The copia retrotransposon and horizontal transfer in Drosophila willistoni

SummaryThe copia element is a retrotransposon that is hypothesized to have been horizontally transferred from Drosophila melanogaster to some populations of Drosophila willistoni in Florida. Here we have used PCR and Southern blots to screen for sequences similar to copia element in South American populations of D. willistoni, as well as in strains previously shown to be carriers of the element. We have not found the canonical copia element in any of these populations. Unlike the P element, which invaded the D. melanogaster genome from D. willistoni and quickly spread worldwide, the canonical copia element appears to have transferred in the opposite direction and has not spread. This may be explained by differences in the requirements for transposition and in the host control of transposition.

Mutations at the Darkener of apricot locus modulate transcript levels of copia and copia-induced mutations in Drosophila melanogaster.

Mutations of the Doa locus of Drosophila melanogaster darken the eye color of the copia-induced white(apricot) (wa) allele and increase the accumulation of white promoter-initiated transcripts encoding functional mRNA. We show here that quantities of transcripts initiated in both long terminal repeats (LTRs) of the specific wa-copia element are increased, and those initiating in the 5' LTR of the element are structurally altered, yielding a slightly shortened transcript. Accumulation of host-initiated transcripts of a copia-induced mutation within the achaete-scute complex, Hairy-wing Ua (HwUa), are reduced by Doa mutations. Finally, we show that homozygosity for Doa mutations increases the accumulation of copia transcripts from the population of elements in the genome. These results suggest that Doa modulates the severity of copia-induced mutations while functioning as a dosage-sensitive modulator of copia transcription.

Dosage compensation of the copia retrotransposon in Drosophila melanogaster.

Dosage compensation in Drosophila has been studied at the steady state RNA level for several single-copy genes; however, an important point is addressed by analyzing a repetitive, transposable element for dosage compensation. The two issues of gene-specific cis control and genomic position can be studied by determining the extent of dosage compensation of a transposable element at different chromosomal locations. To determine whether the multicopy copia transposable element can dosage compensate, we used the X-linked white-apricot (wa) mutation in which a copia element is present. The extent of dosage compensation was determined for the white and copia promoters in larvae and adults in two different genomic locations of the wa allele. We conclude that copia is able to dosage compensate, and that the white promoter and the copia promoter are not coordinate in their dosage compensation abilities when assayed under these various conditions. Thus, two transcriptional units, one within the other, both of which are able to dosage compensate, do so differently in response to developmental stage and genomic position.

Insertion of a copia element 5' to the Drosophila melanogaster alcohol dehydrogenase gene (adh) is associated with altered developmental and tissue-specific patterns of expression.

The Drosophila melanogaster alcohol dehydrogenase gene (adh) is under the control of two separate promoters (proximal and distal) which are preferentially utilized at the larval and adult life stages, respectively. A variant alcohol dehydrogenase allele (RI-42) isolated from a natural population contains a copia retroviral-like transposable element inserted 240 bp upstream from the distal (adult) adh transcriptional start site. Levels of adh transcripts in the RI-42 variant are reduced in tissues and at life stages where copia is actively expressed and are affected in trans- by mutant alleles at the suppressor-of-white-apricot (su(wa] and suppressor-of-forked (su(f] loci. These suppressor genes have no effect on adh expression in wild-type Drosophila.

Partial revertants of the transposable element-associated suppressible allele white-apricot in Drosophila melanogaster: structures and responsiveness to genetic modifiers.

The eye color phenotype of white-apricot (wa), a mutant allele of the white locus caused by the insertion of the transposable element copia into a small intron, is suppressed by the extragenic suppressor suppressor-of-white-apricot (su(wa] and enhanced by the extragenic enhancers suppressor-of-forked su(f] and Enhancer-of-white-apricot (E(wa]. Derivatives of wa have been analyzed molecularly and genetically in order to correlate the structure of these derivatives with their response to modifiers. Derivatives in which the copia element is replaced precisely by a solo long terminal repeat (sLTR) were generated in vitro and returned to the germline by P-element mediated transformation; flies carrying this allele within a P transposon show a nearly wild-type phenotype and no response to either su(f) or su(wa). In addition, eleven partial phenotypic revertants of wa were analyzed. Of these, one appears to be a duplication of a large region which includes wa, three are new alleles of su(wa), two are sLTR derivatives whose properties confirm results obtained using transformation, and five are secondary insertions into the copia element within wa. One of these, waR84h, differs from wa by the insertion of the most 3' 83 nucleotides of the I factor. The five insertion derivatives show a variety of phenotypes and modes of interaction with su[f) and su(wa). The eye pigmentation of waR84h is affected by su(f) and E(wa), but not su(wa). These results demonstrate that copia (as opposed to the interruption of white sequences) is essential for the wa phenotype and its response to genetic modifiers, and that there are multiple mechanisms for the alteration of the wa phenotype by modifiers.

A DETAILED DEVELOPMENTAL AND STRUCTURAL STUDY OF THE TRANSCRIPTIONAL EFFECTS OF INSERTION OF THE COPIA TRANSPOSON INTO THE WHITE LOCUS OF DROSOPHILA MELANOGASTER

ABSTRACT The copia insertion responsible for the wamutation is 3' to the white promotor and in the same transcriptional orientation as white. First, we have analyzed the effects of the wa copia insertion on levels of polyadenylated white transcripts and find large, developmentally programmed effects. Second, we have isolated and sequenced an LTR-excision event involving the copia insertion at wa. This represents the first documented case of an LTR-excision event in Drosophila. This single copia LTR has developmentally programmed effects on white transcript levels qualitatively similar to the intact copia element. Third, we have characterized the structures of white transcripts from wa. We find polyadenylated white transcripts apparently having 3' termini in or near the 3' LTR of the wa copia insertion, as has been reported in limited studies of wa transcription in adults by others. These earlier studies also revealed wa transcripts apparently corresponding to polyadenylated terminus formation in the 5' LTR of the copia transposon; however, our more detailed studies reveal that these transcripts probably have other origins and that little, if any, polyadenylated terminus formation for white transcripts occurs in the 5' LTR of the wa copia insertion. Moreover, we find no polyadenylated terminus formation for white transcripts occurring in the single LTR of the wa LTR-excision product. Fourth, we find that each of three mutant alleles at su(wa) produces elevated levels of several classes of RNAs apparently corresponding to transcriptional readthrough of the wa copia transposon. Elevated levels of one presumptive readthrough transcript were observed previously in one su(wa)1 mutant strain. Fifth, we have confirmed the existence of a transcript initiated in the 3' LTR of the wa copia insertion and find the levels of this transcript to be strongly influenced by developmental stage and genetic background. Lastly, we have analyzed white transcripts produced by the whd 81b11 allele, which carries an insertion of copia in the opposite transcriptional orientation and in a different position than the wa copia insertion. In contrast to the wa copia insertion allele, the whd 81b11 allele produces polyadenylated white transcript levels very similar to the w + case at the stages examined. Moreover, the whd 81b11 copia element apparently produced polyadenylated terminus formation in white transcripts and we observe no effect of the allelic state of su(wa) on apparent readthrough of this stop site. We discuss some possible implications of our results for the properties of retrotransposons as developmentally complex insertional mutagens and for the functional organization of the copia transcription unit.