Frequency-Dependent Selection at the PGM-1 Locus of DROSOPHILA PSEUDOOBSCURA

Frequency-dependent fitness was studied at the Pgm-1 locus of Drosophila pseudoobscura with respect to two fitness components: rate of development and larva-to-adult survival. The Pgm-1 locus is very polymorphic with only two alleles, Pgm-1¹⁰⁰ and Pgm-1¹⁰⁴, occurring at high frequencies. For each of these two alleles, 20 homozygous strains were obtained from a sample of 1,140 wild-inseminated females. First-instar larvae of the two genotypes were combined in a set of eight different frequencies: 0.0, 0.10, 0.25, 0.40, 0.60, 0.75, 0.90, and 1.0. Frequency-dependent fitness effects were observed for the two survival-related fitness components examined: larvae of the less common genotype develop faster and have a higher probability of survival than larvae of the more common genotype. The rate of survival at intermediate genotypic frequencies is similar to that in pure cultures. If selection acted solely as frequency-dependent effects on survival-related components of fitness, the equilibrium frequency of the Pgm-1¹⁰⁰ allele would be 0.615 for a two-genotype system, which fits an observed frequency range for this allele in nature between 0.55 and 0.71. Experimentally created linkage disequilibrium was excluded from the experiment by using a large number of independent strains. It is nevertheless possible that the frequency-dependent selection may not affect the Pgm-1 locus per se, but may reflect a linkage disequilibrium present in the natural population. Even if this were the case, the frequency-dependent selection could affect the frequency of the Pgm-1 alleles in nature.     

Complex Signatures of Natural Selection at the Duffy Blood Group Locus

The Duffy blood group locus (FY) has long been considered a likely target of natural selection, because of the extreme pattern of geographic differentiation of its three major alleles (FY*B, FY*A, and FY*O). In the present study, we resequenced the FY region in samples of Hausa from Cameroon (fixed for FY*O), Han Chinese (fixed for FY*A), Italians, and Pakistanis. Our goals were to characterize the signature of directional selection on FY*O in sub-Saharan Africa and to understand the extent to which natural selection has also played a role in the extreme geographic differentiation of the other derived allele at this locus, FY*A. The data from the FY region are compared with the patterns of variation observed at 10 unlinked, putatively neutral loci from the same populations, as well as with theoretical expectations from the neutral-equilibrium model. The FY region in the Hausa shows evidence of directional selection in two independent properties of the data (i.e., level of sequence variation and frequency spectrum), observations that are consistent with the FY*O mutation being the target. The Italian and Chinese FY data show patterns of variation that are very unusual, particularly with regard to frequency spectrum and linkage disequilibrium, but do not fit the predictions of any simple model of selection. These patterns may represent a more complex and previously unrecognized signature of positive selection.     

In Defense of a Folk Model: The "Skipper Effect" in the Icelandic Cod Fishery

The Icelandic folk model of fishing success emphasizes the importance of the skipper. The validity of this folk model has recently been hotly debated in the social science literature. In this study an attempt is made to estimate the role of the skipper in fishing success. First, we document by a national survey the composition of the folk model of fishing success held by the Icelandic public. Second, drawing on extensive interview data we outline the model of fishing success proposed by skippers themselves. Third, we test this folk science of the skipper by means of visual and statistical analysis against a social science model that emphasizes the role of material factors. The results show that fishing success is not determined by technological and material factors, leaving plenty of room for human factors, such as the skills and knowledge of the skipper. These results are consistent with folk notions of the skipper effect, but contradictory to earlier results of social science. Finally, the results are discussed in a theoretical context with reference to resource management and the nature of folk models.     

Molecular Analysis of Diepoxybutane-Induced Mutations at the rosy Locus of Drosophila melanogaster

We have analyzed at the molecular level diepoxybutane-induced mutants determined to have lesions affecting expression of the ry locus. Of the 21 mutants analyzed here, genetic analysis suggested that five were putative deficiencies involving ry and adjacent lethal loci. However, molecular analysis confirmed that only two of these five putative deficiencies were in fact deletions detectable by the methods used in the analysis. The remaining 16 mutants were viable as homozygotes, suggesting that their lesions were confined to the ry locus. Seven of these 16 intragenic mutants were determined to be deletions of genetic material as evidenced by altered restriction patterns relative to the wild type patterns. Thus, nine of 21 (43%) diepoxybutane-induced mutants are due to deletions ranging in size from approximately 50 base pairs to more than 8 kilobase pairs. Most of the deletions (seven of nine or 78%) are intragenic and less than 250 base pairs in size; it seems that most, if not all, affect coding rather than regulatory sequences.     

