Recombination Can Initiate and Terminate at a Large Number of Sites within the Rosy Locus of Drosophila Melanogaster

This report presents the results of a recombination experiment designed to question the existence of special sites for the initiation or termination of a recombination heteroduplex within the region of the rosy locus. Intragenic recombination events were monitored between two physically separated rosy mutant alleles ry301 and ry2 utilizing DNA restriction site polymorphisms as genetic markers. Both ry301 and ry2 are known from previous studies to be associated with gene conversion frequencies an order of magnitude lower than single site mutations. The mutations are associated with large, well defined insertions located as internal sites within the locus in prior intragenic mapping studies. On the molecular map, they represent large insertions approximately 2.7 kb apart in the second and third exons, respectively, of the XDH coding region. The present study monitors intragenic recombination in a mutant heterozygous genotype in which DNA homology is disrupted by these large discontinuities, greater than the region of DNA homology and flanking both sides of the locus. If initiation/or termination requires separate sites at either end of the locus, then intragenic recombination within the rosy locus of the heterozygote should be eliminated. Contrary to expectation, significant recombination between these sites is seen.     

The Genetic Structure of Natural Populations of Drosophila Melanogaster. Xxii. Comparative Study of DNA Polymorphisms in Northern and Southern Natural Populations

Restriction map variation in four gene regions (Adh, Amy, Pu and Gpdh) was surveyed for 86 second chromosomes from northern (Aomori) and southern (Ogasawara) Japanese populations of Drosophila melanogaster (43 chromosomes from each population). The regions examined cover a total of 62 kilobases. Estimates of nucleotide diversity (pi) were approximately constant across the gene regions and populations examined. The distribution of restriction site polymorphisms was compatible with the expectation from the neutral mutation-random genetic drift hypothesis, but insertion/deletion polymorphisms were not consistent with it. While the two populations shared a majority of restriction site polymorphisms, frequencies of individual restriction site variants were significantly different between the two populations at 7 out of 35 segregating sites. In addition, an insertion in the Amy region was found in 15 chromosomes from the Ogasawara sample but absent in the Aomori sample. A considerable difference was observed in the number of rare insertions and deletions between the two populations. The numbers of aberrations uniquely represented were 16 in the Ogasawara sample and only 3 in the Aomori sample. These findings suggest that the two populations were differentiated from each other to some degree by means of random genetic drift and/or other factors.     

Adh 1 first intron polymorphisms in Argentinean populations of Ceratitis capitata

DNA size polymorphisms were utilized in a study of 24 natural populations of Ceratitis capitata Wiedemann (Diptera: Tephritidae) from Argentina. The first intron of alcohol dehydrogenase 1 gene (Adh₁) was amplified using exon priming intron crossing‐polymerase chain reaction. Three size variants were detected among the 307 samples analyzed. To better differentiate the size variants, further digestion of PCR products with the EcoRI restriction enzyme was carried out. Complete nucleotide sequences of the three‐allele variants were obtained and single changes, insertions, deletions, and EcoRI recognition sites were located. Population allele frequencies were analyzed and a global mean heterozygosity (He) of 0.33 was obtained. In most populations, observed allelic frequencies conformed to Hardy–Weinberg expectations. Significant differences between provinces and sampling sites within these provinces, and among some populations were found. The average number of insects exchanged among populations (Nm) was estimated and high values were observed between Argentina and populations from two African countries (Morocco and Kenya), Australia, and Hawaii (Kauai). Pest introduction sources and dispersion patterns in Argentina are discussed based on these results as well as on available bibliographical data.     

Molecular Genetic Variation in the Centromeric Region of the X Chromosome in Three Drosophila Ananassae Populations. I. Contrasts between the Vermilion and Forked Loci

We have surveyed three natural populations of Drosophila ananassae for restriction map variation at the forked (f) and vermilion (v) loci, using 6-cutter restriction enzymes. Both loci are located in the centromeric region of the X chromosome. Two major conclusions can be drawn from the data. First, we found strong evidence for population subdivision, i.e., significant differences in the frequency distributions of polymorphisms and/or haplotypes between the Burma, India, and Brazil populations. Secondly, the pattern of DNA sequence variation between the two loci is unexpectedly different. The level of nucleotide variation in the v locus region is reduced (relative to f), especially in the Burma population. Furthermore, in contrast to v, we found no insertions/deletions larger than 700 bp and no significant linkage disequilibrium at f. The genetic differentiation among subpopulations can readily be attributed to restricted migration as the predominant evolutionary force. According to population genetics theory, the differences in DNA polymorphisms between the two loci are in qualitative agreement with the hypothesis that recombination is reduced in the v locus region ("centromere effect") but not at f. In order to test this hypothesis directly, we determined the cytogenetic positions of several loci in the centromeric region by in situ hybridization and found by comparison with the genetic map that recombination at v is indeed very low, much lower than at f.     

