Pseudolycoriella hygida (Sauaia and Alves)-An Overview of a Model Organism in Genetics, with New Aspects in Morphology and Systematics

Pseudolycoriella hygida (Sauaia & Alves, 1968) is a sciarid that has been continuously cultured in the laboratory for nearly 60 years. Studies on this species have contributed to the understanding of DNA puffs, which are characteristic of Sciaridae, and to the knowledge of more general aspects of insect biology, including cell death, nucleolar organization, and the role of the hormone ecdysone during molting. The genome of Psl. hygida has now been sequenced, and it is the third publicly available sciarid genome. The aim of this work is to expand the current knowledge on Psl. hygida. The morphology of the adults is revisited. The morphology of larvae and pupae is described, together with the behavior of immature stages under laboratory conditions. Cytogenetic maps of the salivary gland polytene chromosomes are presented, together with a comparative analysis of the mitotic chromosomes of six different sciarid species. Pseudolycoriella hygida was originally described as a species of Bradysia and recently moved to Pseudolycoriella. We examine here the systematic position of Psl. hygida in the latter genus. Our results extend the characterization of an unconventional model organism and constitute an important resource for those working on the cytogenetics, ecology, taxonomy, and phylogenetic systematics of sciarids.

Characterization of a novel Drosophila melanogaster cis-regulatory module that drives gene expression to the larval tracheal system and adult thoracic musculature

In a previous bioinformatics analysis we identified 10 conserved Drosophila melanogaster sequences that reside upstream from protein coding genes (CGs). Here we characterize one of these genomic regions, which constitutes a Drosophila melanogaster cis-regulatory module (CRM) that we denominate TT-CRM. The TT-CRM is 646 bp long and is located in one of the introns of CG32239 and resides about 3,500 bp upstream of CG13711 and about 620 bp upstream of CG12493. Analysis of 646 bp-lacZ lines revealed that TT-CRM drives gene expression not only to the larval, prepupal, and pupal tracheal system but also to the adult dorsal longitudinal muscles. The patterns of mRNA expression of the transgene and of the CGs that lie in the vicinity of TT-CRM were investigated both in dissected trachea and in adult thoraces. Through RT-qPCR we observed that in the tracheal system the pattern of expression of 646 bp-lacZ is similar to the pattern of expression of CG32239 and CG13711, whereas in the thoracic muscles 646 bp-lacZ expression accompanies the expression of CG12493. Together, these results suggest new functions for two previously characterized D. melanogaster genes and also contribute to the initial characterization of a novel CRM that drives a dynamic pattern of expression throughout development.

The Drosophila melanogaster Eip74EF-PA transcription factor directly binds the sciarid BhC4-1 promoter

The DNA puff BhC4-1 gene of Bradysia hygida (Diptera, Sciaridae) is amplified and expressed in the salivary glands at the end of the last larval instar. Even though there are no BhC4-1 orthologs in Drosophila melanogaster, the mechanisms that regulate BhC4-1 gene expression in B. hygida are for the most part conserved in D. melanogaster. The BhC4-1 promoter contains a 129bp (-186/-58) cis-regulatory module (CRM) that drives developmentally regulated expression in transgenic salivary glands at the onset of metamorphosis. Both in the sciarid and in transgenic D. melanogaster, BhC4-1 gene expression is induced by the increase in ecdysone titers that triggers metamorphosis. Genetic interaction experiments revealed that in the absence of the Eip74EF-PA early gene isoform BhC4-1-lacZ levels of expression in the salivary gland are severely reduced. Here we show that the overexpression of the Eip74EF-PA transcription factor is sufficient to anticipate BhC4-1-lacZ expression in transgenic D. melanogaster. Through yeast one-hybrid assays we confirm that the Eip74EF-PA transcription factor directly binds to the 129 bp sciarid CRM. Together, these results contribute to the characterization of an insect CRM and indicate that the ecdysone gene regulatory network that promotes metamorphosis is conserved between D. melanogaster and the sciarid B. hygida.

Beyond DNA puffs: What can we learn from studying sciarids?

