We have found that antifolate treatment enhances the formation of leucine prototrophs in a haploid strain of yeast carrying, on the same chromosome, two different mutant leu2 alleles separated by Escherichia coli plasmid sequences. That this effect is a consequence of thymine nucleotide depletion was verified by the finding that provision of exogenous thymidylate eliminates the increased production of Leu+ colonies. DNA hybridization analysis revealed that recombination, including reciprocal exchange, gene conversion and unequal sister-chromatid crossing over, between the duplicated genes gave rise to the induced Leu+ segregants. Although gene conversion unaccompanied by crossing over was responsible for the major fraction of leucine prototrophs, events involving reciprocal exchange exhibited the largest increase in frequency. These data show that recombination is induced between directly repeated DNA sequences under conditions of thymine nucleotide depletion. In addition, the results of this and previous studies are consistent with the possibility that inhibition of thymidylate biosynthesis in yeast may create a metabolic condition that provokes all forms of mitotic recombination.

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Gene Conversion Alone Accounts for More Than 90% of Recombination Events at the am Locus of Neurospora crassa

Genetics ◽

10.1093/genetics/143.1.129 ◽

1996 ◽

Vol 143 (1) ◽

pp. 129-136 ◽

Cited By ~ 1

Author(s):

Frederick J Bowring ◽

David E A Catcheside

Keyword(s):

Molecular Markers ◽

Neurospora Crassa ◽

Gene Conversion ◽

Holliday Junctions ◽

Crossing Over ◽

Gene Markers ◽

General Significance ◽

Flanking Sequences ◽

Simple Conversion

Abstract We have used closely flanking molecular markers located ~4 kb distal and 6 kb proximal of the am locus to investigate the incidence of crossover events associated with the generation of prototrophic recombinants in a cross heteroallelic am1 am6. Ninety-three percent of prototrophs were generated by events that did not recombine the molecular markers, indicating that simple conversion accounts for the formation of most prototrophs and that associated crossovers are much less frequent (~0.07) than estimated previously using more distant flanking markers. This suggests that conversion and crossing over during meiosis may arise from distinct mechanisms or that if, as is widely supposed, conversion and crossing over result from alternate modes of resolution of Holliday junctions then, at least for the am locus of Neurospora, the mode of resolution is strongly biased in favor of retaining the parental association of flanking sequences. Because estimates of the association of conversion and crossing over based on more distant gene markers are similar for yeast and Neurospora (~0.35), our observation may have general significance.

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MITOTIC CROSSING OVER AND NONDISJUNCTION IN TRANSLOCATION HETEROZYGOTES OF ASPERGILLUS

Genetics ◽

10.1093/genetics/82.4.605 ◽

1976 ◽

Vol 82 (4) ◽

pp. 605-627

Author(s):

Etta Käfer

Keyword(s):

Selective Advantage ◽

The Other ◽

Translocation Chromosome ◽

Crossing Over ◽

Selective Marker ◽

Reciprocal Translocations ◽

Poor Growth ◽

Different Types ◽

Diploid Genotype ◽

One Step

ABSTRACT To analyze mitotic recombination in translocation heterozygotes of A. nidulans two sets of well-marked diploids were constructed, homo- or heterozygous for the reciprocal translocations T1(IL;VIIR) or T2(IL;VIIIR) and heterozygous for selective markers on IL. It was found that from all translocation heterozygotes some of the expected mitotic crossover types could be selected. Such crossovers are monosomic for one translocated segment and trisomic for the other and recovery depends on the relative viabilities of these unbalanced types. The obtained segregants show characteristically reduced growth rates and conidiation dependent on sizes and types of mono- and trisomic segments, and all spontaneously produce normal diploid sectors. Such secondary diploid types either arose in one step of compensating crossing over in the other involved arm, or—more conspicuously—in two steps of nondisjunction via a trisomic intermediate.—In both of the analyzed translocations the segments translocated to IL were extremely long, while those translocated from IL were relatively short. The break in I for T1(I;VII) was located distal to the main selective marker in IL, while that of T2(I;VIII) had been mapped proximal but closely linked to it. Therefore, as expected, the selected primary crossover from the two diploids with T2(I;VIII) in coupling or in repulsion to the selective marker, showed the same chromosomal imbalance and poor growth. These could however be distinguished visually because they spontaneously produced different trisomic intermediates in the next step, in accordance with the different arrangement of the aneuploid segments. On the other hand, from diploids heterozygous for T1(I;VII) mitotic crossovers could only be selected when the selective markers were in coupling with the translocation; these crossovers were relatively well-growing and produced frequent secondary segregants of the expected trisomic, 2n+VII, type. For both translocations it was impossible to recover the reciprocal crossover types (which would be trisomic for the distal segments of I and monosomic for most of groups VII or VIII) presumably because these were too inviable to form conidia.—In addition to the selected segregants of expected types a variety of unexpected ones were isolated. The conditions of selection used favour visual detection of aneuploid types, even if these produce only a few conidial heads and are not at a selective advantage. For T2(I;VIII) these "non-selected" unbalanced segregants were mainly "reciprocal" crossovers of the same phenotype and imbalance as the selected ones. For T1(I;VII) two quite different types were obtained, both possibly originating with loss of the small VII-Itranslocation chromosome. One was isolated when the selective marker in repulsion to T1(I;VII) was used and, without being homo- or hemizygous for the selective marker, it produced stable sectors homozygous for this marker. The other was obtained from both coupling and repulsion diploids and showed a near-diploid genotype; it produced practically only haploid stable sectors of the type expected from monosomics, 2n-1 for the short translocation chromosome.

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