Is genome size influenced by colonization of new environments in dipteran species?

Genome size differences are usually attributed to the amplification and deletion of various repeated DNA sequences, including transposable elements (TEs). Because environmental changes may promote modifications in the amount of these repeated sequences, it has been postulated that when a species colonizes new environments this could be followed by an increase in its genome size. We tested this hypothesis by estimating the genome size of geographically distinct populations of Drosophila ananassae, Drosophila malerkotliana, Drosophila melanogaster, Drosophila simulans, Drosophila subobscura, and Zaprionus indianus, all of which have known colonization capacities. There was no strong statistical differences between continents for most species. However, we found that populations of D. melanogaster from east Africa have smaller genomes than more recent populations. For species in which colonization is a recent event, the differences between genome sizes do not thus seem to be related to colonization history. These findings suggest either that genome size is seldom modified in a significant way during colonization or that it takes time for genome size of invading species to change significantly.

Tissue-specificity of 412 retrotransposon expression in Drosophila simulans and D. melanogaster.

We analyse the expression of the retrotransposon 412 in the soma, testes, and ovaries in populations of Drosophila simulans and D. melanogaster, using RT-PCR and in situ hybridization. We find that expression of 412 is highly variable in the soma, confirming previous findings based on Northern blots. No 412RNA is detected in the ovaries by either in situ hybridization or RT-PCR, in any population of either species. Transcripts are, however, detected in the male germline, which show a very characteristic spatial pattern of 412 expression in primary spermatocytes. There is no relationship between expression of the 412 element in the soma and in the testes in the populations. These findings show that the expression of 412 is independently regulated in the soma and the testes, and this raises the question of the real influence of the somatic transcripts on the organism and on the transposition rate.

Developmental expression of the 412 retrotransposon in natural populations of D. melanogaster and D. simulans.

We analysed the pattern of expression of retrotransposon 412 through developmental stages in various populations of Drosophila simulans and D. melanogaster differing in 412 copy number. We found that the 412 expression pattern varied greatly between populations of both species, indicating that such patterns were not entirely species-specific. In D. simulans, total transcripts increased with number of 412 copies in the chromosomes when this number was low, and then decreased for high copy numbers. D. melanogaster, which has a higher 412 copy number than D. simulans, had overall a lower global 412 expression, but again showed variation in 412 expression pattern between populations. These results suggest that in populations of D. simulans with low 412 copy number, the expression pattern of this element depends not only on copy number but also on host cellular regulatory sequences near which the elements were inserted. In D. simulans populations with high copy number overall transcription was on the contrary globally repressed, as observed in D. melanogaster. A population from Canberra (Australia) which had a very high 412 copy number was found to be associated with very high expression of 412 over all developmental stages, suggesting that the above 412 expression regulation processes are overcome in this population sample. The analysis of hybrids between geographically distinct populations of D. simulans showed that 412 expression was trans-regulated differently according to developmental stages, implying complex interactions between the 412 element and stage-specific host genes.

Distribution of the retrotransposable element 412 in Drosophila species.

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.

Accumulation of transposable elements in the heterochromatin and on the Y chromosome of Drosophila simulans and Drosophila melanogaster.

The elements of the transposon families G, copia, mdg 1, 412, and gypsy that are located in the heterochromatin and on the Y chromosome have been identified by the Southern blotting technique in Drosophila simulans and D. melanogaster populations. Within species, the abundance of such elements differs between transposon families. Between species, the abundance in the heterochromatin and on the Y chromosome of the elements of the same family can differ greatly suggesting that differences within a species are unrelated to structural features of elements. By shedding some new light on the mechanism of accumulation of transposable elements in the heterochromatin, these data appear relevant to the understanding of the long-term interaction between transposable elements and the host genome.

Maintenance of transposable element copy number in natural populations of Drosophila melanogaster and D. simulans.

To investigate the main forces controlling the containment of transposable elements (TE) in natural populations, we analyzed the copia, mdg1, and 412 elements in various populations of Drosophila melanogaster and D. simulans. A lower proportion of insertion sites on the X chromosome in comparison with the autosomes suggests that selection against the detrimental effects of TE insertions is the major force containing TE copies in populations of Drosophila. This selection effect hypothesis is strengthened by the absence of the negative correlation between recombinaiton rate and TE copy number along the chromosomes, which was expected under the alternative ectopic exchange model (selection against the deleterious rearrangements promoted by recombination between TE insertions). A cline in 412 copy number in relation to latitude was observed among the natural populations of D. simulans, with very high numbers existing in some local populations (around 60 copies in a sample from Canberra, Australia). An apparent absence of selection effects in this Canberra sample and a value of transposition rate equal to 1-2 x 10(-3) whatever the population and its copy number agree with the idea of recent but temporarily drastic TE movements in local populations. The high values of transposition rate in D. simulans clearly disfavor the hypothesis that the low amount of transposable elements in this species could result from a low transposition rate.

Environmental stresses and mobilization of transposable elements in inbred lines of Drosophila melanogaster.

Drastic changes in insertion patterns of the mobile elements copia and mdg-1 (a copia-like element) were checked in highly inbred lines of Drosophila melanogaster subjected to various stresses. Flies were treated by factors known for their ability to increase mutation rates of classical genes, and for their action on transcription or transposition of mobile elements: heat shocks at 37 degrees C, dichlorvos, hydrogen peroxide and ecdysterone. The insertion patterns were analyzed in progenies of the treated flies, either on larvae by in situ hybridization of giant salivary gland chromosomes, or on adult flies by Southern blotting interpreted by densitometric analysis. The techniques used made it possible to detect only changes with frequencies of more than 10% of the insertion sites. We show that the copia and mdg-1 elements cannot be radically mobilized in our inbred lines under the stressful conditions used. This absence of 'explosion' of mobilization of transposable elements after the action of external factors suggests that the genomes of our highly inbred lines are relatively protected against environmental stresses.