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.

Transposable element insertion patterns as test of contamination of a Drosophila melanogaster inbred line.

A highly inbred line of Drosophila melanogaster, stable for the insertion pattern of the transposable elements copia and mdg1, was experimentally contaminated by flies from another line. We show that the alien genome income is clearly detectable by the changes induced in the insertion profiles of transposable elements, even twenty generations later.

Genome and stresses: reactions against aggressions, behavior of transposable elements.

The action of stresses on the genome can be considered as responses of cells or organisms to external aggressions. Stress factors are of environmental origin (climatic or trophic) or of genomic nature (introduction of foreign genetic material, for example). In both cases, important perturbations can occur and modify hereditary potentialities, creating new combinations compatible with survival; such a situation may increase the variability of the genome, and allow evolutive processes to take place. The behavior of transposable elements under stress conditions is thus of particular interest, since these sequences are sources of mutations and therefore of genetic variability; they may play an important role in population adaptation. The survey of the available experimental results suggest that, although some examples of mutations and transposable elements movements induced by external factors are clearly described, environmental injuries or introduction of foreign material into a genome are not systematically followed by drastic genomic changes.

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.

Heat shocks do not mobilize mobile elements in genomes of Drosophila melanogaster inbred lines.

Males of three inbred lines of Drosophila melanogaster were heat-shocked 90 min at 37 degrees C. The progenies from treated and untreated males mated with untreated females of the same line were checked for their chromosomal insertion patterns of various mobile elements by either in situ hybridization or Southern blots. No modification in the pattern of insertion of the elements studied was observed after heat treatment. Hence, heating males of our inbred lines did not mobilize mobile elements, contrary to recent reports on other lines of Drosophila melanogaster.

Genome reshuffling of the copia element in an inbred line of Drosophila melanogaster.

Mobile genetic elements are found in the genomes of many organisms, and because of their effects on genes and their ability to induce chromosomal rearrangements they are an important source of genetic variability. Transposition rates are usually found to be low, estimated at around 10(-3) per generation. Higher rates of transposition are observed, however, in crosses between certain strains of Drosophila melanogaster ('hybrid dysgenesis'), which can lead to a dramatic rearrangement of many mobile elements ('transposition bursts'). We have studied the chromosomal distribution of mdg-1 and copia mobile elements in 17 highly inbred lines of D. melanogaster, after 69 generations of sib-mating. Most lines show no changes, but one showed a complete reshuffling of the copia element. We conclude that the transpositions of the copia element in this line occurred rapidly in a few generations. This phenomenon, distinct from 'transposition bursts' in that only copia elements are involved, may account for the instability sometimes observed in inbred lines and may be important in creating genetic variability in highly homozygous populations.

Dietary effects of phytoecdysones in the leek-moth,Acrolepiopsis assectella Zell. (Lepidoptera: Acrolepiidae).

Incorporation of certain phytoecdysones (ecdysterone, polypodine B, and ponasterone A) into a semisynthetic artificial diet induces pathophysiological effects in larvae of the leek-moth (Acrolepiopsis assectella Zell., Acrolepiidae). The effects include lethality of the newly hatched, first-instar larvae; special ecdysial failures associated with the appearance of larvae with two head capsules; and developmental anomalies during metamorphosis. The effective range of dietary ecdysteroid, as evaluated by larval mortality, varies from 25 to 250 ppm. The EC50 value is 100 ppm for polypodine B and 130 ppm for ecdysterone. The dietary effects of the phytoecdysones are similar to the previously observed effects caused by the dried flowers, but not leaves, of the leek plant. However, the active compound of the leek flowers is a saponin.