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The hAT transposons, very abundant in all kingdoms, have a common evolutionary origin probably predating the plant-fungi-animal divergence. In this paper we present their general characteristics. Members of this superfamily belong to Class II transposable elements. hAT elements share transposase, short terminal inverted repeats and eight base-pairs duplication of genomic target. We focus on hAT elements in Drosophila, especially hobo. Its distribution, dynamics and impact on genome restructuring in laboratory strains as well as in natural populations are reported. Finally, the evolutionary history of hAT elements, their domestication and use as transgenic tools are discussed.

Drosophila melanogaster resistance against the parasitoid wasp Leptopilina boulardi is under the control of a single gene (Rlb), with two alleles, the resistant one being dominant. Using strains bearing deletions, we previously demonstrated that the 55E2-E6; 55F3 region on chromosome 2R is involved in the resistance phenomenon. In this paper, we first restricted the Rlb containing region by mapping at the molecular level the breakpoints of the Df(2R)Pc66, Df(2R)P34 and Df(2R)Pc4 deficiencies, using both chromosomal in situ hybridization and Southern analyses. The resistance gene was localized in a 100 kb fragment, predicted to contain about 10 different genes. Male recombination genetic experiments were then performed, leading to identification of two possible candidates for the Rlb gene. Potential involvement of one of this genes, edl/mae, is discussed.

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.

This review deals with the differences between Drosophila melanogaster and Drosophila simulans in their mitotic and polytene chromosomes. The description of the mitotic karyotypes of D. melanogaster and D. simulans is mainly based on the methods that allow to differentiate their euchromatin from their heterochromatin: banding patterns, distribution of satellite DNAs and location of the rDNA. The polytene chromosomes karyotypes are known for many years to differ by a major paracentric inversion on chromosome 3 and minor few differences. The main difference take place in their chromosomal polymorphism: D. melanogaster is highly polymorphic while D. simulans has long been known to be a monomorphic species. In fact, despite worldwide studies of natural populations for both species, only 14 unique inversions have been described for D. simulans while more than 500 inversions are already known for D. 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.

In situ hybridization is particularly appropriate for mapping specific DNA sequences on polytene chromosomes of Drosophila and other dipterans. This technique is based on the recognition and binding of one labeled sequence (the probe) to homologous sequences on chromosomes fixed on a microscope slide. The probes are labeled with biotin or other nonradioactive products, and the probe signal can be detected as a thin line on the chromosomes, following the shape of the classical Giemsa-stained chromosome bands, thus allowing the detection of TE insertions within the range of 50 to 200 kb. In our laboratory we work on many individuals from natural populations, and as a result we process high numbers of slides hybridized with various DNA probes of transposable elements every day. Therefore, the in situ hybridization technique we use is a simplification of earlier published protocols. This chapter presents our simplified standard in situ hybridization protocol for labeling polytene chromosomes of Drosophila with biotin and a fluorescence stain (FISH).

Cyclophilins are enzymes catalyzing the cis-trans isomerization of peptidyl-prolyl bonds and belong to the enzyme class of peptidyl-prolyl cis-trans isomerases (PPIases), which includes two more families (FK506 binding proteins and parvulins). We report the characterization of a novel cyclophilin (Moca-cyp) isolated from Drosophila melanogaster. The single-copy Moca-cyp gene, which is localized on chromosome 3R, was cloned and sequenced. The sequence alignment of the gene against Moca-cyp cDNA allowed us to define its intron/exon structure and to identify a variant cDNA corresponding to an alternatively spliced mRNA. By embryo in situ RNA hybridization and immunostaining, we show that the expression of Moca-cyp is regulated during embryogenesis of Drosophila. The 120-kDa nuclear Moca-cyp protein belongs to a subfamily of large cyclophilins sharing structural and enzymatic features: their highly conserved N-terminal PPIase domain is extended by a positively charged and divergent C-terminal tail. Compared with cyclophilin 18, the enzymatic activity carried by the PPIase domain of Moca-cyp is low, exhibits characteristic substrate specificity, and shows a reduced sensitivity to the drug cyclosporin A (CsA). The reduced affinity for CsA is one of the typical features linking members of this subfamily and is probably the consequence of two amino acid substitutions within their active site. Another structural feature shared by members of this subfamily is a conserved polypeptidic segment ("moca" domain) that we report for the first time. The moca domain is located within the C-terminal tail and is the exclusive hallmark of a group of large cyclophilins found in multicellular organisms of the animal kingdom.

