Stability of Tandem Repeats in the Drosophila Melanogaster HSR-Omega Nuclear RNA

The Drosophila melanogaster Hsr-omega locus produces a nuclear RNA containing >5 kb of tandem repeat sequences. These repeats are unique to Hsr-omega and show concerted evolution similar to that seen with classical satellite DNAs. In D. melanogaster the monomer is ~280 bp. Sequences of 191/2 monomers differ by 8 +/- 5% (mean +/- SD), when all pairwise comparisons are considered. Differences are single nucleotide substitutions and 1-3 nucleotide deletions/insertions. Changes appear to be randomly distributed over the repeat unit. Outer repeats do not show the decrease in monomer homogeneity that might be expected if homogeneity is maintained by recombination. However, just outside the last complete repeat at each end, there are a few fragments of sequence similar to the monomer. The sequences in these flanking regions are not those predicted for sequences decaying in the absence of recombination. Instead, the fragmentation of the sequence homology suggests that flanking regions have undergone more severe disruptions, possibly during an insertion or amplification event. Hsr-omega alleles differing in the number of repeats are detected and appear to be stable over a few thousand generations; however, both increases and decreases in repeat numbers have been observed. The new alleles appear to be as stable as their predecessors. No alleles of less than ~5 kb nor more than ~16 kb of repeats were seen in any stocks examined. The evidence that there is a limit on the minimum number of repeats is consistent with the suggestion that these repeats are important in the function of the unusual Hsr-omega nuclear RNA.     

Population Genetics of Y-Chromosome Short Tandem Repeats in Humans

Eight human short tandem repeat polymorphisms (STRs) also known as microsatellites—DYS19, DYS388, DYS390, DYS391, DYS392, DYS393, DYS389I, and DYS389II, mapping in the Y chromosome—were analyzed in two Iberian samples (Basques and Catalans). Allele frequency distributions showed significant differences only for DYS392. Fst and gene diversity index (D) were estimated for the Y STRs. The values obtained are comparable to those of autosomal STR if corrections for the smaller effective population size on the Y chromosome are taken into account. This suggests that Y-chromosome microsatellites might be as useful as their autosomal counterparts to both human population genetics and forensics. Our results also reinforce the hypothesis that selective sweeps in the Y chromosome in recent times are unlikely. Haplotypes combining five of the loci were constructed for 71 individuals, showing 29 different haplotypes. A haplotype tree was constructed, from which an estimate of 7,000 to 60,000 years for the age of the Y-chromosome variation in Iberia was derived, in accordance with previous estimates obtained with mtDNA sequences and nuclear markers. Received: 3 January 1997 / Accepted: 25 April 1997     

Structure of the Y Chromosomal Su(ste) Locus in Drosophila Melanogaster and Evidence for Localized Recombination among Repeats

The structure of the Suppressor of Stellate [Su(Ste)] locus on the Drosophila melanogaster Y chromosome was examined by restriction analysis of both native and cloned genomic DNA. The locus consists of short subarrays of tandem repeats separated by members of other moderately repeated families. Both size variants and restriction variants proved to be common. Most repeats fell into two size classes--2.8 and 2.5 kb--but other size variants were also observed. Restriction variants showed a strong tendency to cluster, both at the gross level where some variants were present in only one of three subintervals of the locus, and at the fine level, where repeats from the same phage clone were significantly more similar than repeats from different clones. Restriction variants were shared freely among repeats of different size classes; however, size variants appeared to be randomly distributed among phage clones. These data indicate that recombination among tandem Su(Ste) repeats occurs at much higher frequencies between close neighbors than distant ones. In addition, they suggest that gene conversion rather than sister chromatid exchange may be the primary recombinational mechanism for spreading variation among repeats at the Su(Ste) locus.     

Targeted Transposition at the Vestigial Locus of Drosophila Melanogaster

Targeted transposition is the replacement of one P element with another. We are exploiting this unique property of P elements to study the complex regulatory domain of the Dopa decarboxylase (Ddc) gene in Drosophila melanogaster. P element constructs targeted to the same site in the genome will be subjected to the same position effect. This allows the subtle effects typical of most mutations in the Ddc regulatory region to be measured in the absence of the variable influences of position effects which are associated with the current method of germline transformation. We have investigated some of the parameters affecting targeted transposition of a Ddc transposon, P[Ddc], into a P element allele at the vestigial locus. These events were detected by an increased mutant vg phenotype. The location of the donor transposon in cis or in trans to the target had little effect on the frequency of targeting. Likewise, the mobility of different donor elements, as measured by their rate of transposition to a different chromosome, varied nearly 20-fold, while the rate of targeted transposition was very similar between them. All targeted alleles were precise replacements of the target P element by P[Ddc], but in several cases the donor was inserted in the opposite orientation. The targeted alleles could be described as the result of a replicative, conversion-like event.     

Cytogenetic Analysis of Segregation Distortion in Drosophila Melanogaster: The Cytological Organization of the Responder (Rsp) Locus

The segregation distortion phenomenon occurs in Drosophila melanogaster males carrying an SD second chromosome and an SD+ homolog. In such males the SD chromosome is transmitted to the progeny more frequently than the expected 50% because of an abnormal differentiation of the SD+-bearing sperms. Three major loci are involved in this phenomenon: SD and Rsp, associated with the SD and SD+ chromosome, respectively, and E(SD). In the present work we performed a cytogenetic analysis of the Rsp locus which was known to map to the centromeric heterochromatin of the second chromosome. Hoechst- and N-banding techniques were used to characterize chromosomes carrying Responder insensitive (Rspi), Responder sensitive (Rsps) and Responder supersensitive (Rspss) alleles. Our results locate the Rsp locus to the h39 region of 2R heterochromatin. This region is a Hoechst-bright, N-banding negative heterochromatic block adjacent to the centromere. Quantitative variations of the h39 region were observed. The degree of sensitivity to Sd was found to be directly correlated with the physical size of that region, demonstrating that the Rsp locus is composed of repeated DNA.     

Historicity and the Population Genetics of Drosophila Melanogaster and D. Simulans

We summarize data showing that there is population structure in African populations of Drosophila from the melanogaster-simulans complex. In D. melanogaster, population structuring is found at individual loci, but is obscured by population structuring for large inversions that simultaneously affect several loci. In D. simulans, molecular polymorphism at the X-linked vermilion locus suggests that different groups of populations have been geographically isolated for some time. Invading populations are probably derived from different areas in Africa. European populations originate from an east African population that was probably not at a demographic equilibrium. The origin of the Antilles population is apparently different and is as yet unknown. In south-western France, populations from these two species undergo different population structuring at the scale of a few kilometres: D. melanogaster makes up a large panmictic population, whereas D. simulans forms a metapopulation that is divided into smaller demes.