Biased gene conversion is not occurring among rDNA repeats in the Brassica triangle

Genome ◽  
1996 ◽  
Vol 39 (1) ◽  
pp. 150-154 ◽  
Author(s):  
Elizabeth R. Waters ◽  
Barbara A. Schaal
Keyword(s):  
Genetic Variation ◽  
Key Words ◽  
Gene Conversion ◽  
Relative Frequency ◽  
Brassica Species ◽  
Multigene Families ◽  
Crossing Over ◽  
Common Phenomenon ◽  
Unequal Crossing Over ◽  
Biased Gene Conversion

Hybridization is a common phenomenon that results in complex genomes. How ancestral genomes interact in hybrids has long been of great interest. Recombination among ancestral genomes may increase or decrease genetic variation. This study examines rDNA from members of the Brassica triangle for evidence of gene conversion across ancestral genomes. Gene conversion is a powerful force in the evolution of multigene families. It has previously been shown that biased gene conversion can act to homogenize rDNA repeats within hybrid genomes. Here, we find no evidence for biased gene conversion or unequal crossing over across ancestral genomes in allotetraploid Brassica species. We suggest that, while basic genomic processes are shared by all organisms, the relative frequency of these processes and their evolutionary importance may differ among lineages. Key words : Brassica, rDNA, gene conversion, allotetraploids.

scholarly journals UNEQUAL CROSSING OVER AT THE rRNA TANDON AS A SOURCE OF QUANTITATIVE GENETIC VARIATION IN DROSOPHILA

Genetics ◽  
1980 ◽  
Vol 95 (3) ◽  
pp. 727-742 ◽  
Author(s):  
R Frankham ◽  
D A Briscoe ◽  
R K Nurthen
Keyword(s):  
Genetic Variation ◽  
Selection Response ◽  
Crossing Over ◽  
Wild Type ◽  
X Chromosomes ◽  
Unequal Crossing Over ◽  
Quantitative Genetic ◽  
Y Chromosomes ◽  
Homozygous Line ◽  
Minimum Estimate

ABSTRACT Abdominal bristle selection lines (three high and three low) and controls were founded from a marked homozygous line to measure the contribution of sex-linked "mutations" to selection response. Two of the low lines exhibited a period of rapid response to selection in females, but not in males. There were corresponding changes in female variance, in heritabilities in females, in the sex ratio (a deficiency of females) and in fitness, as well as the appearance of a mutant phenotype in females of one line. All of these changes were due to bb alleles (partial deficiencies for the rRNA tandon) in the X chromosomes of these lines, while the Y chromosomes remained wild-type bb+. We argue that the bb alleles arose by unequal crossing over in the rRNA tandon.—A prediction of this hypothesis is that further changes can occur in the rRNA tandon as selection is continued. This has now been shown to occur.—Our minimum estimate of the rate of occurrence of changes at the rRNA tandon is 3 × 10-4. As this is substantially higher than conventional mutation rates, the questions of the mechanisms and rates of origin of new quantitative genetic variation require careful re-examination.

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

Genetics ◽  
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.

scholarly journals Genetic variation in small multigene families

1981 ◽  
Vol 37 (2) ◽  
pp. 133-149 ◽  
Author(s):  
Tomoko Ohta
Keyword(s):  
Genetic Variation ◽  
Natural Selection ◽  
Globin Gene ◽  
Gene Organization ◽  
Crossing Over ◽  
Random Genetic Drift ◽  
Cycle Model ◽  
Unequal Crossing Over ◽  
Biological Phenomena ◽  
Chromosomal Recombination

SUMMARYIn order to understand the evolution of genetic systems in which two genes are tandemly repeated (small multigene family) such as has been recently found in the haemoglobin α loci of primates, haemoglobin β loci of mouse and rarbit and other proteins, a population genetics approach was used. Special reference was made to the probarility of gene identity (identity coefficient), when unequal crossing-over is continuously occurring as well as random genetic drift, inter-chromosomal recombination and mutation. Two models were studied, cycle and selection models. The former assumes that unequal crossing-over occurs in cycles of duplication and deletion, and that the equilibrium identity coefficients were obtained. The latter is based on more realistic biological phenomena, and in this model it is assumed that natural selection is responsible for eliminating chromosomes with extra or deficient gene dose. Unequal crossing-over, inter-chromosomal recombination and natural selection lead to a duplication-deletion balance, which can then be treated as though it were a cycle model. The basic parameter is the rate of duplication-deletion which is shown to be approximately equal to 2(u + 2β)X, where u is the unequal crossing-over rate, 2β is the inter-chromosomal recombination rate and X is the frequency of chromosomes with three genes or of that with one gene. Genetic variation of the globin gene family, of which gene organization is known in most detail, is discussed in the light of the present analyses.