DNA fingerprinting based on microsatellite-anchored fragment length polymorphisms,and isolation of sequence-specific PCR markers in lupin (Lupinus angustifolius L.)

We report a method of microsatellite-anchored fragment length polymorphisms for DNA fingerprinting. The method combines the concept of AFLP and the microsatellite-anchor primer technique. Genomic DNA was digested by one restriction enzyme MseI. One AFLP adaptor (MseI adaptor) was ligated onto the restriction fragments. DNA fingerprints were produced by PCR using one microsatellite-anchor primer in combination with one MseI-primer. The method allows co-amplification of over 100 DNA fragments containing microsatellite motifs per PCR. Polymorphisms detected from lupin by this method included those arising from variation in the number of microsatellite repeat units targeted by the microsatellite-anchor primers, from variation on the annealing sites for the SSR-anchor primers, from insertions/deletions outside the SSR region, and from variation in restriction sites. The first three types of polymorphisms were readily converted into sequence-specific PCR markers suitable for marker-assisted breeding.     

Linkage disequilibria and the site frequency spectra in the su(s) and su(w(a)) regions of the Drosophila melanogaster X chromosome

Over the last decade, surveys of DNA sequence variation in natural populations of several Drosophila species and other taxa have established that polymorphism is reduced in genomic regions characterized by low rates of crossing over per physical length. Parallel studies have also established that divergence between species is not reduced in these same genomic regions, thus eliminating explanations that rely on a correlation between the rates of mutation and crossing over. Several theoretical models (directional hitchhiking, background selection, and random environment) have been proposed as population genetic explanations. In this study samples from an African population (n = 50) and a European population (n = 51) were surveyed at the su(s) (1955 bp) and su(w(a)) (3213 bp) loci for DNA sequence polymorphism, utilizing a stratified SSCP/DNA sequencing protocol. These loci are located near the telomere of the X chromosome, in a region of reduced crossing over per physical length, and exhibit a significant reduction in DNA sequence polymorphism. Unlike most previously surveyed, these loci reveal substantial skews toward rare site frequencies, consistent with the predictions of directional hitchhiking and random environment models and inconsistent with the general predictions of the background selection model (or neutral theory). No evidence for excess geographic differentiation at these loci is observed. Although linkage disequilibrium is observed between closely linked sites within these loci, many recombination events in the genealogy of the sampled alleles can be inferred and the genomic scale of linkage disequilibrium, measured in base pairs between sites, is the same as that observed for loci in regions of normal crossing over. We conclude that gene conversion must be high in these regions of low crossing over.     

Polymorphism of mitochondrial genome noncoding regions in the three Kazakh populations inhabited different areas of Kazakhstan and in the samples of DNA from ancient people of Kazakhstan Altai

Polymorphism of major noncoding region of mitochondrial DNA (mtDNA D-loop, 528 bp in length) from the three modem kazakh populations and from DNA samples of ancient people inhabited modern Kazakhstani Altai were studied. PCR and RFLP analysis of 13 sites of restriction--BamHI, EcoRV, Sau3AI (1 restriction site), KpnI (2 sites), HaeIII (3 sites), RsaI (5 restriction sites), were carried out. The distribution of each site frequencies was determined. Nucleotide diversity (h) and genetic distance between different kazakh population and other populations of world were estimated. The same RFLP analysis of the mitochondrial DNA control region was carried out for the paleogenomic samples. It was shown that two samples of ancient mitochondrial DNA were monomorphous throughout all analyzed restriction sites.     

An investigation of the cause of low variability on the fourth chromosome of Drosophila melanogaster.

