bluuebunny/crossref_metadata_2025 huggingface dataset:
The pheromone receptors inhibit the pheromone response pathway in Saccharomyces cerevisiae by a process that is independent of their associated G alpha protein.
Authors: Hirsch J P , Cross F R
Published: January 2021

Dominant mutations at the DAF2 locus confer resistance to the cell-cycle arrest that normally occurs in MATa cells exposed to alpha-factor. One of these alleles, DAF2-2, has also been shown to suppress the constitutive signaling phenotype of null alleles of the gene encoding the alpha subunit of the G protein involved in pheromone signaling. These observations indicate that DAF2-2 inhibits transmission of the pheromone response signal. The DAF2-2 mutation has two effects on the expression of a pheromone inducible gene, FUS1. In DAF2-2 cells, FUS1 RNA is present at an increased basal level but is no longer fully inducible by pheromone. Cloning of DAF2-2 revealed that it is an allele of STE3, the gene encoding the a-factor receptor. STE3 is normally an alpha-specific gene, but is inappropriately expressed in a cells carrying a STE3DAF2-2 allele. The two effects of STE3DAF2-2 alleles on the pheromone response pathway are the result of different functions of the receptor. The increased basal level of FUS1 RNA is probably due to stimulation of the pathway by an autocrine mechanism, because it required at least one of the genes encoding a-factor. Suppression of a null allele of the G alpha subunit gene, the phenotype associated with the inhibitory function of STE3, was independent of a-factor. This suppression was also observed when the wild-type STE3 gene was expressed in a cells under the control of an inducible promoter. Inappropriate expression of STE2 in alpha cells was able to suppress a point mutation, but not a null allele, of the G alpha subunit gene. The ability of the pheromone receptors to block the pheromone response signal in the absence of the G alpha subunit indicates that these receptors interact with another component of the signal transduction pathway.
Insertional mutations in the yeast HOP1 gene: evidence for multimeric assembly in meiosis.
Authors: Friedman D B , Hollingsworth N M , Byers B
Published: January 2021

The HOP1 gene of Saccharomyces cerevisiae has been shown to play an important role in meiotic synapsis. In this study we analyzed the mechanism of this function by phenotypic characterization of novel in-frame linker-insertion mutations located at various sites throughout the HOP1 coding sequence. Among 12 mutations found to cause defects in meiotic recombination and spore viability, three were temperature-sensitive for the spore viability defect. Although substantial meiotic recombination was found for these conditional alleles at the restrictive temperature, the level of exchange measured in spo13 meiosis was reduced in some of the monitored intervals, indicating that nondisjunction resulting from a deficit in crossing over could account for SPO13 spore inviability. Intragenic complementation between linker-insertion alleles was assessed by testing the viability of spores generated from heteroallelic diploids after SPO13 meiosis. Complex patterns of complementation and enhancement of the spore-inviability phenotype indicate that HOP1 functions in a multimeric complex. In addition, the ability of alleles which map near the carboxyl terminus to complement several other alleles provides evidence for a functional domain in this region of the protein. Two previously identified multicopy suppressors of the conditional hop1-628ts allele were tested for their effects in cells bearing the linker-insertion hop1 alleles. Overexpression of REC104 from a 2 mu plasmid was shown to enhance the spore viability of every allele tested, including a hop1 disruption allele. On the other hand, suppression by overexpression of RED1 from a 2 mu plasmid was found only for allele hop1-628ts. Surprisingly, similar overexpression of RED1 in strains bearing several other conditional hop1 linker-insertion alleles caused enhanced spore lethality. This finding, in conjunction with the evidence for a carboxy-terminal domain, provides new insight into the nature of interactions between the HOP1 and RED1 products in meiosis.
Identification and characterization of Aspergillus nidulans mutants defective in cytokinesis.
Authors: Harris S D , Morrell J L , Hamer J E
Published: January 2021

Filamentous fungi undergo cytokinesis by forming crosswalls termed septa. Here, we describe the genetic and physiological controls governing septation in Aspergillus nidulans. Germinating conidia do not form septa until the completion of their third nuclear division. The first septum is invariantly positioned at the basal end of the germ tube. Block-and-release experiments of nuclear division with benomyl or hydroxyurea, and analysis of various nuclear division mutants demonstrated that septum formation is dependent upon the third mitotic division. Block-and-release experiments with cytochalasin A and the localization of actin in germlings by indirect immunofluorescence showed that actin participated in septum formation. In addition to being concentrated at the growing hyphal tips, a band of actin was also apparent at the site of septum formation. Previous genetic analysis in A. nidulans identified four genes involved in septation (sepA-D). We have screened a new collection of temperature sensitive (ts) mutants of A. nidulans for strains that failed to form septa at the restrictive temperature but were able to complete early nuclear divisions. We identified five new genes designated sepE, G, H, I and J, along with one additional allele of a previously identified septation gene. On the basis of temperature shift experiments, nuclear counts and cell morphology, we sorted these cytokines mutants into three phenotypic classes. Interestingly, one class of mutants fails to form septa and fails to progress past the third nuclear division. This class of mutants suggests the existence of a regulatory mechanism in A. nidulans that ensures the continuation of nuclear division following the initiation of cytokinesis.
Change in genetic variance under selection in a self-fertilizing population.
Authors: Hayashi T , Ukai Y
Published: January 2021