Polydnaviruses, obligatorily associated with endoparasitoid wasps, are unique in that their segmented genome is composed of multiple double-stranded DNA circles. We present here the first cytological evidence that virus segments are integrated in the wasp genome, obtained by using in situ hybridization of virus probes with viral sequences in the chromosomes of a wasp from the braconid family of hymenopterans.

When 41 populations from Africa (south of the Sahara) and Indian Ocean islands were analysed for their chromosomal inversion polymorphism, 34 rearrangements were found, including the four common cosmopolitans (In(2L)t, In(2R)NS, In(3L)P and In(3R)P), four rare cosmopolitans (In(2L)NS, In(3R)C, In(3R)Mo and In(3R)K) and six African polymorphic ('recurrent') endemics. Mean inversion frequencies per major autosome arm were positively and, generally, highly correlated to each other. There was no altitudinal nor latitudinal cline of inversion frequency, except for one African polymorphic endemic. Significant longitudinal clines were detected for In(2L)t, In(3L)P and In(3R)K; in all cases, inversion frequencies decreased eastward. Principal components analysis and ANOVA made it possible to distinguish three groups of populations. A high level of polymorphism was found in populations from west tropical Africa. The other low altitude populations from the mainland were moderately polymorphic, whereas the lowest levels of polymorphism were those of high altitude populations and of Indian Ocean islands. Moreover, some regional and local differentiation was also found. The frequency of unique autosomal inversions was not different from those found in Asia, Australia and America, but was significantly higher than that in Europe and North Africa. A West-East differentiation was also observed for the African polymorphic endemics. The present geographic pattern suggests a long, patchy evolution with restricted gene flow, followed by the modern period with numerous recent migrations linked to human transportation.

The ref(2)P gene is involved in the control of sigma rhabdovirus multiplication in Drosophila melanogaster. ref(2)P activity is also necessary for male fertility. However, in one-third of laboratory strains tested, males that lacked ref(2)P activity were fertile. In all such strains studied, the male sterility phenotype was abolished due to the presence of a particular allele at the Su(P) locus, at 73B1-2. These spontaneous suppressor alleles were dominant. We were able to induce dominant suppressor alleles at the Su(P) locus by X-ray mutagenesis and hybrid dysgenesis, suggesting that null alleles of Su(P) confer the dominant suppressor phenotype. The Su(P) gene was cloned by P element tagging. The P element-tagged alleles identified a Su(P) transcript as a 1.4-kb mRNA produced in the soma of both males and females, which is also abundant in ovaries.

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.

The P[MTW] transposon carries a functional MtnA (metallothionein) gene and a miniwhite reporter gene. When P[MTW] was transformed into Drosophila, many lines were found to show position-dependent expression patterns of the miniwhite or the MtnA transgene. Identification of several of the target genes indicated that this construct behaves as an enhancer or silencer trap. For instance, expression of at least one reporter transgene was shown to correlate with that of the endogenous gene in the case of insertions in Ultrabithorax, four-jointed, and the iroquois complex. The frequency of patterns recovered with P[MTW] is higher than that reported for P[LacW], suggesting that P[MTW] has unusual properties. The possibility of biased insertion of P[MTW] was assayed by screening a sample of 66 MTW lines for modifiers of the extra sex comb phenotype caused by a hypomorphic allele of polyhomeotic. Seven modifiers were recovered, which could be ranked in two classes: genes involved in leg morphogenesis (including four-jointed and spitz), and genes of the Polycomb- or trithorax-Group, including trithorax and batman, a new gene which encodes a product with a BTB/POZ domain. Taken together, these results indicate that P[MTW] allows the tagging of patterning and cell determination genes, and thus provides a useful tool for identifying new developmental functions.