The fourth chromosome of Drosophila melanogaster lacks meiotic recombination. There is also a lack of nucleotide variation on the chromosome. This lack of variation could have been caused by a recent selective sweep, by background selection, or by a combination of these two forces. It should be possible to differentiate between the two mechanisms by studying the frequencies of polymorphic sites on the chromosome: a selective sweep should have resulted in low-frequency polymorphisms, whereas higher frequency polymorphisms would indicate the action of background selection. We have analyzed retrotransposable element insertions on the fourth chromosome in 11 strains of D. melanogaster. The polymorphisms found have a range of frequencies, with the presence of some insertions with high frequencies suggesting that the lack of variation is the result of background selection. We summarize the data using two statistics: the number of sites shared by more than one of the sample of 11 chromosomes (internal sites) and the mean number of transposable element differences in presence or absence between the sampled chromosomes. Simulations indicate that a selective sweep occurring more than 15,000 (0.03N) generations ago cannot be ruled out from the number of internal sites, although the number of differences between the chromosomes suggests either background selection or a sweep occurring more than 60,000 (0.12N) generations ago. Our results show no homoplasies and are thus consistent with no recombination occurring on the chromosome. The difficulties of distinguishing between the models using polymorphism data are discussed.     

PCR method for generating multiple mutations at adjacent sites

Procedures to introduce point mutations, restriction sites and insert or delete DNA fragments are very important tools to study protein function. We describe here two-step PCR-based method for generating single or multiple mutations, insertions and delections in a small region of the sequence. In the first step, a unique restriction site is introduced near the part of DNA sequence to be changed, without changing the amino acid sequence. For this step, one of the methods already described can be used. In the second step, mutations are introduced using mutagenic primers containing the unique restriction site from the first step at the 5′ end, paired with a universal primer crossing another unique restriction site present originally in the sequence. The method is very simple, economic and rapid. In comparison with the traditionalin vitro mutagenesis methods, one can generate large numbers of mutated plasmids in hours.     

Transforming growth factor-alpha: characterization of the BamHI, RsaI, and TaqI polymorphic regions.

We have characterized the nature of structural alleles of the transforming growth factor-alpha (TGF alpha) locus by restriction-enzyme digestion with BamHI, RsaI, and TaqI. The BamHI polymorphic site is located within exon VI, which codes for the 3' untranslated region. The two BamHI alleles differ by a single point mutation at the restriction site. The RsaI and TaqI polymorphic sites are located within intron V. The two alleles differ at the restriction site, either by a point mutation (RsaI) or by a 4-bp deletion (TaqI). This analysis permits us to devise a PCR method coupled with restriction digestions to directly identify the TGF alpha polymorphisms. Analysis of 99 Caucasian controls has revealed a highly significant (P < .001) association between the RsaI and the BamHI genotype. The frequency of the rare BamHI allele was significantly higher (P < .001) in transformed cell lines (.30) than in controls (.076).     

A locus on chromosome 11p with multiple restriction site polymorphisms.

We have discovered and characterized a new polymorphic locus on chromosome 11p, D11S12, defined by an arbitrary genomic DNA segment cloned in the plasmid pADJ762. Four different polymorphic restriction sites with minor allele frequencies greater than 5% are revealed by Southern hybridization of this probe and its derivatives to digests of human DNAs. These include two MspI sites, a TaqI site, and a BclI site. The frequencies of the common haplotypes at this locus have been determined in a Utah population. Significant linkage disequilibrium has been demonstrated to exist between some pairs of polymorphic sites. A molecular map of this region has been determined, and the polymorphic sites have been localized. Comparison of physical separation with degree of linkage disequilibrium reveals an interesting case where an MspI site and a TaqI site that are separated by 6.8 kilobases (kb) show a greater degree of disequilibrium with each other than they do with two polymorphic sites located between them. One of the two interior sites is a BclI site that is approximately 0.2 kb away from the TaqI site but shows the same degree of disequilibrium with the TaqI site as with the MspI site 6.7 kb away. Although there is significant linkage disequilibrium at this locus, there are four major haplotypes with frequencies of 5% or greater, and the polymorphic information content (PIC) of this locus is .64.     

Coding region deletions associated with the major form of rDNA interruption in Drosophila

The nucleotide sequences at and around the termini of 5 kb type 1 interruptions in three separate clones of D. melanogaster rDNA repeats have been determined, and have been compared with the sequence of the corresponding region of an insertion-free rDNA repeat. All three interrupted rDNA repeats contain a small deletion of 28S rRNA coding material at the left coding/insertion sequence junction. A second deletion was found in one of the three clones, ad other aberrations were suggested by the results of restriction enzyme digestions of unfractionated rDNA. The termini of 5 kb type 1 rDNA insertions in D. melanogaster were also compared with the corresponding regions of 28S rDNA interruptions in D. virilis: the insertion site is identical in the two species, but the termini of the two species' interruptions show no homology. I sequenced a 1.1 kb region of the 5 kb type 1 D. melanogaster rDNA interruption that covers the sequences of the 1 kb and 0.5 kb insertions. There is 98% homology between the rightmost 1 kb of the 5 kb interruption and the sequences of the shorter insertions. Data suggest that Drosophila rDNA interruptions arose as a transposable element, and that divergence had included length alterations generated by unequal crossing over.     

