Developmental expression of the 412 retrotransposon in natural populationsof D-melanogaster and D-simulans

We analysed the pattern of expression of retrotransposon 412 through develo pmental stages in various populations of Drosophila simulans and B. melanog aster differing in 412 copy number. We found that the 412 expression patter n varied greatly between populations of both species, indicating that such patterns were not entirely species-specific. In D. simulans, total transcri pts 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 4 12 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 on ly on copy number but also on host cellular regulatory sequences near which the elements were inserted. In D. Simulans populations with high copy numb er overall transcription was on the contrary globally repressed. as observe d in D. melanogaster. a population from Canberra (Australia) which had a ve ry high 412 copy number was found to be associated with very high expressio n of 412 over all developmental stages, suggesting that the above 412 expre ssion regulation processes are overcome in this population sample. The anal ysis of hybrids between geographically distinct populations of D. simulans showed that 412 expression was trans-regulated differently according to dev elopmental stages, implying complex interactions between the 412 element an d stage-specific host genes.