The haploid complement consists of seven metacentric chromosomes in most diprionid species but has evolved to n = 8 by fission in Neodiprion abietis. This fission generated a small telocentric chromosome and a large pseudoacrocentric chromosome with a short arm carrying a satellite. In situ hybridization indicated that the location of the rRNA gene cluster corresponds to the whole short arm. This suggests that (i) the breaking point was located close to an rRNA gene cluster, and (ii) fission was associated with growth of rDNA. These results suggest rDNA as a preferential breaking point but with a role in the healing of naked chromosome ends.

The finding of new melanogaster sister species may help us in understanding more about how the emergence of genetic novelties, particularly in insular habitats, can result in speciation. Here we report on the discovery of Drosophila santomea, which is the first melanogaster sibling found off West-equatorial Africa, on São Tomé, one of the Gulf of Guinea islands. Although the eight other melanogaster sister species are remarkably conservative in their morphology except for their terminalia, the new find has a morphological trait distinguishing it from all of these: a pure yellow body coloration of both sexes without the normal black abdominal banding. Evidence from the terminalia, polytene and mitotic chromosomes, period gene and allozymes are provided indicating that it is nonetheless the nearest relative of Drosophila yakuba with which it coexists on the island. The new find is a clear-cut taxon as shown by the production of sterile male hybrids, eventually with developmental defects, in both directions of cross with yakuba and by the existence of an altitudinal divide accompanied by a hybrid zone at mid-elevation on the island. Molecular and karyotypic data further support this conclusion. In contrast to the significant divergence of their nuclear DNAs, an intriguing similarity in their cytochrome b sequences was observed indicating a recent coalescence common to santomea, yakuba and also teissieri cytoplasms. These were shown to harbour the same Wolbachia endosymbiotic bacteria which could possibly be responsible for mitochondrial DNA hitchhiking across the species barrier.

Insect hosts can survive infection by parasitoids using the encapsulation phenomenon. In Drosophila melanogaster the abilities to encapsulate the wasp species Leptopilina boulardi and Asobara tabida each involve one major gene. Both resistance genes have been precisely localized on the second chromosome, 35 centimorgans apart. This result clearly demonstrates the involvement of at least two separate genetic systems in Drosophila resistance to parasitoid wasps. The resistance genes to L. boulardi and A. tabida are not clustered as opposed to many plant resistance genes to pathogens cloned to date.

Drosophila melanogaster larvae usually react against eggs of the parasitoid wasp Leptopilina boulardi by surrounding them with a multicellular melanotic capsule. The genetic determinism of this response has been studied previously using susceptible (non-capsule-forming) and resistant (capsule-forming) strains. The results suggest that differences in their encapsulation response involve a single gene, resistance to Leptopilina boulardi (Rlb), with two alleles, the resistant one being dominant. Rlb confers specific protection against Leptopilina boulardi and is thus probably involved in parasitoid recognition. Recent studies have localized this gene on the right arm of the second chromosome and our aim was to precisely determine its genetic and molecular location. Using strains bearing deletions, we demonstrated that resistance to Leptopilina boulardi is conferred by the 55C; 55F3 region and that the 55E2-E6; F3 region is particularly involved. A physical map of the 55C; 56A region was then constructed, based on a set of overlapping cosmid and P1 phage clones. Using single and double digests, cross hybridization of restriction fragments, and location of genetically mapped genes and STSs, a complete, five-enzyme restriction map of this 830-kb region was obtained.

Copy numbers of sequences homologous to the Drosophila melanogaster retrotransposable element 412, their distribution between the chromosome arms and the chromocenter, and whether they contain full-size copies were analyzed for 55 species of the Drosophila genus. Element 412 insertion sites were detected on the chromosome arms of D. melanogaster, Drosophila simulans, and a few species of the obscura group, but the chromocenter was labeled in almost all species. The presence of element 412 sequences in the majority of species shows that this element has a long evolutionary history in Drosophilidae, although it may have recently invaded the chromosomes in some species, such as D. simulans. Differences in copy number between species may be due to population size or specific endogenous or environmental factors and may follow the worldwide invasion of the species. Putative full-length copies were detected in the chromocenters of some species with no copies on the chromosome arms, suggesting that the chromocenter may be a shelter for such copies and not only for deleted ones.

