Control of gene activities in the polytene chromosomes of Chironomus tentans by ecdysone and juvenile hormone

The mode of action of juvenile hormone and ecdysone: towards an epi-endocrinological paradigm?

In some insect species, two sites of juvenile hormone (JH) synthesis have been reported: the very well documented corpora allata that secrete JH for "general use", and the reproductive system, in particular the male accessory glands, in which the function of the sometimes huge amounts of JH (e.g. in Hyalophora cecropia) remains to be clarified. A recent finding in Schistocerca gregaria, namely that suppression of the ecdysteroid peak preceding a molt by RNAi of the Halloween genes spook, phantom and shade does not impede normal molting, challenges the (never experimentally proven) classical concept that such a peak is causally linked to a molt. Recent developments in epigenetic control of gene expression in both the honey bee and in locusts suggest that, in addition to the classical scheme of hormone-receptor (membrane- and/or nuclear) mode of action, there may be a third way. Upon combining these and other orphan data that do not fit in the commonly accepted textbook schemes, we here advance the working hypothesis that both JH and ecdysone might be important but overlooked players in epigenetic control of gene expression, in particular at extreme concentrations (peak values or total absence). In this review, we put forward how epi-endocrinology can complement classical arthropod endocrinology.

Molecular effects of endocrine-disrupting chemicals on the Chironomus riparius estrogen-related receptor gene

Endocrine-disrupting chemicals (EDCs) mimic the action of endogenous estrogen hormones; consequently, they can interfere with the endocrine systems of a variety of organisms. There is global concern regarding the potential impacts of EDCs on the aquatic environment. To evaluate the effects of EDCs on the estrogen-related receptors (ERR) of Chironomus, we characterized full-length cDNA sequences of the ERR gene from Chironomus riparius. The complete cDNA sequence of the ERR gene was found to be 1332bp in length. The results of our phylogenetic analysis demonstrated that C. riparius ERR was most closely related to that of mosquitoes. The basal level of ERR mRNA was expressed abundantly during different life-history stages, with the exception of adult males. Additionally, ERR gene expression was upregulated significantly in C. riparius exposed to bisphenol A (BPA) and 4-nonylphenol (NP) at all concentrations assayed after 24h of exposure. The ERR gene was significantly upregulated following short periods of exposure to di(2-ethylhexyl)phthalate (DEHP) concentrations of only 50mgL(-1). However, under long-term exposure conditions, ERR expression was induced to a significant degree after BPA, NP, and DEHP exposure at all concentrations assayed. Furthermore, the luciferase reporter gene assay revealed increased ERR expression following exposure to these compounds. Collectively, these findings indicate that EDCs influence the expression of ERR in Chironomus species.

Farnesoid secretions of dipteran ring glands: what we do know and what we can know

Harnessing of the Drosophila genetic system toward ascertaining the molecular endocrinology of higher dipteran (cyclorrhaphan) larval development has been a goal for over 70 years, beginning with the data left to us by pioneer researchers from the classical endocrine era. The results of their experiments evidence numerous ring gland activities that are parsimoniously explained as arising from secretions of the larval corpora allatal cells. Utilization of those data toward an understanding of molecular endocrinology of cyclorrhaphan metamorphosis has not yet achieved its hoped for fruition, in part due to a perceived difficulty in identifying larval targets of the molecule "methyl epoxyfarnesoate" (=juvenile hormone III). However, as is reviewed here, it is important to maintain a conceptual distinction between "the target of JH III"Versus "the target(s) of products secreted by the larval corpora allatal cells of ring glands." Recent advances have been made on the identity, regulation and reception of ring gland farnesoid products. When these advances are evaluated together with the above data from the classical endocrine era, there is a new opportunity to frame experimental hypotheses so as to discern underlying mechanisms on cyclorrhaphan larval-pupal metamorphosis that have been heretofore intractable. This paper reconsiders a number of evidenced physiological targets of secretions of corpora allatal cells of the larval ring gland, and places them in the context of more recent biochemical and molecular advances in the field.