Members of the Sciaridae family attracted the interest of researchers because of the demonstration that the DNA puff regions, which are formed in the salivary gland polytene chromosomes at the end of the fourth larval instar, constitute sites of developmentally regulated gene amplification. Besides contributing to a deeper understanding of the process of gene amplification, the study of sciarids has also provided important insights on other biological processes such as sex determination, programmed cell death, insect immunity, telomere maintenance, and nucleolar organizing regions (NOR) formation. Open questions in sciarids include among others, early development, the role of noncoding RNAs in gene amplification and the relationship between gene amplification and transcription in DNA puff forming regions. These and other questions can now be pursued with next generation sequencing techniques and experiments using RNAi experiments, since this latter technique has been shown to be feasible in sciarids. These new perspectives in the field of sciarid biology open the opportunity to consolidate sciarid species as important emerging models. genesis 54:361-378, 2016. © 2016 Wiley Periodicals, Inc.

Amplification and expression of a salivary gland DNA puff gene in the prothoracic gland of Bradysia hygida (Diptera: Sciaridae)

The DNA puff BhC4-1 gene, located in DNA puff C4 of Bradysiahygida, is amplified and expressed in the salivary gland at the end of the fourth larval instar as a late response to the increase in 20-hydroxyecdysone titer that triggers metamorphosis. Functional studies revealed that the mechanisms that regulate BhC4-1 expression in the salivary gland are conserved in transgenic Drosophila. These studies also led to the identification of a cis-regulatory module that drives developmentally regulated expression of BhC4-1-lacZ in the prothoracic gland cells of the ring gland, a compound organ which in Drosophila results from the fusion of the prothoracic glands, the corpus allatum and the corpus cardiacum. Here we have investigated the occurrence of BhC4-1 expression in B. hygida prothoracic glands. We report the identification of the B. hygida prothoracic gland and demonstrate that it releases ecdysone. Using RT-qPCR, western blots and immunolocalization experiments, we demonstrate that the BhC4-1 mRNA and the BhC4-1 protein are both expressed in the B. hygida prothoracic glands at the same time that DNA puff C4 is formed in the salivary gland. We also show that BhC4-1 is concomitantly amplified 4.8-fold in the prothoracic gland and 23-fold in the salivary gland. Our results reveal the occurrence of stage specific expression of a DNA puff gene in the prothoracic glands of B. hygida, and extend previous studies that have shown that DNA puff genes expression is not restricted to the salivary gland. In addition, the description of stage specific gene amplification in the prothoracic glands of B. hygida constitutes the first demonstration that gene amplification in Diptera might occur concomitantly in two different tissues in the same developmental stage.

Functional characterization of the sciarid BhC4-1 core promoter in transgenic Drosophila

Background

Core promoters are cis-regulatory modules to which bind the basal transcriptional machinery and which participate in the regulation of transcription initiation. Although core promoters have not been extensively investigated through functional assays in a chromosomal context, the available data suggested that the response of a given core promoter might vary depending on the promoter context. Previous studies suggest that a (-57/+40) fragment constitutes the core promoter of the BhC4-1 gene which is located in DNA puff C4 of the sciarid fly Bradysia hygida. Here we tested this (-57/+40) fragment in distinct regulatory contexts in order to verify if promoter context affects its core promoter activity.

Results

Consistent with the activity of a core promoter, we showed that in the absence of upstream regulatory sequences the (-57/+40) fragment drives low levels of reporter gene mRNA expression throughout development in transgenic Drosophila. By assaying the (-57/+40) fragment in two distinct regulatory contexts, either downstream of the previously characterized Fbp1 enhancer or downstream of the UAS element, we showed that the BhC4-1 core promoter drives regulated transcription in both the germline and in various tissues throughout development. Furthermore, the use of the BhC4-1 core promoter in a UAS construct significantly reduced salivary gland ectopic expression in third instar larvae, which was previously described to occur in the context of the GAL4/UAS system.

Conclusions

Our results from functional analysis in transgenic Drosophila show that the BhC4-1 core promoter drives gene expression regardless of the promoter context that was assayed. New insights into the functioning of the GAL4/UAS system in Drosophila were obtained, indicating that the presence of the SV40 sequence in the 3' UTR of a UAS construct does not preclude expression in the germline. Furthermore, our analysis indicated that ectopic salivary gland expression in the GAL4/UAS system does not depend only on sequences present in the GAL4 construct, but can also be affected by the core promoter sequences in the UAS construct. In this context, we propose that the sciarid BhC4-1 core promoter constitutes a valuable core promoter which can be employed in functional assays in insects.