In this study we show how the genetic variance of a quantitative trait changes in a self-fertilizing population under repeated cycles of truncation selection, with the analysis based on the infinitesimal model in which it is assumed that the trait is determined by an infinite number of unlinked loci without epistasis. The genetic variance is reduced not as a consequence of the genotypic frequency change but due to the build-up of linkage disequilibrium under truncation selection in this model. We assume that the order of the genotypic contribution from each locus is n-1/2, where n is the number of loci involved, and investigate the change in linkage disequilibrium resulting from selection and self-fertilization using genotypic frequency dynamics in order to analyze the change in the genetic variance. Our analysis gives recurrence relations of genetic variance among the succeeding generations for the three cases of gene action, i.e., purely additive action, pure dominance without additive effect and the presence of both additive effect and dominance, respectively. Numerical examples are also given as a check on the recurrence formulas.
Unusual properties of regulatory DNA from the Drosophila engrailed gene: three "pairing-sensitive" sites within a 1.6-kb region.
Authors: Kassis J A
Published: January 2021

We have previously shown that a 2-kb fragment of engrailed DNA can suppress expression of a linked marker gene, white, in the P element vector CaSpeR. This suppression is dependent on the presence of two copies of engrailed DNA-containing P elements (P[en]) in proximity in the Drosophila genome (either in cis or in trans). In this study, the 2-kb fragment was dissected and found to contain three fragments of DNA which could mediate white suppression [called "pairing-sensitive sites" (PS)]. A PS site was also identified in regulatory DNA from the Drosophila escargot gene. The eye colors of six different P[en] insertions in the escargot gene suggest an interaction between P[en]-encoded and genome-encoded PS sites. I hypothesize that white gene expression from P[en] is repressed by the formation of a protein complex which is initiated at the engrailed PS sites and also requires interactions with flanking genomic DNA. Genes were sought which influence the function of PS sites. Mutations in some Polycomb and trithorax group genes were found to affect the eye color from some P[en] insertion sites. However, different mutations affected expression from different P[en] insertion sites and no one mutation was found to affect expression from all P[en] insertion sites examined. These results suggest that white expression from P[en] is not directly regulated by members of the Polycomb and trithorax group genes, but in some cases can be influenced by them. I propose that engrailed PS sites normally act to promote interactions between distantly located engrailed regulatory sites and the engrailed promoter.
Variegated phenotype and developmental methylation changes of a maize allele originating from epimutation.
Authors: Das O P , Messing J
Published: January 2021

Two instances of genetic transmission of spontaneous epimutation of the maize P-rr gene were identified. Transmission gave rise to two similar, moderately stable alleles, designated P-pr-1 and P-pr-2, that exhibited Mendelian behavior. Both isolates of P-pr conditioned a variable and variegated phenotype, unlike the uniform pigmentation conditioned by P-rr. Extensive genomic analysis failed to reveal insertions, deletions or restriction site polymorphisms between the new allele and its progenitor. However, methylation of the P gene was increased in P-pr relative to P-rr, and was greatly reduced (though not lost) in a revertant to uniform pigmentation. Variability in pigmentation conditioned by P-pr correlated with variability in transcript levels of the P gene, and both correlated inversely with variability in its methylation. Part of the variability in methylation could be accounted for by a developmental decrease in methylation in all tissues of plants carrying P-pr. We hypothesize that the variegated phenotype results from a general epigenetic pathway which causes a progressive decrease in methylation and increase in expression potential of the P gene as a function of cell divisions in each meristem of the plant. This renders all tissues chimeric for a functional gene; chimerism is visualized as variegation only in pericarp due to the tissue specificity of P gene expression. Therefore, this allele that originates from epimutation may exemplify an epigenetic mechanism for variegation in maize.
Mapping quantitative trait loci in crosses between outbred lines using least squares.
Authors: Haley C S , Knott S A , Elsen J M
Published: January 2021

The use of genetic maps based upon molecular markers has allowed the dissection of some of the factors underlying quantitative variation in crosses between inbred lines. For many species crossing inbred lines is not a practical proposition, although crosses between genetically very different outbred lines are possible. Here we develop a least squares method for the analysis of crosses between outbred lines which simultaneously uses information from multiple linked markers. The method is suitable for crosses where the lines may be segregating at marker loci but can be assumed to be fixed for alternative alleles at the major quantitative trait loci (QTLs) affecting the traits under analysis (e.g., crosses between divergent selection lines or breeds with different selection histories). The simultaneous use of multiple markers from a linkage group increases the sensitivity of the test statistic, and thus the power for the detection of QTLs, compared to the use of single markers or markers flanking an interval. The gain is greater for more closely spaced markers and for markers of lower information content. Use of multiple markers can also remove the bias in the estimated position and effect of a QTL which may result when different markers in a linkage group vary in their heterozygosity in the F1 (and thus in their information content) and are considered only singly or a pair at a time. The method is relatively simple to apply so that more complex models can be fitted than is currently possible by maximum likelihood. Thus fixed effects of background genotype can be fitted simultaneously with the exploration of a single linkage group which will increase the power to detect QTLs by reducing the residual variance. More complex models with several QTLs in the same linkage group and two-locus interactions between QTLs can similarly be examined. Thus least squares provides a powerful tool to extend the range of crosses from which QTLs can be dissected whilst at the same time allowing flexible and realistic models to be explored.
Complex frameshift mutations mediated by plasmid pKM101: mutational mechanisms deduced from 4-aminobiphenyl-induced mutation spectra in Salmonella.
Authors: Levine J G , Schaaper R M , DeMarini D M
Published: January 2021

