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

Abstract

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.

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

“…In D. simulans, a very high level of 412 expression was observed in the populations from Canberra (40-fold higher than that of the competitor), Madeira (21-fold) and Eden (18-fold), and the lowest level of expression was detected in the African population from Zimbabwe (0.2 times the competitor amount). This very low expression level for 412 element in Zimbabwe is consistent with the absence of the 412 transcript in previous Northern blot experiments (Borie et al, 2000). In D. melanogaster, the somatic expression of 412 was higher in the population from Valence than in Canton or St Cyprien.…”

“…Since RT-PCR followed by radioactive detection of bands is a very sensitive method, these data suggest that 412 expression was strongly repressed in the ovaries. Even the Canberra population, which is known to have a very high expression of the 412 retrotransposon in whole females (Borie et al, 2000), did not exhibit any detectable 412 transcript in the ovaries.…”

“…Probe 1 (Figure 1) corresponded to an internal 850 bp Mlu-NI fragment of the 412 element located in the middle of the amplified region, and was used to detect both 412 and the competitor. Quantification was performed on the two expected fragments as described previously (Borie et al, 2000). Probe 2 (Figure 1) was 249 bp long and was generated by PCR between positions 71-319 of the 412 amplified fragment.…”

“…Because the 412 element is expressed at the same level in whole males and females (Borie et al, 2000), we analysed 412 expression in pooled male and female carcasses (Figure 3b). Table 1 shows the estimated 412 RNA amount in soma and testes for all the populations tested.…”

“…However, it has been shown that the copia transcript level differs in natural populations of D. melanogaster (Csink and McDonald, 1990), and that the amount of 412 transcripts during Drosophila development and in adults differs in different D. simulans populations according to the 412 copy number, whereas little or no expression is detected in D. melanogaster adults (Borie et al, 2000). No male/female differences in the amount of 412 transcripts have been detected for the whole-body in D. simulans, although sex-related differences may exist for 412 expression in germlines, as previously reported for the copia retrotransposon (Pasyukova et al, 1997).…”