Evolution of the chromosomal location of rDNA genes in two Drosophila species subgroups: ananassae and melanogaster.

The evolution of the chromosomal location of ribosomal RNA gene clusters and the organization of heterochromatin in the Drosophila melanogaster group were investigated using fluorescence in situ hybridization and DAPI staining to mitotic chromosomes. The investigation of 18 species (11 of which were being examined for the first time) belonging to the melanogaster and ananassae subgroups suggests that the ancestral configuration consists of one nucleolus organizer (NOR) on each sex chromosome. This pattern, which is conserved throughout the melanogaster subgroup, except in D. simulans and D. sechellia, was observed only in the ercepeae complex within the ananassae subgroup. Both sex-linked NORs must have been lost in the lineage leading to D. varians and in the ananassae and bipectinata complexes, whereas new sites, characterized by intra-species variation in hybridization signal size, appeared on the fourth chromosome related to heterochromatic rearrangements. Nucleolar material is thought to be required for sex chromosome pairing and disjunction in a variety of organisms including Drosophila. Thus, either remnant sequences, possibly intergenic spacer repeats, are still present in the sex chromosomes which have lost their NORs (as observed in D. simulans and D. sechellia), or an alternative mechanism has evolved.

Maintenance of a large pericentric inversion generated by the hobo transposable element in a transgenic line of Drosophila melanogaster.

The impact of the hobo transposable element in the global reorganization of the Drosophila melanogaster genome has been investigated in transgenic lines generated by the injection of hobo elements into the Hikone strain, which lacked them previously. Extensive surveys of transgenic lines followed for 250 generations have identified 13 inversions with hobo inserts at most breakpoints. One of these inversions is pericentric on chromosome 2. It has been maintained in the line where it was discovered and in several sublines at frequencies from 0.19 to 0.45, generating stable chromosomal polymorphisms, similar to cosmopolitan paracentric inversions in natural populations. Individuals homozygous for this inversion were viable and fertile, allowing the creation of a new homozygous strain.

Dynamics of the hobo transposable element in transgenic lines of Drosophila melanogaster.

The impact of the hobo transposable element in global reorganization of the Drosophila melanogaster genome has been investigated in transgenic lines generated by injection of hobo elements into the Hikone strain, which lacked them. In the present extensive survey, the chromosomal distribution of hobo insertion sites in the line 28 was found to be homogeneous and similar for all chromosomal arms, except 3L, when compared with other transgenic lines. However, some original features were observed in this line at the genetic and chromosomal levels. Several hotspots of insertion sites were observed on the X, second and third chromosomes. Five sites with a high frequency of hobo insertions were present on the 3L arm in most individuals tested, suggesting the action of selection for hobo element in some sites. The presence of doublets or triplet was also observed, implying that hobo inserts can show local jumps or insertions in preferred regions. This local transposition occurred independently in 11 specific genomic regions in many individuals and generations. The dynamics of this phenomenon were analysed across generations. These results support the use of the hobo system as an important tool in fundamental and applied Drosophila genetics.

Hobo transposons causing chromosomal breakpoints.

Several laboratory surveys have shown that transposable elements (TEs) can cause chromosomal breaks and lead to inversions, as in dysgenic crosses involving P-elements. However, it is not presently clear what causes inversions in natural populations of Drosophila. The only direct molecular studies must be taken as evidence against the involvement of mobile elements. Here, in Drosophila lines transformed with the hobo transposable element, and followed for 100 generations, we show the appearance of five different inversions with hobo inserts at breakpoints. Almost all breakpoints occurred in hobo insertion sites detected in previous generations. Therefore, it can be assumed that such elements are responsible for restructuring genomes in natural populations.

Allele-specific population structure of Drosophila melanogaster alcohol dehydrogenase at the molecular level.

The history of the Drosophila melanogaster alcohol dehydrogenase (ADH) Fast/Slow polymorphism was studied by recording molecular variation and inversion polymorphism in 233 chromosomes from European and African populations. Silent molecular variation in the Slow allele was very different between standard chromosomes and chromosomes bearing the In(2L)t inversion. Within populations, inverted Slow haplotypes were more variable than standard Slow haplotypes. Between populations, geographical structure was almost nonexistent for inverted Slow haplotypes and highly significant for standard Slow. All Fast haplotypes occurred on standard chromosomes. They showed little variation within and between populations. They were highly significantly closer to standard Slow haplotypes from Europe. These results suggest that the current range of Fast and In(2L)t Slow haplotypes is recent and that an older genetic differentiation between populations was followed by allele-specific gene flow.

Chromosomal distribution and population dynamics of the 412 retrotransposon in a natural population of Drosophila melanogaster.

The localization of the insertion sites of the 412 retrotransposable element was analysed by in situ hybridization to the polytene chromosomes of the genomes of males from a natural population of Drosophila melanogaster. Non-parametric statistical tests do not reveal any particular distribution of the insertion sites over the chromosomes, suggesting an apparently random distribution of the 412 element. Aggregation and dispersion tests were highly significant with data of copy number (when all genomes are pooled, many copies may be at a given site), suggesting the existence of sites with high insertion frequency. Comparison with other data from the literature confirms the tendency for a low proportion of insertions on the X chromosome in comparison with the autosomes, a result in agreement with selection acting against the detrimental effect of the 412 element insertions.

Population dynamics of the copia, mdg1, mdg3, gypsy, and P transposable elements in a natural population of Drosophila melanogaster.