Unequal crossing-over and gene conversion at the amplifiedCUP1 locus of yeast

1990 ◽  
Vol 222 (2-3) ◽  
pp. 304-310 ◽  
Author(s):  
Juliet W. Welch ◽  
Daniel H. Maloney ◽  
Seymour Fogel
Keyword(s):  
Gene Conversion ◽  
Crossing Over ◽  
Unequal Crossing Over

scholarly journals Recombination and Spontaneous Mutation at the Major Cluster of Resistance Genes in Lettuce (Lactuca sativa)

Genetics ◽  
2001 ◽  
Vol 157 (2) ◽  
pp. 831-849
Author(s):  
Doris B Chin ◽  
Rosa Arroyo-Garcia ◽  
Oswaldo E Ochoa ◽  
Rick V Kesseli ◽  
Dean O Lavelle ◽  
...  
Keyword(s):  
Gene Conversion ◽  
Resistance Genes ◽  
Spontaneous Mutation ◽  
Large Chromosome ◽  
Crossing Over ◽  
Unequal Crossing Over ◽  
Chimeric Genes ◽  
Recessive Mutations ◽  
Major Cluster ◽  
Genetic Mechanisms

Abstract Two sets of overlapping experiments were conducted to examine recombination and spontaneous mutation events within clusters of resistance genes in lettuce. Multiple generations were screened for recombinants using PCR-based markers flanking Dm3. The Dm3 region is not highly recombinagenic, exhibiting a recombination frequency 18-fold lower than the genome average. Recombinants were identified only rarely within the cluster of Dm3 homologs and no crossovers within genes were detected. Three populations were screened for spontaneous mutations in downy mildew resistance. Sixteen Dm mutants were identified corresponding to spontaneous mutation rates of 10–3 to 10–4 per generation for Dm1, Dm3, and Dm7. All mutants carried single locus, recessive mutations at the corresponding Dm locus. Eleven of the 12 Dm3 mutations were associated with large chromosome deletions. When recombination could be analyzed, deletion events were associated with exchange of flanking markers, consistent with unequal crossing over; however, although the number of Dm3 paralogs was changed, no novel chimeric genes were detected. One mutant was the result of a gene conversion event between Dm3 and a closely related homolog, generating a novel chimeric gene. In two families, spontaneous deletions were correlated with elevated levels of recombination. Therefore, the short-term evolution of the major cluster of resistance genes in lettuce involves several genetic mechanisms including unequal crossing over and gene conversion.

Mathematical models for the evolution of multigene families by unequal crossing over

Nature ◽  
1977 ◽  
Vol 265 (5592) ◽  
pp. 304-310 ◽  
Author(s):  
Alan S. Perelson ◽  
George. I. Bell
Keyword(s):  
Mathematical Models ◽  
Multigene Families ◽  
Crossing Over ◽  
Unequal Crossing Over

scholarly journals New rust resistance specificities associated with recombination in the Rp1 complex in maize.

Genetics ◽  
1995 ◽  
Vol 141 (1) ◽  
pp. 373-381 ◽  
Author(s):  
T E Richter ◽  
T J Pryor ◽  
J L Bennetzen ◽  
S H Hulbert
Keyword(s):  
Gene Conversion ◽  
Resistance Genes ◽  
Partial Resistance ◽  
Rust Resistance ◽  
Crossing Over ◽  
Unequal Crossing Over ◽  
Genetic Reassortment ◽  
Marker Exchange

Abstract We address the question of whether genetic reassortment events, including unequal crossing over and gene conversion, at the Rp1 complex are capable of generating novel resistance specificities that were not present in the parents. Some 176 events involving genetic reassortment within the Rp1 complex were screened for novel resistance specificities with a set of 11 different rust biotypes. Most (150/176) of the events were susceptible to all tested rust biotypes, providing no evidence for new specificities. Eleven events selected as double-resistant recombinants, when screened with the 11 test biotypes, showed the combined resistance of the two parental types consistent with a simple recombination and pyramiding of the parental resistances. Nine events selected either as having partial resistance or complete susceptibility to a single biotype possessed resistance to a subset of the biotypes that the parents were resistant to, suggesting segregation of resistance genes present in the parental Rp1 complex. Four events gave rise to novel specificities being resistant to at least one rust biotype to which both parents were susceptible. All four had flanking marker exchange, demonstrating that crossing over within the Rp1 complex is associated with the appearance of new rust resistance specificities.