Restriction polymorphism of the major noncoding region of mtDNA in the indigenous population of the TUVA republic]

Mitochondrial DNA sequence variation was examined in three rural populations of the indigenous inhabitants from the Tuva Republic. The frequencies of restriction sites within the D-loop region of mtDNA were determined. The three populations studied demonstrated similar patterns of mtDNA polymorphism. Like other Siberian populations, Tuvinians were characterized by high frequencies of the HaeIII 16517 and AspS9I (Cfr13I) 16516 restriction sites (about 75%). Moreover, in Tuvinians, a relatively low (71 to 81%) frequency of the KpnI 16129 restriction site was observed. The frequency of the mitotype differing from the Cambridge sequence by the HaeIII 16517 and KpnI 16129 sites in Tuvinians was higher than in Mongols and Russians. The features of mtDNA polymorphism point to the similarity between Tuvinians and other Siberian ethnic groups (Sel'kups in particular). This can be explained by the contribution of the Samoyed component, along with the Turkic and Mongoloid ones, to the formation of the Tuvinian ethnic group.     

DNA replication and chromatin structure of simian virus 40 insertion mutants.

Insertion of DNA segments into the nuclease-sensitive region of simian virus 40 alters both replication efficiency and chromatin structure. Mutants containing large insertions between the simian virus 40 origin of replication (ori site) and the 21-base-pair repeated sequences replicated poorly when assayed by transfection into COS-1 cells. Replication of mutants with shorter insertions was moderately reduced. This effect was cis-acting and independent of the nucleotide sequence of the insert. The nuclease-sensitive chromatin structure was retained in these mutants, but the pattern of cleavage sites was displaced in the late direction from the ori site. New cleavage sites appeared within the inserted sequences, suggesting that information specifying the nuclease-sensitive chromatin structure is located on the late side of the inserts. Accessibility to BglI (which cleaves within the ori site) was reduced in the larger insertion mutants. These results support the conclusion that efficient function of the viral origin of replication is correlated with its proximity to an altered chromatin structure.     

Demonstration of sequence variations in the promoter region of the human cystatin C gene.

Four point mutations in the promoter region of the human cystatin C gene have been detected by direct sequencing of polymerase chain reaction (PCR) amplified DNA. The four base changes are all localized within a short segment of 85 base pairs. Three cystatin C gene alleles could be defined with respect to these promoter mutations; one with the sequence previously published, one carrying three of the mutations and one with all four base substitutions. Two of the observed mutations are involved in a novel Sst II polymorphism and another generates a new Dde I restriction site. A PCR-based assay for analysis of these Sst II and Dde I sites was designed and used to demonstrate Mendelian inheritance of the polymorphisms as well as to determine the frequencies of the cystatin C gene alleles in the population.     

Molecular genetic variation in the centromeric region of the X chromosome in three Drosophila ananassae populations. II. The Om(1D) locus

Restriction-site and sequence-length polymorphism in the Om(1D) locus region on the X chromosome in Drosophila ananassae was investigated for three natural populations (from Burma, India, and Brazil), by using hexanucleotide-recognizing restriction enzymes. The estimates of average heterozygosity per nucleotide (pi) were 0.0085, 0.0043, and 0.0004 for the Burma, India, and Brazil populations, respectively, and the average frequencies of insertions/deletions were 0.078, 0.054, and 0.007/chromosome/kb. While the pi values at this locus are similar to the estimates obtained from other euchromatic loci in D. ananassae or in other Drosophila species, the frequencies of insertions/deletions are much higher than those previously reported from Drosophila. The exceptionally high frequencies of length polymorphisms in the Burmese sample and, to a lesser extent, in the Indian sample indicate that the hypermutability of Om(1D), caused by the frequent insertion of the transposable element tom, may be due to locus-specific rather than to tom element-specific properties. The low level of nucleotide variation in the Brazilian population seems to be due to a recent bottleneck of population size. This population was apparently founded in recent years by a small number of individuals and has been relatively isolated ever since.