We used colony probe hybridization and polymerase chain reaction/DNA sequence analysis to determine the mutations in approximately 2,400 4-aminobiphenyl (4-AB) +S9-induced revertants of the -1 frameshift allele hisD3052 and of the base-substitution allele hisG46 of Salmonella typhimurium. Most of the mutations occurred at sites containing guanine, which is the primary base at which 4-AB forms DNA adducts. A hotspot mutation involving the deletion of a CG or GC within the sequence CGCGCGCG accounted for 100 and 99.9%, respectively, of the reversion events at the hisD3052 allele in the pKM101 plasmid-minus strains TA1978 (uvr+) and TA1538 (delta uvrB). In strain TA98 (delta uvrB, pKM101), which contained the SOS DNA repair system provided by the pKM101 plasmid, approximately 85% of the revertants also contained the hotspot deletion; the remaining approximately 15% contained one of two types of mutations: (1) complex frameshifts that can be described as a -2 or +1 frameshift and an associated base substitution and (2) deletions of the CC or GG sequences that flank the hotspot site (CCGCGCGCGG). We propose a misincorporation/slippage model to account for these mutations in which (1) pKM101-mediated misincorporation and translesion synthesis occurs across a 4-AB-adducted guanine; (2) the instability of such a mispairing and/or the presence of the adduct leads to strand slippage in a run of repeated bases adjacent to the adducted guanine; and (3) continued DNA synthesis from the slipped intermediate produces a frameshift associated with a base substitution. This model readily accounts for the deletion of the CC or GG sequences flanking the hotspot site, indicating that these mutations are, in fact, complex mutations in disguise (i.e., cryptic complex frameshifts). The inferred base-substitution specificity associated with the complex frameshifts at the hisD3052 allele (primarily G.C-->T.A transversions) is consistent with the finding that 4-AB induced primarily G.C-->T.A transversions at the hisG46 base-substitution allele. The model also provides a framework for understanding the different relative mutagenic potencies of 4-AB at the two alleles in the various DNA repair backgrounds of Salmonella.
A screen for nonconditional dauer-constitutive mutations in Caenorhabditis elegans.
Authors: Malone E A , Thomas J H
Published: January 2021

In Caenorhabditis elegans, formation of the developmentally arrested dauer larva is induced by high levels of a constitutively secreted pheromone. Synergy between two groups of incompletely penetrant dauer-constitutive (Daf-c) mutations has recently led to a proposal that these two groups of genes are partially redundant and function in two parallel pathways that regulate dauer formation. A possible weakness in this reasoning is that the mutations used to identify the synergy were specifically obtained as incompletely penetrant mutations. Here we use screens to identify new Daf-c alleles without any requirement for partial penetrance. Nevertheless, 22 of the 25 new mutations are incompletely penetrant mutations in 6 previously identified genes. Among these are mutations in daf-8 and daf-19, genes for which only one mutation had been previously identified. Also included in this group are three daf-1 alleles that do not exhibit the maternal rescue characteristic of other daf-1 alleles. Two of the 25 new mutations are fully penetrant and are alleles of daf-2, the one gene in which a fully penetrant mutation had been found earlier. Finally, one of the 25 new mutations is semidominant, temperature-sensitive, and identifies a new gene, daf-28. The results demonstrate that an incompletely penetrant Daf-c phenotype is characteristic of mutations in most Daf-c genes other than daf-2. This finding strengthens the hypothesis that a branched genetic pathway controls dauer formation.
Synonymous codon usage in Drosophila melanogaster: natural selection and translational accuracy.
Authors: Akashi H
Published: January 2021

I present evidence that natural selection biases synonymous codon usage to enhance the accuracy of protein synthesis in Drosophila melanogaster. Since the fitness cost of a translational misincorporation will depend on how the amino acid substitution affects protein function, selection for translational accuracy predicts an association between codon usage in DNA and functional constraint at the protein level. The frequency of preferred codons is significantly higher at codons conserved for amino acids than at nonconserved codons in 38 genes compared between D. melanogaster and Drosophila virilis or Drosophila pseudoobscura (Z = 5.93, P < 10(-6)). Preferred codon usage is also significantly higher in putative zinc-finger and homeodomain regions than in the rest of 28 D. melanogaster transcription factor encoding genes (Z = 8.38, P < 10(-6)). Mutational alternatives (within-gene differences in mutation rates, amino acid changes altering codon preference states, and doublet mutations at adjacent bases) do not appear to explain this association between synonymous codon usage and amino acid constraint.
Polygenic mutation in Drosophila melanogaster: estimates from response to selection of inbred strains.
Authors: Mackay T F , Fry J D , Lyman R F , Nuzhdin S V
Published: January 2021

