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.