By examining expression of arc in different mutant embryos, it was determined that transcription factors known to be required for patterning and maintenance of various developing epithelia control arc expression in those domains. tll and hkb, which are required to pattern the posterior 15% of the embryo, control arc expression in the posterior midgut primordium. fkh, which appears to act as a maintenance, or permissive, transcription factor, is required for expression of arc throughout the gut. byn, which is required for hindgut development and specifies its central domain (the large intestine), controls expression of arc in the elongating hindgut. Kr and cut, required for evagination and extension of the Malpighian tubule buds control expression of arc in the tubule primordia (Liu, 2000).
Expression of arc during embryogenesis was characterized by in situ hybridization and antibody staining. ARC mRNA and Arc protein exhibit the same expression pattern, with earlier expression of the ARC mRNA. Immediately after egg laying, a considerable store of uniformly distributed maternal ARC mRNA is detected. Presumably as a result of differential degradation along the anterior/posterior axis, this maternal mRNA is rapidly converted to an anterior/posterior gradient. Zygotic arc is expressed dynamically in multiple domains in the embryo; regions expressing arc are either about to undergo morphogenesis or are in the process of carrying out morphogenetic movements such as invagination, elongation, or convergent extension. Specifically, zygotic arc is first detected at the posterior midgut primordium, prior to and during its invagination. arc is next expressed in the Malpighian tubule primordium, prior to the evagination of the tubule buds; this expression increases, becoming very strong during bud evagination and tubule elongation. While expression of arc in the posterior midgut primordium and Malpighian tubule primordia takes place prior to the invagination of these tissues, arc is expressed in other tubules (hindgut, foregut, salivary gland, tracheae) after they have formed and while they are elongating (Liu, 2000).
The arc mutation aP, a P element insert in line l(2)k11011, is a hypomorph, since its wing phenotype and eye phenotype are less severe than those of a null allelic combination. Molecularly, aP is an insertion of the P lacZ element at position 111 of the arc transcription unit. Using quantitative RT-PCR, it has been found that the total ARC mRNA level in aP homozygotes is only 15% that of the wild type. While the level of each of the three transcripts is significantly reduced, that of transcripts I and II is most severely reduced. These quantitative expression data are consistent with the genetic evidence that aP is a hypomorph (Liu, 2000).
The tightly regulated pattern of arc expression in morphogenetically active epithelia during embryogenesis suggests that arc is involved in the development of these tissues. However, complete removal of arc activity (both maternally and zygotically), has no detectable effect on epithelial development in the embryo. Therefore, double mutants between arc and a number of candidate genes were made, to ask if any novel phenotypes that would reveal overlapping or redundant function could be uncovered. Genes that might function redundantly with arc are canoe (cno), polychaetoid (pyd), and discs lost (dlt: now redefined as Drosophila Patj) since, like arc, they encode PDZ domain proteins localized to the adherens junction. However, no additional phenotypes were found in flies completely lacking arc function and homozygous for a visible allele of cno, heterozygous for both cno and pyd, or heterozygous for dlt. Genes that might interact with arc include those encoding proteins that have either an S/T-X-V carboxyl-terminal motif or PDZ domain(s) that can potentially interact with the PDZ domains in Arc and those encoding components of adherens junctions. The Toll receptor is expressed in a pattern similar to that of arc and has a carboxy-terminal SDV motif. Generation of transheterozygotes between arc and Tl, however, do not reveal an obvious phenotype, nor does lack of Tl activity appear to have an effect on arc expression in embryos. Similarly, generation of transheterozygotes of arc and baz, which encodes an apically localized PDZ domain protein, does not reveal any interaction. Finally, decreased dosage of arm (which encodes the Drosophila adherens junction component beta-catenin) in an arc null background does not enhance the visible phenotypes associated with loss of function of arc (Liu, 2000).
The effect of ectopic expression of arc in various tissues at different developmental stages (from embryo to pupa), was examined by using the GAL4/UAS binary system. A GAL4 inducible promoter, in which five UAS enhancers are fused upstream to the hsp70 basal promoter, was inserted upstream of the endogenous arc gene by P element replacement. This allele with UAS enhancers upstream of the arc gene was designated aUAS. aUAS homozygous flies develop normal wings and eyes, indicating that the hsp70 basal promoter in the P element insert can substitute for the original arc promoter. The aUAS allele allows arc to be expressed in specific patterns as combined with different GAL4 lines. Surprisingly, strong ectopic expression of arc in the engrailed expression domains, driven by en-GAL4, does not result in any detectable phenotype. In contrast, ubiquitous expression of arc driven by Actin 5C-GAL4 results in pupal lethality; these lethal pupae exhibit rough eyes with fused ommatidia, but appear otherwise morphologically normal. Consistent with this result, ectopic expression of arc in all cells behind the morphogenetic furrow in the eye imaginal disc also results in a rough eye phenotype with fused ommatidia. In addition, arc ectopic expression results in eyes that are larger than wild-type (Liu, 2000).
Bridges, C. B., and Morgan, T. H. (1919). Contributions to the genetics of Drosophila melanogaster. Publ. Carnegie Instn. 278: 123-304
Liu, X. and Lengyel. J. A. (2000). Drosophila arc encodes a novel adherens junction-associated PDZ domain protein required for wing and eye development. Dev. Biol. 221: 419-434.
date revised: 10 June 2000
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