Programmed cell death during metamorphosis in the blow-fly Calliphora vomitoria

During metamorphosis, the salivary glands of the blow-fly undergo programmed cell death. Data is presented indicating that this programmed cell death does not in many respects emulate classical apoptosis. The cells are seen to vacuolate and swell rather than condense and shrink. There appears to be a transient enhancement in autophagy and an increase in acid phosphatase activity. This is followed by the characteristic appearance of ribosomal and extracisternal sources of the enzyme leading to autolysis. There appears to be no lysosomal leakage of acid phosphatase. As in apoptosis, the mitochondria persist until the cell fragments. The nucleus, however, does not show the distinct chromatin margination and blebbing that is typical of apoptosis. These changes are compared with necrotic changes induced by experimental anoxia. Overall the results show that a programmed cell death distinct from classical apoptosis is taking place.

Immunological studies on the developmental and chromosomal distribution of ecdysteroid receptor protein in Chironomus tentans

Antisera were raised against different domains of a putative ecdysteroid receptor (cEcRH) of Chironomus tentans. All the antisera reacted with a 68,000 dalton protein exhibiting DNA binding properties. Additionally, we were able to demonstrate that the antisera immunoprecipitate protein which binds a radioactively labeled ecdysteroid (Ec), i.e., [3H]ponasterone A, with high specificity. These properties indicate that the antisera recognize specifically an endogenous ecdysteroid receptor protein (cEcR) in C. tentans cells and thus are suitable for the following quantitative and qualitative immunological and immunohistochemical investigations. The cellular level of cEcR varies during development, and it is particularly low in oligopausing larvae. In polytene chromosomes of prepupal salivary glands, cEcR is located at approximately 50 transcriptionally active loci. These loci include both early ecdysteroid (Ec)-inducible puff sites, such as the locus containing the gene coding for the homolog of the E75 protein in Drosophila melanogaster, as well as late Ec-inducible puff-sites. The latter group comprises a locus of a gene specifying the homolog of the D. melanogaster ultraspiracle protein. However, loci of genes coding for salivary gland secretory proteins (e.g., Balbiani ring forming chromosome regions) do not specifically react with the antisera. Thus, the developmental regulation of these genes is not directly controlled by Ec. Polytene chromosomes of oligopausing larvae show hardly any loci that contain cEcR. The few detected correspond, with few exceptions, to the most potent cEcR binding sites found in prepupae.

Effects of DMSO on the structure and function of polytene chromosomes of Chironomus

Dimethylsulfoxide (DMSO) controlled puff induction and repression (or non-induction) in larval polytene chromosomes of Chironomus tentans were studied for the case of the Balbiani rings (BR). A characteristic reaction pattern, involving BR 1, BR 2, and BR 3, all in salivary gland chromosome IV was found. In vivo exposure of 4th instar larvae (not prepupae) to 10%n DMSO at 18 degrees C first evokes an over-stimulation of BR 3 while DMSO-stimulation of puffing at BR 1 and BR 2 always follows that of BR 3. After removal of the drug, a rapid uniform collapse of all puffs occurs, thus more or less restoring the banding pattern of all previously decondensed chromosome segments. Recovery proceeds as BR's and other puffs reappear. By observing the restoration, one can locate the site from which a BR (puff) originates. BR 2, which is normally the most active non-ribosomal gene locus in untreated larvae, here serves as an example. As the sizes of BR 3, BR 1 and BR 2 change, so do the quantities of the transcriptional products in these gene loci (and vice versa), as estimated electron-microscopically in ultrathin sections and autoradiographically in squash preparations. In autoradiograms, the DMSO-stimulated BRs exhibit the most dense concentration of silver grains and therefore the highest rate of transcriptional activity. In DMSO-repressed BRs (and other puffs) the transcription of the locus specific genes is not completely shut off. In chromosomes from nuclei with high labelling intensities the repressed BRs (and other puffs) always exhibit a low level of 3H-uridine incorporation in vivo. The absence of cytologically visible BR (puff) formation therefore does not necessarily indicate complete transcriptional inactivity. Typically, before the stage of puff formation the 3H-uridine labelling first appears in the interband-like regions.