Molecular Basis of Polymorphism at the Esterase-5b Locus in Drosophila Pseudoobscura

Sequence variation was studied in a 2.2-kb region encompassing the esterase-5B locus in Drosophila pseudoobscura from two California populations. In these populations, two common electrophoretic classes and many less frequent variants occur, and it was formerly shown by KEITH (1983) that allele frequencies differed from random distribution under an infinite allele model. Nucleotide polymorphisms were determined in 16 sequences representing 14 electrophoretic classes. There was no significant sequence differentiation between populations, and both synonymous and nonsynonymous polymorphisms are distributed homogeneously along the sequence. The data show that the two major electrophoretic classes are heterogeneous at the amino acid level with no diagnostic amino acid(s) distinguishing them. At the nucleotide level, members of one major class are more similar to members of other electrophoretic classes than they are to each other. It appears that random combinations of the neutral amino acid polymorphisms and other undefined physical properties of the proteins generate the different electrophoretic classes and maintain considerable variation at Est-5B.     

Molecular Analysis of 36 Mutations at the Mouse Pink-Eyed Dilution (P) Locus

Thirty-six radiation- or chemically induced homozygous-lethal mutations at the p locus in mouse chromosome 7 have been analyzed at 17 loci defined by molecular probes to determine the types of lesions, numbers of p-region markers deleted or rearranged, regions of overlap of deletion mutations, and genetic distances between loci. A linear deletion map of the [Myod1, Ldh3]-[Snrpn, Znf127] region has been constructed from the molecular analyses of the p-locus deletions. The utility of these deletions as tools for the isolation and characterization of the genes specifying the neurological, reproductive, and developmental phenotypes genetically mapped to this region will grow as more detailed molecular analyses continue.     

Spontaneous Mutation at the Mtr Locus in Neurospora: The Molecular Spectrum in Wild-Type and a Mutator Strain

Sequence analysis of 34 mtr mutations has yielded the first molecular spectrum of spontaneous mutants in Neurospora crassa. The great majority of the mutations are base substitutions (48%) or deletions (35%). In addition, sequence analysis of the entire mtr region, including the 1472-base pair open reading frame and 1205 base pairs of flanking DNA, was performed in both the Oak Ridge and Mauriceville strains of Neurospora, which are known to be divergent at the DNA level. Sixteen sequence differences between these two strains have been found in the mtr region, with 13 of these in DNA flanking the open reading frame. The differences consisted of base substitutions and small frameshifts at monotonic runs. This set of sequence differences has allowed a comparison of mutations in unselected DNA to those mutations that produce a phenotypic signal. We have isolated a mutator strain (mut-1) of Neurospora in which the spontaneous mutation rate at various loci is as much as 80-fold higher than in the non-mutator (wild type). Twenty-one mtr mutations in the mutator background have been sequenced and compared to the non-mutator spectrum, revealing a striking increase in -1 frameshift mutations. These frameshifts occur exclusively within or adjacent to monotonic runs and can be explained by small slippage events during DNA replication. This argues for a role of the mut-1 gene in this process.     

Apparent Differential Selection at an Isozyme Locus between Queens and Workers of the Ant APHAENOGASTER RUDIS

The frequencies of two alleles at a cathodal malate dehydrogenase locus in populations of A. rudis were studied in 47 colonies from three localities in Georgia and one in New Jersey. The male gene frequencies of the New Jersey and one Georgia locality differ significantly. All the queens at one Georgia locality were heterozygotes. This genotypic distribution differs strongly from that of the workers, in which approximately equal numbers of heterozygotes and homozygotes were found. The heterozygote excess among queens but not their worker progeny suggests differential selection between these castes. This study also shows that rudis colonies consist of a single, once-mated queen and her worker progeny.     

Dynamics of Correlated Genetic Systems. I. Selection in the Region of the Glued Locus of DROSOPHILA MELANOGASTER

The dynamical behavior of chromosomal segments undergoing strong selection was investigated in four replicate populations of Drosophila melanogaster. This was accomplished by following the joint behavior of allozyme markers at the loci phosphoglucomutase and esterase C, adjacent to the recessive lethal locus Glued, during and following the course of selection against Glued. The results show strong selection at other loci in the region of the marked segment. Examination of the joint dynamic of the two markers indicates that there must be more than one, and probably several, selected loci in the region under observation, with large epistatic effects. The mode of selection on the segment often results in excess heterozygosity at the markers, but does not appear to be constant in time. It is concluded that the density of selective effects in the region under study is substantial.     