Earlier cytological investigations characterize the family Diprionidae with a modal chromosome number of seven. This study shows that Diprion pini and D. similis have 14 acrocentric chromosomes (n = 14 for haploid males and 2n = 28 for diploid females). In D. pini, rRNA genes are located on the satellite and the round short arm of the chromosome carrying this segment. We studied six populations with no evidence of chromosomal polymorphism at the species level. Our results disagree with those published previously and reopen the question of karyotype evolution. Chromosome morphology, ISH and C-banding results support the hypothesis of chromosome number doubling by centric fission and not by polyploidization, followed by the growth of short arms by means of pericentromeric DNA amplification.

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.

We describe a gene from Drosophila melanogaster related to the alpha-amylase gene Amy. This gene, which exists as a single copy, was named Amyrel. It is strikingly divergent from Amy because the amino acid divergence is 40%. The coding sequence is interrupted by a short intron at position 655, which is unusual in amylase genes. Amyrel has also been cloned in Drosophila ananassae, Drosophila pseudoobscura, and Drosophila subobscura and is likely to be present throughout the Sophophora subgenus, but, to our knowledge, it has not been detected outside. Unexpectedly, there is a strong conservation of 5' and 3' flanking regions between Amyrel genes from different species, which is not the case for Amy and which suggests that selection acts on these regions. In contrast to the Amy genes, Amyrel is transcribed in larvae of D. melanogaster but not in adults. However, the protein has not been detected yet. Amyrel evolves about twice as fast as Amy in the several species studied. We suggest that this gene could result from a duplication of Amy followed by accelerated and selected divergence toward a new adaptation.

This study is an attempt to trace the fate of hobo elements in the genomes of E strains of Drosophila melanogaster that have been transfected with pHFL1, a plasmid containing an autonomous hobo. Such long-term population studies (over 105 generations) could be very useful for better understanding the population and genomic dynamics of transposable elements and their pattern of insertions. Molecular analyses of hobo elements in the transfected lines were performed using Southern blots of XhoI-digested genomic DNAs. The complete element was observed in all six injected lines. In two lines we observed, at generation 100, two deleted elements, which did not correspond to Th1 and Th2. The results obtained by the in situ method show that the number of hybridization sites increases in each line and prove that the hobo element may be amplified in an RM genome. The hobo activity does not seem to be systematically correlated with the number of hobo elements. After generation 85, the evolution of the hobo element's insertion site number depends on the injected line. In all lines, the total number of insertions remains quite small, between 0 and 11. Hobo elements are located on each of the chromosomal arms. We describe 'hotspots'-insertion sites present in all lines and in all generations. On the 3R arm, a short inversion appeared once at generation 85.

The transposable element hobo has been introduced into the previously empty Drosophila melanogaster strain Hikone so that its dynamics can be followed and it can be compared with the P element. Five transformed lines were followed over 58 generations. The results were highly dependent on the culture temperature, the spread of hobo element being more efficient at 25 degrees C. The multiplication of hobo sequences resulted in a change in the features of these lines in the hobo system of hybrid dysgenesis. The number of hobo elements remained low (two to seven copies) and the insertions always corresponded to complete sequences. Our findings suggest that, despite their genetic similarities, P and hobo elements differ in many aspects, such as mobility and regulation mechanisms.

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.

The I factor is a LINE-like transposable element responsible for the I-R system of hybrid dysgenesis in Drosophila melanogaster. Inducer strains of this species contain several I factors whereas reactive strains do not. I factors are stable in inducer strains, but transpose at high frequency in the germ-line of females, known as SF females, produced by crossing reactive females and inducer males. Various abnormalities occur in SF females, most of which result from this high rate of transposition. We report here that recombination is increased in the germ-line of these females. This is a new characteristic of the I-R system of hybrid dysgenesis that might also be associated with transposition of the I factor.