Replicated divergent artificial selection for abdominal and sternopleural bristle number from a highly inbred strain of Drosophila melanogaster resulted in an average divergence after 125 generations of selection of 12.0 abdominal and 8.2 sternopleural bristles from the accumulation of new mutations affecting bristle number. Responses to selection were highly asymmetrical, with greater responses for low abdominal and high sternopleural bristle numbers. Estimates of VM, the mutational variance arising per generation, based on the infinitesimal model and averaged over the responses to the first 25 generations of selection, were 4.32 x 10(-3) VE for abdominal bristle number and 3.66 x 10(-3) VE for sternopleural bristle number, where VE is the environmental variance. Based on 10 generations of divergent selection within lines from generation 93, VM for abdominal bristle number was 6.75 x 10(-3) VE and for sternopleural bristle number was 5.31 x 10(-3) VE. However, estimates of VM using the entire 125 generations of response to selection were lower and generally did not fit the infinitesimal model largely because the observed decelerating responses were not compatible with the predicted increasing genetic variance over time. These decelerating responses, periods of response in the opposite direction to artificial selection, and rapid responses to reverse selection all suggest new mutations affecting bristle number on average have deleterious effects on fitness. Commonly observed periods of accelerated responses followed by long periods of stasis suggest a leptokurtic distribution of mutational effects for bristles.
Identification of genes required for normal pheromone-induced cell polarization in Saccharomyces cerevisiae.
Authors: Chenevert J , Valtz N , Herskowitz I
Published: January 2021

In response to mating pheromones, cells of the yeast Saccharomyces cerevisiae adopt a polarized "shmoo" morphology, in which the cytoskeleton and proteins involved in mating are localized to a cell-surface projection. This polarization is presumed to reflect the oriented morphogenesis that occurs between mating partners to facilitate cell and nuclear fusion. To identify genes involved in pheromone-induced cell polarization, we have isolated mutants defective in mating to an enfeebled partner and studied a subset of these mutants. The 34 mutants of interest are proficient for pheromone production, arrest in response to pheromone, mate to wild-type strains, and exhibit normal cell polarity during vegetative growth. The mutants were divided into classes based on their morphological responses to mating pheromone. One class is unable to localize cell-surface growth in response to mating factor and instead enlarges in a uniform manner. These mutants harbor special alleles of genes required for cell polarization during vegetative growth, BEM1 and CDC24. Another class of mutants forms bilobed, peanut-like shapes when treated with pheromone and defines two genes, PEA1 and PEA2. PEA1 is identical to SPA2. A third class forms normally shaped but tiny shmoos and defines the gene TNY1. A final group of mutants exhibits apparently normal shmoo morphology. The nature of their mating defect is yet to be determined. We discuss the possible roles of these gene products in establishing cell polarity during mating.
Analysis of a detailed genetic linkage map of Lactuca sativa (lettuce) constructed from RFLP and RAPD markers.
Authors: Kesseli R V , Paran I , Michelmore R W
Published: January 2021

A detailed genetic map has been constructed from the F2 population of a single intraspecific cross of Lactuca sativa (n = 9). It comprises 319 loci, including 152 restriction fragment length polymorphism (RFLP), 130 random amplified polymorphic DNA (RAPD), 7 isozyme, 19 disease resistance, and 11 morphological markers. Thirteen major, four minor linkage groups and several unlinked markers are identified for this genome which is estimated to be approximately 1950 cM. RFLP and RAPD markers show similar distributions throughout the genome and identified similar levels of polymorphism. RAPD loci were much quicker to identify but more difficult to order. Procedures for generating accurate genetic maps and their limitations are described.
Analysis of autoregulation at the level of pre-mRNA splicing of the suppressor-of-white-apricot gene in Drosophila.
Authors: Zachar Z , Chou T B , Kramer J , Mims I P , Bingham P M
Published: January 2021

The Drosophila suppressor-of-white-apricot [su(wa)] protein regulates/modulates at least two somatic RNA processing events. It is a potent regulator of its own expression. We report here new studies of this autoregulatory circuit. Among other things, our studies show the following. First, new evidence that su(wa) expression is autoregulated at the level of pre-mRNA splicing is reported. su(wa) protein represses accumulation of the fully spliced su(wa) mRNA encoding it and promotes accumulation of high levels of incompletely spliced su(wa) pre-mRNA. Second, the fully spliced su(wa) mRNA is sufficient for all known su(wa) genetic functions indicating that it encodes the sole su(wa) protein. Third, the incompletely spliced su(wa) pre-mRNAs resulting from autoregulation are not translated (probably as a result of nuclear retention) and apparently represent nonfunctional by-products. Fourth, the special circumstances of su(wa) expression during oogenesis allows maternal deposition exclusively of fully spliced su(wa) mRNA. Fifth, su(wa) protein immunolocalizes to nuclei consistent with its being a direct regulator of pre-mRNA processing. We discuss the implications of our results for mechanisms of splicing regulation and for developmental control of su(wa) expression.
Recovery of dominant, autosomal flightless mutants of Drosophila melanogaster and identification of a new gene required for normal muscle structure and function.
Authors: Cripps R M , Ball E , Stark M , Lawn A , Sparrow J C
Published: January 2021