scholarly journals AN EXTENSION OF A MODEL FOR THE EVOLUTION OF MULTIGENE FAMILIES BY UNEQUAL CROSSING OVER

Genetics ◽  
1979 ◽  
Vol 91 (3) ◽  
pp. 591-607
Author(s):  
Tomoko Ohta
Keyword(s):  
Population Genetics ◽  
Diffusion Equation ◽  
Multigene Family ◽  
Gene Families ◽  
Multigene Families ◽  
Crossing Over ◽  
Homologous Chromosomes ◽  
Unequal Crossing Over ◽  
Random Frequency ◽  
Two Measures

ABSTRACT Evolution of a multigene family is studied from the standpoint of population genetics. It is assumed that the multigene family is undergoing continuous interchromosomal unequal crossing over, mutation and random frequency drift. The equilibrium properties of the probability of gene identity (clonality) are investigated, using two measures: identity probability within and between chromosomes. The measures represent homogeneity of genes within a family in one chromosome and $milarity of gene families between two homologous chromosomes. The means, the variances and the covariance of these two measures of identity probability are obtained by using the diffusion equation method. It is shown that the means and the variances are generally smaller than those predicted in the previous model assuming intrachromosomal (sister chromatid) unequal crossing over (OHTA 1978a,b).

Structural variation and dynamics of the nuclear ribosomal intergenic spacer region in key members of the Gibberella fujikuroi species complex

Genome ◽  
2013 ◽  
Vol 56 (4) ◽  
pp. 205-213 ◽  
Author(s):  
Salvador Mirete ◽  
Belén Patiño ◽  
Miguel Jurado ◽  
Covadonga Vázquez ◽  
María Teresa González-Jaén
Keyword(s):  
Gene Conversion ◽  
Species Complex ◽  
Intergenic Spacer ◽  
Gibberella Fujikuroi ◽  
Crossing Over ◽  
Repeat Elements ◽  
Unequal Crossing Over ◽  
Intergenic Spacer Region ◽  
Species Specific ◽  
Gibberella Fujikuroi Species Complex

The intergenic spacer (IGS) region of the ribosomal DNA was cloned and sequenced in eight species within the Gibberella fujikuroi species complex with anamorphs in the genus Fusarium, a group that includes the most relevant toxigenic species. DNA sequence analyses revealed two categories of repeated elements: long repeats and short repeats of 125 and 8 bp, respectively. Long repeats were present in two copies and were conserved in all the species analyzed, whereas different numbers of short repeat elements were observed, leading to species-specific IGS sequences with different length. In Fusarium subglutinans and Fusarium nygamai, these differences seemed to be the result of duplication and deletion events. Here, we propose a model based on unequal crossing over that can explain these processes. The partial IGS sequence of 22 Fusarium proliferatum isolates was also obtained to study variation at the intraspecific level. The results revealed no differences in terms of number or pattern of repeated elements and detected frequent gene conversion events. These results suggest that the homogenization observed at the intraspecific level might not be achieved primarily by unequal crossing-over events but rather by processes associated with recombination such as gene conversion events.

Evidence for the involvement of gene conversion in meiotic instability of the Rp1 rust resistance genes of maize

Genome ◽  
1994 ◽  
Vol 37 (5) ◽  
pp. 742-746 ◽  
Author(s):  
Gongshe Hu ◽  
Scot Hulbert
Keyword(s):  
Gene Conversion ◽  
Rust Resistance ◽  
Fragment Length Polymorphism ◽  
Crossing Over ◽  
Resistance Locus ◽  
Unequal Crossing Over ◽  
Rust Resistance Genes ◽  
Family Restriction ◽  
Restriction Fragment Length ◽  
Recombination Gene

Rp1 is a complex rust resistance locus of maize. Susceptible recombinants can be observed at variable frequencies from different Rp1 heterozygotes and homozygotes by crossing-over and unequal crossing-over. In addition, susceptible progeny have been observed from one heterozygote that were not associated with crossing-over, as indicated by flanking marker analysis. We analyzed testcross progeny from this cross (Rp1-C/Rp1-J) and a related cross (Rp1-F/Rp1-J) to identify noncrossover type derivatives, to estimate their frequency, and to determine the mechanism by which they arise. Testcross progeny were screened sequentially with two different rust isolates to assay each gene individually. Both susceptible noncrossover types and those with the combined resistance of each parent were identified in approximately equal frequencies. Patterns of recombination indicated that the noncrossover type derivatives arose by gene conversion as opposed to mutation or intrachromosomal crossing-over. While gene conversion events were observed frequently in both crosses (approx. 7 × 10−4), they were less frequent than crossover derivatives. Key words: Puccinia, disease resistance, recombination, gene family, restriction fragment length polymorphism.

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