Unequal crossing over at the rRNA locus as a source of quantitative genetic variation

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.

Download Full-text

Genetic variation in small multigene families

Genetics Research ◽

10.1017/s0016672300020115 ◽

1981 ◽

Vol 37 (2) ◽

pp. 133-149 ◽

Cited By ~ 7

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.

Download Full-text

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

Genome ◽

10.1139/g96-020 ◽

1996 ◽

Vol 39 (1) ◽

pp. 150-154 ◽

Cited By ~ 25

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.

Download Full-text

Mapping Unequal Crossing over Hotspot Region of Simple Sequence Repeat in Maize

ACTA AGRONOMICA SINICA ◽

10.3724/sp.j.1006.2009.00958 ◽

2009 ◽

Vol 35 (5) ◽

pp. 958-961

Author(s):

Ji-Hua TANG ◽

Xi-Qing MA ◽

Wen-Tao TENG ◽

Jian-Bing YAN ◽

Jing-Rui DAI ◽

...

Keyword(s):

Simple Sequence Repeat ◽

Crossing Over ◽

Sequence Repeat ◽

Unequal Crossing Over ◽

Simple Sequence

Download Full-text

Maintenance of Quantitative Genetic Variation

10.1093/oso/9780198830870.003.0028 ◽

2018 ◽

Author(s):

Bruce Walsh ◽

Michael Lynch

Keyword(s):

Genetic Variation ◽

Quantitative Genetics ◽

Stabilizing Selection ◽

Quantitative Genetic ◽

Wide Range

One of the major unresolved issues in quantitative genetics is what accounts for the amount of standing genetic variation in traits. A wide range of models, all reviewed in this chapter, have been proposed, but none fit the data, either giving too much variation or too little apparent stabilizing selection.

Download Full-text

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.

Download Full-text

The Molecular Basis of Quantitative Genetic Variation in Central and Secondary Metabolism in Arabidopsis

Genetics ◽

10.1093/genetics/149.2.739 ◽

1998 ◽

Vol 149 (2) ◽

pp. 739-747 ◽

Cited By ~ 7

Author(s):

Thomas Mitchell-Olds ◽

Deana Pedersen

Keyword(s):

Enzyme Activity ◽

Genetic Variation ◽

Secondary Metabolism ◽

Genetic Correlations ◽

Theoretical Models ◽

Model Systems ◽

Activity Levels ◽

Significant Qtls ◽

Cis Acting ◽

Quantitative Genetic

Abstract To find the genes controlling quantitative variation, we need model systems where functional information on physiology, development, and gene regulation can guide evolutionary inferences. We mapped quantitative trait loci (QTLs) influencing quantitative levels of enzyme activity in primary and secondary metabolism in Arabidopsis. All 10 enzymes showed highly significant quantitative genetic variation. Strong positive genetic correlations were found among activity levels of 5 glycolytic enzymes, PGI, PGM, GPD, FBP, and G6P, suggesting that enzymes with closely related metabolic functions are coregulated. Significant QTLs were found influencing activity of most enzymes. Some enzyme activity QTLs mapped very close to known enzyme-encoding loci (e.g., hexokinase, PGI, and PGM). A hexokinase QTL is attributable to cis-acting regulatory variation at the AtHXK1 locus or a closely linked regulatory locus, rather than polypeptide sequence differences. We also found a QTL on chromosome IV that may be a joint regulator of GPD, PGI, and G6P activity. In addition, a QTL affecting PGM activity maps within 700 kb of the PGM-encoding locus. This QTL is predicted to alter starch biosynthesis by 3.4%, corresponding with theoretical models, suggesting that QTLs reflect pleiotropic effects of mutant alleles.

Download Full-text