To identify further mutations affecting muscle function and development in Drosophila melanogaster we recovered 22 autosomal dominant flightless mutations. From these we have isolated eight viable and lethal alleles of the muscle myosin heavy chain gene, and seven viable alleles of the indirect flight muscle (IFM)-specific Act88F actin gene. The Mhc mutations display a variety of phenotypic effects, ranging from reductions in myosin heavy chain content in the indirect flight muscles only, to reductions in the levels of this protein in other muscles. The Act88F mutations range from those which produce no stable actin and have severely abnormal myofibrillar structure, to those which accumulate apparently normal levels of actin in the flight muscles but which still have abnormal myofibrils and fly very poorly. We also recovered two recessive flightless mutants on the third chromosome. The remaining five dominant flightless mutations are all lethal alleles of a gene named lethal(3)Laker. The Laker alleles have been characterized and the gene located in polytene bands 62A10,B1-62B2,4. Laker is a previously unidentified locus which is haplo-insufficient for flight. In addition, adult wild-type heterozygotes and the lethal larval trans-heterozygotes show abnormalities of muscle structure indicating that the Laker gene product is an important component of muscle.
Quantitative genetics of sex ratio traits in the parasitic wasp, Nasonia vitripennis.
Authors: Orzack S H , Gladstone J
Published: January 2021

We detected significant parent-offspring regressions for the first sex ratio (the sex ratio produced by a female in a fresh host) and the second sex ratio (the sex ratio produced by a female in a previously parasitized host) in the parasitic wasp, Nasonia vitripennis. For both traits, estimates of the narrow-sense heritability range from approximately 0.05 to approximately 0.15 (depending on how the data are analyzed). The study population was derived from isofemale strains created from wasps captured in a single bird nest. The same population exhibited no significant parent-offspring regression for the brood sizes associated with the first and second sex ratios. There may be a significant negative parent-offspring regression for diapause proportion in the first sex ratio broods. The estimates of the genetic correlations between first and second sex ratios are positive although almost all are not significantly different from 0.0. To our knowledge, this study is the first "fine-scale" analysis of genetic variation for sex ratio traits in any species of insect. Such studies are an essential part of the assessment of the validity of claims that sex ratio traits are locally optimal.
Mutation load under vegetative reproduction and cytoplasmic inheritance.
Authors: Kondrashov A S
Published: January 2021

For reasons that remain unclear, even multicellular organisms usually originate from a single cell. Here I consider the balance between deleterious mutations and selection against them in a population with obligate vegetative reproduction, when every offspring is initiated by more than one cell of a parent. The mutation load depends on the genomic deleterious mutation rate U, strictness of selection, number of cells which initiate an offspring n, and the relatedness among the initial cells. The load grows with increasing U, n and strictness of selection, and declines when an offspring is initiated by more closely related cells. If Un > 1, the load under obligate vegetative reproduction may be substantially higher than under sexual or asexual reproduction, which may account for its rarity. In nature obligate vegetative reproduction seems to be more common and long term in taxa whose cytological features ensure a relatively low load under it. The same model also describes the mutation load under two other modes of inheritance: (1) uniparental transmission of organelles and (2) reproduction by division of multinuclear cells, where each daughter cell receives many nuclei. The load declines substantially when the deleterious mutation rate per organelle genome gets lower or when the number of nuclei in a cell sometimes drops. This may explain the small sizes of organelle genomes in sexual lineages and the presence of karyonic cycles in asexual unicellular multinuclear eukaryotes.
Tetrahymena thermophila mutants defective in the developmentally programmed maturation and maintenance of the rDNA minichromosome.
Authors: Kapler G M , Orias E , Blackburn E H
Published: January 2021

The abundant rDNA minichromosome of Tetrahymena thermophila is generated by a series of developmentally controlled processing steps, termed rDNA maturation, during the formation of the new macronucleus in conjugating cells. rDNA maturation involves excision of a region encoding the single copy rRNA gene (rDNA) from its germline location, rearrangement of the rDNA into a palindromic minichromosome, de novo telomere addition, and amplification to approximately 10(4) copies. The rDNA is maintained at this high level in vegetatively growing cells. Using a previously developed genetic scheme for studying rDNA maturation and maintenance, we report the isolation of a new class of mutants defective for rDNA maturation. Several new rDNA maintenance mutants were also obtained. The maturation mutant, rmm10, is severely defective for the production of both monomeric and palindromic rDNA in the developing macronucleus. The mm10 mutation is recessive-lethal and cis-acting. None of the previously identified DNA sequence elements that control rDNA maturation or maintenance is mutated in rmm10. Therefore, additional cis-acting sequence elements must be required for rDNA maturation. Based on our current understanding of rDNA maturation processes, we suggest that the rmm10 mutation affects rDNA excision rather than subsequent rDNA amplification/replication.
A reevaluation of data from competitive tests shows high levels of heterosis in Drosophila melanogaster.
Authors: Latter B D , Sved J A
Published: January 2021