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.

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.

The invasion kinetics of hobo transposable element in the Drosophila melanogaster genome was studied by in situ hybridization on the polytene chromosomes. Six independent lines of Drosophila melanogaster flies that had been previously transformed by microinjection of the pHFL1 plasmid containing a complete hobo element were followed over 50 generations. We observed that hobo elements were scattered on each of the chromosome arms, with more insertion sites on the 3R arm. The total number of insertion sites remains quite small, between four and six, at generation 52. On the 2R arm, a short inversion appeared once at generation 52. Most of the integration sites reported here were already described for several transposons but some of them appear to be hotspots for hobo elements.

Hobo elements are a family of transposable elements found in Drosophila melanogaster and its three sibling species: D. simulans, D. mauritiana and D. sechellia. Studies in D. melanogaster have shown that hobo may be mobilized, and that the genetic effects of such mobilizations included the general features of hybrid dysgenesis: mutations, chromosomal rearrangements and gonadal dysgenis in F1 individuals. At the evolutionary level some hobo-hybridizing sequences have also been found in the other members of the melanogaster subgroup and in many members of the related montium subgroup. Surveys of older collected strains of D. melanogaster suggest that complete hobo elements were absent prior to 50 years ago and that they have recently been introduced into this species by horizontal transfer. In this paper we review our findings and those of others, in order to precisely describe the geographical distribution and the evolutionary history of hobo in the D. melanogaster complex. Studies of the DNA sequences reveal a different level of divergence between the group D. melanogaster, D. simulans and D. mauritiana and the fourth species D. sechellia. The hypothesis of multiple transfers in the recent past into the D. melanogaster complex from a common outside source is discussed.

The number and organization of amylase genes in Drosophila ananassae were investigated through classical genetic methods and in situ and filter hybridizations. At least four genes may be active in D. ananassae, organized as two independent pairs of closely linked copies on the 2L and 3L chromosomal arms. Several other species of the D. ananassae subgroup were studied and show the same chromosomal locations, suggesting an ancient duplication event. However, the number of Amy copies seems to be higher in the D. ananassae multigene family, and there is a striking intraspecific molecular differentiation.

We describe herein, a new unstable mutant of the vestigial locus, isolated from a French natural population. From this mutant vestigial almost (vgal) wild-type flies (vgal+) and extreme vg phenotypes (vge) arose spontaneously without genomic shock. The occurrence of vgal+ or vge alleles depends mostly on the breeding temperature; vgal+ revertants arose principally at low temperature (21 degrees C) and vge at 28 degrees C. These events occur mainly in the male germ line and the phenomenon appears to be premeiotic. Our results with in situ hybridization experiments and Southern blots show that the vgal mutation is due to a 2 kb DNA insertion, which is a deleted hobo element. Genetic and molecular analyses show that two distinct events may underly the wild-type revertants. One is the excision of the resident hobo element, the other a further deletion (about 300 bp in the example characterized herein). The vge mutation is probably due to a deletion of vestigial sequences flanking the hobo insertion.

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.

The occurrence of active, or autonomous, mariner elements was investigated by crossing white-peach mutant Drosophila simulans females with wild-type males from various geographic origins. From a total of 194 experimental crosses only 17 failed to produce progeny with eye mosaicism (MOS, i.e. pigmented spots in otherwise white-peach eyes). Therefore, active mariner elements inducing somatic excision of the copy inserted at the white locus are abundant in all populations sampled. In the experimental crosses the frequency of mosaic offspring ranged from 0 to 100%, showing that the phenotypic expression is highly variable. The MOS phenotype, measured by the number of spots on the eyes, is quite variable within the progeny of single crosses. Although a difference was observed in the average MOS score (percentage of mosaic flies) between northern and southern populations of France, there was no indication of long range variation between geographic populations. Neither was there a systematic difference between recently collected populations and samples kept several years as isofemale lines.