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

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.

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

Nature ◽

10.1038/272080a0 ◽

1978 ◽

Vol 272 (5648) ◽

pp. 80-81 ◽

Cited By ~ 24

Author(s):

R. FRANKHAM ◽

D. A. BRISCOE ◽

R. K. NURTHEN

Keyword(s):

Genetic Variation ◽

Crossing Over ◽

Unequal Crossing Over ◽

Quantitative Genetic

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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.

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Unequal crossing-over within the B duplication of Drosophila melanogaster: a molecular analysis

Genetics Research ◽

10.1017/s0016672300029165 ◽

1991 ◽

Vol 57 (2) ◽

pp. 105-111 ◽

Cited By ~ 3

Author(s):

Stuart I. Tsubota

Keyword(s):

Drosophila Melanogaster ◽

Transposable Element ◽

Molecular Analysis ◽

Recombination Event ◽

Tandem Duplication ◽

Molecular Data ◽

Crossing Over ◽

Wild Type ◽

Unequal Crossing Over ◽

Extreme Form

SummaryThe B mutation is associated with a tandem duplication of 16A1–16A7. It is unstable, mutating to wild type and to a more extreme form at a frequency of one in 1000 to 3000. The reversion to wild type is associated with the loss of one copy of the duplication, whereas the mutation to extreme B is associated with a triplication of the region. The instability of B has been attributed to unequal crossing-over between the two copies of the duplication. Recent molecular data show that there is a transposable element, B104, between the two copies of the duplication and support the hypothesis that this element generated the duplication via a recombination event. These data suggest that unequal crossing-over within the duplication may not be the cause of the instability of B. Instead, the instability may be caused by a recombination event involving the B104 element. This issue was addressed using probes for the DNA on either side of the B104 element at the B breakpoint. All of the data indicate that the B104 element is not involved in the instability of B and support the original unequal crossing-over model.

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Evolution of Fitness in Experimental Populations of Vesicular Stomatitis Virus

Genetics ◽

10.1093/genetics/142.3.673 ◽

1996 ◽

Vol 142 (3) ◽

pp. 673-679 ◽

Cited By ~ 3

Author(s):

Santiago F Elena ◽

Fernando González-Candelas ◽

Isabel S Novella ◽

Elizabeth A Duarte ◽

David K Clarke ◽

...

Keyword(s):

Population Dynamics ◽

Genetic Variation ◽

Genetic Analysis ◽

Vesicular Stomatitis Virus ◽

Wild Type ◽

Quantitative Genetic ◽

Vesicular Stomatitis ◽

Continuous Population ◽

Genetic Fitness ◽

Viral Clone

Abstract The evolution of fitness in experimental clonal populations of vesicular stomatitis virus (VSV) has been compared under different genetic (fitness of initial clone) and demographic (population dynamics) regimes. In spite of the high genetic heterogeneity among replicates within experiments, there is a clear effect of population dynamics on the evolution of fitness. Those populations that went through strong periodic bottlenecks showed a decreased fitness in competition experiments with wild type. Conversely, mutant populations that were transferred under the dynamics of continuous population expansions increased their fitness when compared with the same wild type. The magnitude of the observed effect depended on the fitness of the original viral clone. Thus, high fitness clones showed a larger reduction in fitness than low fitness clones under dynamics with included periodic bottleneck. In contrast, the gain in fitness was larger the lower the initial fitness of the viral clone. The quantitative genetic analysis of the trait “fitness” in the resulting populations shows that genetic variation for the trait is positively correlated with the magnitude of the change in the same trait. The results are interpreted in terms of the operation of Muller's ratchet and genetic drift as opposed to the appearance of beneficial mutations.

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Recombination between the X and Y chromosomes and the Sxr region of the mouse

Genetics Research ◽

10.1017/s0016672300030925 ◽

1992 ◽

Vol 60 (3) ◽

pp. 175-184 ◽

Cited By ~ 6

Author(s):

Anne McLaren ◽

Elizabeth Simpson ◽

Colin E. Bishop ◽

Michael J. Mitchell ◽

Susan M. Darling

Keyword(s):