We have analyzed the results from a range of procedures designed to measure the fitness under competitive conditions of inbred strains of Drosophila melanogaster, specifically strains which are homozygous for chromosome 2. All methods show a substantial reduction in fitness, ranging from an estimated 70-80% for single generation competition tests to 80-90% for a multiple generation population cage procedure. Furthermore, inbreeding through brother-sister mating reduces fitness by a comparable amount when allowance is made for the expected degree of homozygosity.
The SUP35 omnipotent suppressor gene is involved in the maintenance of the non-Mendelian determinant [psi+] in the yeast Saccharomyces cerevisiae.
Authors: Ter-Avanesyan M D , Dagkesamanskaya A R , Kushnirov V V , Smirnov V N
Published: January 2021

The SUP35 gene of yeast Saccharomyces cerevisiae encodes a 76.5-kD ribosome-associated protein (Sup35p), the C-terminal part of which exhibits a high degree of similarity to EF-1 alpha elongation factor, while its N-terminal region is unique. Mutations in or overexpression of the SUP35 gene can generate an omnipotent suppressor effect. In the present study the SUP35 wild-type gene was replaced with deletion alleles generated in vitro that encode Sup35p lacking all or a part of the unique N-terminal region. These 5'-deletion alleles lead, in a haploid strain, simultaneously to an antisuppressor effect and to loss of the non-Mendelian determinant [psi+]. The antisuppressor effect is dominant while the elimination of the [psi+] determinant is a recessive trait. A set of the plasmid-borne deletion alleles of the SUP35 gene was tested for the ability to maintain [psi+]. It was shown that the first 114 amino acids of Sup35p are sufficient to maintain the [psi+] determinant. We propose that the Sup35p serves as a trans-acting factor required for the maintenance of [psi+].
Drosophila alcohol dehydrogenase polymorphism and carbon-13 fluxes: opportunities for epistasis and natural selection.
Authors: Freriksen A , de Ruiter B L , Scharloo W , Heinstra P W
Published: January 2021

The influence of genetic variations in Drosophila alcohol dehydrogenase (ADH) on steady-state metabolic fluxes was studied by means of 13C NMR spectroscopy. Four pathways were found to be operative during 8 hr of ethanol degradation in third instar larvae of Drosophila. Seven strains differed by 18-25% in the ratio between two major pathway fluxes, i.e., into glutamate-glutamine-proline vs. lactate-alanine-trehalose. In general, Adh genotypes with higher ADH activity exhibit a twofold difference in relative carbon flux from malate into lactate and alanine vs. alpha,alpha-trehalose compared to low ADH activity genotypes. Trehalose was degraded by the pentose-phosphate shunt. The pentose-phosphate shunt and malic enzyme could supply NADPH necessary for lipid synthesis from ethanol. Lactate and/or proline synthesis may maintain the NADH/NAD+ balance during ethanol degradation. After 24 hr the flux into trehalose is increased, while the flux into lipids declines in AdhF larvae. In AdhS larvae the flux into lipids remains high. This co-ordinated nature of metabolism and the genotype-dependent differences in metabolic flux may form the basis for various epistatic interactions and ultimately for variations in organismal fitness.
Genetic instability at the agouti locus of the mouse (Mus musculus). I. Increased reverse mutation frequency to the Aw allele in A/a heterozygotes.
Authors: Sandulache R , Neuhäuser-Klaus A , Favor J
Published: January 2021

We have compiled the reverse mutation rate data to the white bellied agouti (Aw) allele in heterozygous A/a mice and shown it to be increased by a factor of at least 350 in comparison to the reverse mutation rate in homozygous a/a mice. Employing tightly linked flanking restriction fragment length polymorphism DNA markers, we have shown that reversion to Aw is associated with crossing over in the vicinity of the agouti locus. The non-agouti (a) allele has been recently shown to contain an 11-kb insert within the first intron of the agouti gene. Together with our present results, these observations suggest possible mechanisms to explain the reversion events.
Roles of ruvA, ruvC and recG gene functions in normal and DNA damage-inducible replication of the Escherichia coli chromosome.
Authors: Asai T , Kogoma T
Published: January 2021

Induction of the SOS response in Escherichia coli activates normally repressed DNA replication which is termed inducible stable DNA replication (iSDR). We previously demonstrated that initiation of iSDR requires the products of genes, such as recA, recB and recC, that are involved in the early stages of homologous recombination. By measuring the copy number increase of the origin (oriM1) region on the chromosome, we show, in this study, that initiation of iSDR is stimulated by mutations in the ruvA, ruvC and recG genes which are involved in the late stages of homologous recombination. Continuation of iSDR, on the other hand, is inhibited by these mutations. The results suggest that Holliday recombination intermediates, left on the chromosome due to abortive recombination, arrest replication fork movement. Low levels of iSDR and sfiA (sulA) gene expression were also observed in exponentially growing ruvA, ruvC and recG mutants, suggesting that the SOS response is chronically induced in these mutants. We propose that replication forks are arrested in these mutants, albeit at a low frequency, even under the normal (uninduced) conditions.
Microbial evolution in a simple unstructured environment: genetic differentiation in Escherichia coli.
Authors: Rosenzweig R F , Sharp R R , Treves D S , Adams J
Published: January 2021