The insertion site polymorphism of the copia, mdg1, mdg3, gypsy, and P transposable elements was analysed by in situ hybridization to the polytene chromosomes in genomes of males from a natural population of Drosophila melanogaster. Parameters of various theoretical models of the population biology of transposable elements were estimated from our data, and different hypotheses explaining TE copy number containment were tested. The copia, mdg1 and gypsy elements show evidence for a deficiency of insertions on the X chromosomes, a result consistent with selection against the mutational effects of insertions. On the contrary, mdg3 and P copy numbers fit a neutral model with a balance between regulated transposition and excisions. There is no strong evidence of a systematic accumulation of elements in the distal and proximal regions of the chromosomes where crossing over and ectopic exchanges are reduced. For all chromosome arms but 3L, however, the TE site density increases from the proximal to the distal parts of the chromosomes (the centromeric regions were excluded in this analysis) with sometimes a sharp decrease in density at the extreme tip, following in part the exchange coefficient. The way the copy number of TEs is contained in genomes depends thus on the element considered, and on various forces acting simultaneously, indicating that models of TE dynamics should include details of each element.

High rate of movement of one (mdg3) out of four transposable elements in a natural population of Drosophila melanogaster.

We have analysed by in situ hybridization the insertion site polymorphism of the copia, mdg1, mdg3, and P transposable elements in diploid genomes of wild males from a natural population of Drosophila melanogaster. The values of observed average degree of individual heterozygosity for all elements except mdg3 deviate statistically from the values expected by site frequencies, revealing a tendency toward homozygosity as if drift or a structuration in the population was operating. The high degree of heterozygosity for mdg3 which in addition does not deviate from the expected value, suggests that recent high rate of movements has strongly countered the population host effect on this element. Hence, heterozygosity for TEs may well reflect the history of the population, and reveals temporary high rate of site movement existing in natural populations.

Molecular variation of Adh and P6 genes in an African population of Drosophila melanogaster and its relation to chromosomal inversions.

Four-cutter molecular polymorphism of Adh and P6, and chromosome inversion polymorphism of chromosome II were investigated in 95 isogenic lines of an Ivory Coast population of Drosophila melanogaster, a species assumed to have recently spread throughout the world from a West African origin. The P6 gene showed little linkage disequilibrium with the In(2L)t inversion, although it is located within this inversion. This suggests that the inversion and the P6 locus have extensively exchanged genetic information through either double crossover or gene conversion. Allozymic variation in ADH was in linkage disequilibrium with In(2L)t and In(2R)NS inversions. Evidence suggests either that inversion linkage with the Fast allele is selectively maintained, or that this allele only recently appeared. Molecular polymorphism at the Adh locus in the Ivory Coast is not higher than in North American populations. New haplotypes specific to the African population were found, some of them connect the "WaS-like" haplotypes found at high frequencies in the United States to the other slow haplotypes. Their relation with In(2L)t supports the hypothesis that WaS recently recombined away from an In(2L)t chromosome which may be the cause of its divergence from the other haplotypes.

Lack of underdominance in a naturally occurring pericentric inversion in Drosophila melanogaster and its implications for chromosome evolution.

In(2LR)PL is a large pericentric inversion polymorphic in populations of Drosophila melanogaster on two Indian Ocean islands. This polymorphism is puzzling: because crossing over in female heterokaryotypes produces inviable zygotes, such inversions are thought to be underdominant and should be quickly eliminated from populations. The observed fixation for such inversions among related species has led to the idea that genetic drift can cause chromosome evolution in opposition to natural selection. We found, however, that In(2LR)PL is not underdominant for fertility, as heterokaryotypic females produce perfectly viable eggs. Genetic analysis shows that the lack of underdominance results from the nearly complete absence of crossing over in the inverted region. This phenomenon is probably caused by mechanical and not genetic factors, because crossing over is not suppressed in In(2LR)PL homokaryotypes. Our observations do not support the idea that the fixation of pericentric inversions among closely related species implies the action of genetic drift overcoming strong natural selection in very small populations. If chromosome arrangements vary in their underdominance, it is those with the least disadvantage as heterozygotes, like In(2LR)PL, that will be polymorphic or fixed in natural populations.

Abnormal genetic segregation associated with the presence of a Y. w+ chromosome in Drosophila melanogaster.

We have observed an abnormal genetic segregation in the progeny of crosses between males of the F71 (y wa/Y.w+) strain and females of various strains carrying marker mutations on their chromosome 2. The Y.w+ chromosome, previously described as possibly being associated with a translocation of the 22D region of chromosome 2, was shown to carry the 21A1-22E4 tip of the 2L chromosome. One chromosome 2 of F71 had a deletion of this region. The abnormal genetic segregation observed in the progeny of different crosses can be explained both by the partial lethality (which becomes severe in some homogeneous genetic backgrounds) due to trisomy of the 21A1-22E4 chromosome 2 fragment and by the lethality associated with monosomy of this 21A1-22E4 segment.

Distribution and conservation of the foldback transposable element in Drosophila.

Foldback elements are a family of transposable elements described in Drosophila melanogaster. The members of this dispersed repetitive family have terminal inverted repeats that sometimes flank a central region. The inverted repeats of all the family members are homologous. The study of the distribution and conservation of the foldback elements in different Drosophila species shows that this distribution is different from that of the hybrid dysgenesis systems (PM and IR). Sequences homologous to foldback elements were observed by Southern blots and in situ hybridization in all species of the melanogaster subgroup and in some species of the montium and takahashii subgroups. The element was probably already present before the radiation of these subgroups. No evidence of horizontal transmission of the foldback element could be observed.