Y Chromosome ◽

X Chromosome ◽

Recombination Frequency ◽

Male Meiosis ◽

Homologous Region ◽

Crossing Over ◽

Female Meiosis ◽

Unequal Crossing Over ◽

Y Chromosomes ◽

Autosome Translocation

SummaryThe Sxr (sex-reversed) region that carries a copy of the mouse Y chromosomal testis-determining gene can be attached to the distal end of either the Y or the X chromosome. During male meiosis, Sxr recombined freely between the X and Y chromosomes, with an estimated recombination frequency not significantly different from 50% in either direction. During female meiosis, Sxr recombined freely between the X chromosome to which it was attached and an X-autosome translocation. A male mouse carrying the original Sxra region on its Y chromosome, and the shorter Sxrb variant on the X, also showed 50% recombination between the sex chromosomes. Evidence of unequal crossing-over between the two Sxr regions was obtained: using five markers deleted from Sxrb, 3 variant Sxr regions were detected in 159 progeny (1·9%). Four other variants (one from the original cross and three from later generations) were presumed to have been derived from illegitimate pairing and crossing-over between Sxrb and the homologous region on the short arm of the Y chromosome. The generation of new variants throws light on the arrangement of gene loci and other markers within the short arm of the mouse Y chromosome.

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A cytological study of two closely related blackfly species: Cnephia dacotensis and Cnephia ornithophilia (Diptera: Simuliidae)

Canadian Journal of Zoology ◽

10.1139/z75-196 ◽

1975 ◽

Vol 53 (11) ◽

pp. 1622-1637 ◽

Cited By ~ 14

Author(s):

William S. Procunier

Keyword(s):

Light Microscopy ◽

Polytene Chromosome ◽

Sex Chromosomes ◽

Cytological Study ◽

Molecular Model ◽

Crossing Over ◽

X Chromosomes ◽

Synaptonemal Complexes ◽

Y Chromosomes

Cnephia dacotensis and C. ornithophilia are two closely related blackfly species. Their polytene chromosome complements differ in three interspecific inversions, in separate inversion polymorphisms, and the presence of specific sex chromosomes of C. dacotensis. These sex chromosomes comprise four structurally different X chromosomes and two structurally different Y chromosomes. A molecular model is proposed for their evolution. Light microscopy revealed that both species have male achiasmate meiosis. Male larvae of C. dacotensis lack synaptonemal complexes. Random association of inversions IIIS-1 and IIIL-1 and the presence of synaptonemal complexes indicate that crossing-over occurs in the female.

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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.

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Wild worm embryogenesis harbors ubiquitous polygenic modifier variation

eLife ◽

10.7554/elife.09178 ◽

2015 ◽

Vol 4 ◽

Cited By ~ 45

Author(s):

Annalise B Paaby ◽

Amelia G White ◽

David D Riccardi ◽

Kristin C Gunsalus ◽

Fabio Piano ◽

...

Keyword(s):

Genetic Variation ◽

Caenorhabditis Elegans ◽

Gene Function ◽

Natural Populations ◽

Complex Trait ◽

Genetic Modifiers ◽

Wild Type ◽

Cryptic Genetic Variation ◽

Quantitative Genetic ◽

In The Wild

Embryogenesis is an essential and stereotypic process that nevertheless evolves among species. Its essentiality may favor the accumulation of cryptic genetic variation (CGV) that has no effect in the wild-type but that enhances or suppresses the effects of rare disruptions to gene function. Here, we adapted a classical modifier screen to interrogate the alleles segregating in natural populations of Caenorhabditis elegans: we induced gene knockdowns and used quantitative genetic methodology to examine how segregating variants modify the penetrance of embryonic lethality. Each perturbation revealed CGV, indicating that wild-type genomes harbor myriad genetic modifiers that may have little effect individually but which in aggregate can dramatically influence penetrance. Phenotypes were mediated by many modifiers, indicating high polygenicity, but the alleles tend to act very specifically, indicating low pleiotropy. Our findings demonstrate the extent of conditional functionality in complex trait architecture.

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The Nature of Quantitative Genetic Variation in Drosophila. II* Average Dominance of Abdominal Bristle Polygenes

Australian Journal of Biological Sciences ◽

10.1071/bi9740683 ◽

1974 ◽

Vol 27 (6) ◽

pp. 683 ◽

Cited By ~ 4

Author(s):

R Frankham

Keyword(s):

Genetic Variation ◽

Selection Response ◽

Base Population ◽

Quantitative Genetic

A comparison of 13 abdominal bristle selection lines with their base population and with reciprocal Fls between the selection lines and the base population was carried out. There was no significant directional contribution of maternally inherited factors to selection response.

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