Populations of Escherichia coli initiated with a single clone and maintained for long periods in glucose-limited continuous culture, become polymorphic. In one population, three clones were isolated and by means of reconstruction experiments were shown to be maintained in stable polymorphism, although they exhibited substantial differences in maximum specific growth rates and in glucose uptake kinetics. Analysis of these three clones revealed that their stable coexistence could be explained by differential patterns of the secretion and uptake of two alternative metabolites acetate and glycerol. Regulatory (constitutive and null) mutations in acetyl-coenzyme A synthetase accounted for different patterns of acetate secretion and uptake seen. Altered patterns in glycerol uptake are most likely explained by mutations which result in quantitative differences in the induction of the glycerol regulon and/or structural changes in glycerol kinase that reduce allosteric inhibition by effector molecules associated with glycolysis. The evolution of resource partitioning, and consequent polymorphisms which arise may illustrate incipient processes of speciation in asexual organisms.
Site-directed mutations altering the CAAX box of Ste18, the yeast pheromone-response pathway G gamma subunit.
Authors: Whiteway M S , Thomas D Y
Published: January 2021

The STE18 gene encodes the gamma subunit of the G protein which functions in the Saccharomyces cerevisiae pheromone-response pathway. The STE18 gene product undergoes a post-translational processing at the carboxyl terminus directed by the CCAAX box motif CCTLM110. A variety of site-directed mutations of this sequence have been constructed to test the role of this motif on Ste18 function. Mutations which change or eliminate the cysteine at position 107 abolish Ste18-dependent mating, and thus the cysteine (C107) is essential for Ste18 function. However, inactivation of the prenyltransferase by disruption of DPR1 has only a minor effect on Ste18-dependent mating. Mutation of cysteine 106 to serine significantly reduces but does not eliminate Ste18 function. Deletion of the C-terminal TLM sequence or modification of the ultimate methionine to lysine, arginine or leucine, all changes which do not affect the CAAX box cysteines, have only minor effects on Ste18-dependent mating. Intriguingly, these latter mutations dramatically compromise Ste18 function in cells which are deleted for Gpa1, the alpha subunit of the G protein. In addition, overexpression of these mutant versions of STE18 causes a dominant negative phenotype and inhibits the constitutive mating response generated by GPA1 deletion in cells which contain a functional STE18 gene. These results suggest that the C terminus of Ste18 and the Gpa1 protein have overlapping roles in some aspect of yeast G protein function such as membrane targeting.
Genetic analysis of Dictyostelium slug phototaxis mutants.
Authors: Darcy P K , Wilczynska Z , Fisher P R
Published: January 2021

Mapping and complementation analysis with 17 phototaxis mutations has established 11 complementation groups phoA-phoK distributed over six linkage groups. Statistical calculations from the complementation data yielded 17 as the maximum likelihood estimate of the number of pho genes assuming all loci are equally mutable. Most of the phototaxis mutants were found to exhibit bimodal phototaxis and all were found to be impaired in positive thermotaxis supporting convergence of the photosensory and thermosensory pathways. The thermotaxis mutant HPF228 was unaltered in phototaxis suggesting that the mutation in this strain affects a gene product whose site of action is before the convergence of the two pathways. Other phenotypes such as multiple tip formation by aggregates, stumpy fruiting bodies with short or absent stalks and short migration were associated with some pho alleles suggesting multiple biological roles for some gene products important in phototransduction.
Suppressors of recB mutations in Salmonella typhimurium.
Authors: Benson N R , Roth J
Published: January 2021

Using a screen that directly assesses transductional proficiency, we have isolated suppressors of recB mutations in Salmonella typhimurium. The alleles of sbcB reported here are phenotypically distinct from those isolated in Escherichia coli in that they restore recombination proficiency (Rec+), resistance to ultraviolet light (UVR), and mitomycin C resistance (MCR) in the absence of an accompanying sbcCD mutation. In addition the sbcB alleles reported here are co-dominant to sbcB+. We have also isolated insertion and deletion mutants of the sbcB locus. These null mutations suppress only the UVS phenotype of recB mutants. We have also isolated sbcCD mutations, which map near proC. These sbcCD mutations increase the viability, recombination proficiency and MCR of both the transductional recombination suppressors (sbcB1 & sbcB6) and the sbcB null mutations. S. typhimurium recB sbcB1 sbcCD8 strains are 15-fold more recombination proficient than wild-type strains. The increase in transductants in these strains is accompanied by a loss of abortive transductants suggesting that these fragments are accessible to the mutant recombination apparatus. Using tandem duplications, we have constructed sbcB merodiploids and found that, in a recB mutant sbcCD+ genetic background, the sbcB+ allele is dominant to sbcB1 for transductional recombination but co-dominant for UVR and MCR. However, in a recB sbcCD8 genetic background, the sbcB1 mutation is co-dominant to sbcB+ for all phenotypes. Our results lead us to suggest that the SbcB and SbcCD proteins have roles in RecBCD-dependent recombination.
X-linked elements associated with negative segregation distortion in the SD system of Drosophila melanogaster.
Authors: Hiraizumi Y , Albracht J M , Albracht B C
Published: January 2021