Selection at a Single Locus Leads to Widespread Expansion of Toxoplasma gondii Lineages That Are Virulent in Mice

Pathogenicity differences among laboratory isolates of the dominant clonal North American and European lineages of Toxoplasma gondii are largely controlled by polymorphisms and expression differences in rhoptry secretory proteins (ROPs). However, the extent to which such differences control virulence in natural isolates of T. gondii, including those from more diverse genetic backgrounds, is uncertain. We elucidated the evolutionary history and functional consequences of diversification in the serine/threonine kinase ROP18, a major virulence determinant in the mouse model. We characterized the extent of sequence polymorphism and the evolutionary forces acting on ROP18 and several antigen-encoding genes within a large collection of natural isolates, comparing them to housekeeping genes and introns. Surprisingly, despite substantial genetic diversity between lineages, we identified just three principal alleles of ROP18, which had very ancient ancestry compared to other sampled loci. Expression and allelic differences between these three alleles of ROP18 accounted for much of the variation in acute mouse virulence among natural isolates. While the avirulent type III allele was the most ancient, intermediate virulent (type II) and highly virulent (type I) lineages predominated and showed evidence of strong selective pressure. Out-group comparison indicated that historical loss of an upstream regulatory element increased ROP18 expression, exposing it to newfound diversifying selection, resulting in greatly enhanced virulence in the mouse model and expansion of new lineages. Population sweeps are evident in many genomes, yet their causes and evolutionary histories are rarely known. Our results establish that up-regulation of expression and selection at ROP18 in T. gondii has resulted in three distinct alleles with widely different levels of acute virulence in the mouse model. Preservation of all three alleles in the wild indicates they are likely adaptations for different niches. Our findings demonstrate that sweeping changes in population structure can result from alterations in a single gene.     

Molecular Analysis of a Transposon-Induced Deletion of the Nivea Locus in Antirrhinum Majus

The transposable element Tam3 of Antirrhinum majus is capable of causing large-scale chromosomal restructuring. It induced a large deletion at the nivea locus, to produce the allele niv-:529. The deletion removed the entire nivea coding region while the element remains intact with the potential to induce further rearrangements. Genetic experiments showed that the endpoint of the deletion (called x) is closely linked to nivea. The DNA sequences of niv-:529, a genomic excision of Tam3 from niv-:529, and the original genomic position of x have been determined. These data suggest that the deletion could have resulted from an abortive transposition or through breakage and religation.     

中国田鼠卵巢细胞gpt基因自发和亚砷酸钠诱发突变的分子分析

本文研究了亚砷酸钠对CHO-AS52细胞gpt基因的致突变作用。实验结果表明,亚砷酸钠能诱发该基因发生突变,且其突变频率随砷浓度的增加而增高。PCR分析指出,绝大多数亚砷酸钠诱发的CHO-AS52突变体的gpt基因完全缺失。在CHO-AS52细胞自发的、50μmol/L和100μmol/L亚砷酸钠诱发的突变体中,gpt基因完全缺失者所占比率分别为36.00％、54.72％及66.67％。对亚砷酸钠诱发的非缺失型gpt基因突变的PCR产物直接进行DNA序列分析表明,在9个突变细胞克隆中,有2个发生移码突变,其余7个突变细胞克隆的gpt基因结构未发现改变,碱基的改变可能发生在基因启动子区。     

Molecular Variation at the Vermilion Locus in Geographically Diverse Populations of Drosophila Melanogaster and D. Simulans

We surveyed nucleotide variation at vermilion in population samples of Drosophila melanogaster from Africa, Asia and the Americas to test the hypothesis that the vermilion gene was a target of balancing selection and to improve our understanding of geographic differentiation. Patterns of polymorphism and divergence showed no evidence for non-neutral evolution. However, the frequency spectrum of polymorphic sites in some non-African samples departed from the neutral equilibrium expectation. Furthermore, there were high levels of linkage disequilibrium in non-African samples, despite apparently high rates of crossing over in the vermilion region. In the absence of comparable data from other loci in these same population samples, we cannot determine whether the unusual patterns of variation at vermilion reflect demographic as opposed to locus-specific events. We found surprisingly high levels of differentiation at vermilion between U.S. and Congo samples of D. simulans. In light of previously published allozyme and mtDNA data that provided no evidence for significant differentiation between African and non-African D. simulans populations, the vermilion data raise the possibility that both mtDNA and allozymes have been influenced by selection.