Three elements, M1, M2 and M3, found in a special X chromosome, supp-X(SD), modify the degree and direction of segregation distortion in the SD system of Drosophila melanogaster. The first element, M1, is located between the y and the cv loci, probably close to the y locus. The second element, M2, is located near the cv locus and the third element, M3, is located between the y and the car loci. The M1 element appears to cause a relatively small amount of reduction in the rate of recovery of the SD-72, but not the cn bw, chromosome from SD-72/cn bw males, when raised at 27.5 degrees. The M2 and the M3 elements cause considerable decrease in the recovery rate of the SD-72 chromosome, whereas they increase the recovery rate of the cn bw chromosome. The amount of decrease is nearly the same as the amount of increase for each element. Some type of "switch" mechanism in the directions of distortion is suggested for each of these two elements and their effects appear to be approximately additive.
Directed replacement of mt A by mt a-1 effects a mating type switch in Neurospora crassa.
Authors: Chang S , Staben C
Published: January 2021

To test the functions of a mating type genes, we developed an efficient strategy to select transformants of Neurospora crassa in which resident A mating type DNA was replaced by cloned DNA from the mt a idiomorph. Cloned a idiomorphic DNA could specify all functions, including fertility, of a mating type, but only when it replaced A DNA at the mating type locus. Only the mt a-1 region of the a idiomorph was necessary in order to specify a mating type. Gene replacement events involved the homologous sequences flanking the unique mating type idiomorphic DNA, resulting in apparently isogenic a and A strains. These isogenic strains were fertile when crossed with one another, indicating that no determinants outside the transforming DNA are necessary for fertility as a and that no host sequences of A strains interfere with fertility as a. One a replacement strain bore a duplication of the transforming mt a-1 and hph DNA. The duplication strain had unexpected properties. Although mating type segregated 1:1 in crosses of this strain to A, the duplicated regions were efficiently altered during the sexual process to generate a single copy in the progeny. No progeny were recovered that had undergone RIP (repeat induced point mutation) sufficient to inactivate the mt a-1 gene. We infer that the mt a-1 gene is necessary and sufficient to specify a mating type identity in all vegetative and sexual activities. Mt a-1 may also play an essential role in ascosporogenesis after fertilization.
Mapping of DBR1 and YPK1 suggests a major revision of the genetic map of the left arm of Saccharomyces cerevisiae Chromosome XI.
Authors: Simchen G , Chapman K B , Caputo E , Nam K , Riles L , Levin D E , Boeke J D
Published: January 2021

The Saccharomyces cerevisiae dbr1 mutation has been mapped on the left arm of chromosome XI. XIL is a chromosome arm that was until now rather sparsely populated with accurately mapped markers. On the basis of physical data, the overall order of markers is inverted relative to the existing genetic map of XI. We present tetrad analyses using a variety of markers on XI that indicate that the existing genetic map of XIL should be inverted, at least for the strains in which our mapping was carried out, and probably for other S. cerevisiae strains.
Genetic epilepsy model derived from common inbred mouse strains.
Authors: Frankel W N , Taylor B A , Noebels J L , Lutz C M
Published: January 2021

The recombinant inbred mouse strain, SWXL-4, exhibits tonic-clonic and generalized seizures similar to the commonest epilepsies in humans. In SWXL-4 animals, seizures are observed following routine handling at about 80 days of age and may be induced as early as 55 days by rhythmic gentle tossing. Seizures are accompanied by rapid, bilateral high frequency spike cortical discharges and followed by a quiescent post-ictal phase. Immunohistochemistry of the immediate early gene products c-Fos and c-Jun revealed abnormal activation within cortical and limbic structures. The seizure phenotype of SWXL-4 can be explained and replicated fully by the inheritance of susceptibility alleles from its progenitor strains, SWR/J and C57L/J. Outcrosses of SWXL-4 with most other common inbred strains result in F1 hybrids that have seizure at least as frequently as SWXL-4 itself. Quantitative trait locus mapping reveals a seizure frequency determinant, Szf1, near the pink-eyed dilution locus on chromosome 7, accounting for up to 32% of the genetic variance in an F2 intercross between SWXL-4 and the linkage testing strain ABP/Le. These studies demonstrate that common strains of mice such as SWR and C57L contain latent epilepsy susceptibility alleles. Although the inheritance of susceptibility may be complex, these results imply that a number of potentially important and practical, noninvasive models for this disorder can be constructed and studied in crosses between common mouse strains.
Epistasis can facilitate the evolution of reproductive isolation by peak shifts: a two-locus two-allele model.
Authors: Wagner A , Wagner G P , Similion P
Published: January 2021

The influence of epistasis on the evolution of reproductive isolation by peak shifts is studied in a two-locus two-allele model of a quantitative genetic character under stabilizing selection. Epistasis is introduced by a simple multiplicative term in the function that maps gene effects onto genotypic values. In the model with only additive effects on the trait, the probability of a peak shift and the amount of reproductive isolation are always inversely related, i.e., the higher the peak shift rate, the lower the amount of reproductive isolation caused by the peak shift. With epistatic characters there is no consistent relationship between these two values. Interestingly, there are cases where both transition rates as well as the amount of reproductive isolation are increased relative to the additive model. This effect has two main causes: a shift in the location of the transition point, and the hybrids between the two alternative optimal genotypes have lower average fitness in the epistatic case. A review of the empirical literature shows that the fitness relations resulting in higher peak shift rates and more reproductive isolation are qualitatively the same as those observed for genes causing hybrid inferiority.