Heat shock phenomena in Chironomus tentans I. In vivo effects of heat, overheat, and quenching on salivary chromosome puffing

Incubation of 4th instar larvae of Chironomus tentans at elevated temperatures leads in salivary and Malpighian chromosomes to the appearance of 4-5 new puffs. Previously present puffs, particularly Balbiani rings in salivary chromosomes, become drastically reduced. The reactions of region IV-5C and Balbiani ring 1 and 2 in salivary glands are quantitatively analyzed. Statistically significant heat shock effects are observed already after 5 min and reach a maximum between 30 and 60 min. The effective temperature range is small (between 33 to 40 degrees C) with an optimum at 37 degrees C. Above 40 degrees C, i.e., at overheat shock temperatures, heat shock reactions are suppressed. Larvae heat or overheat shocked for 1-7 h or 15-30 min, respectively, survive when returned to normal culturing temperatures. The recovery from heat shock of the puffing pattern occurs in two phases: a fast one (10-20 min) and a slow one (up to 5 h) sometimes separated by a period of backlash. Quenching of overheat shocked larvae does not result in a delayed heat shock reaction.

Change in nuclear potassium electrochemical activity and puffing of potassium-sensitive salivary chromosome regions during Chironomus development

Changes in nuclear K+ electrochemical activity and total nuclear K+ content in salivary glands of Chironomus tentans were measured with ion-selective microelectrodes based on valinomycin and with flameless atomic absorption spectrometry, respectively. The K+ activity increased by a factor of 2.6 and the total K+, by a factor of 1.5 as oligopausing larvae developed into prepupae. The extent of decondensation (puffing) of K+-sensitive regions in the polytene chromosomes underwent a parallel increase during this developmental event. In vitro culture of glands from oligopausing larvae resulted in similar changes with respect to nuclear K+ activity and puffing.

Effect of galactose treatment in the puffing pattern of Chironomus thummi Balbiani rings

Galactose feeding of Chironomus thummi larvae induces the regression of Balbiani ring c (BRc) and the full expansion of BRb, both localized in the IV salivary gland chromosome. This effect coincides with that described on BR2 and BR1 of Ch. pallidivittatus and Ch. tentans. The puffing changes of BRb and BRc throughout develoment have been studied and also show identical variations as in BR1 and BR2 of Ch. pallidivittatus and Ch. tentans. The similar behaviour of BRb and BR1, and of BRc and BR2 respectively after galactose treatment and throughout development strongly suggests that these BRs play the same physiological role in the three Chironomus species, with BRb = BR1 and BRc= BR2.

Correlated changes of Balbiani ring expansion and secretory protein synthesis in larval salivary glands of Chironomus tentans

Salivary glands of various stages of the last larval instar of Chironomus tentans were quantitatively analyzed with respect to the expansion of their Balbiani rings (B1, B2, B3) by a fast green staining procedure as well as to the rate of synthesis of their secretory proteins (S1, S2, S3) by a scintillation counting procedure of electrophoretic fractions. The extent of expansion of B1, B2 and B3 correlates positively with the rate of synthesis of S3, S2 and S1, respectively. With B1 and S3 these parameters undergo a parallel and developmentally specific change being rather depressed in intermolt, and particularly in diapausing animals.

The transcripts of Balbiani rings from Chironomus thummi. Giant RNA molecules with messenger characteristics

Experimental results on the transcription of Balbiani rings BR 1 and BR 2 of Chironomus thummi salivary gland chromosomes are presented. The DNA of Balbiani ring 2, which is the most active puff in larvae, is transcribed into large RNA molecules of about 22 X 10(6) D which resist degradation by heating, formamide or urea treatment. The asymmetrical distribution of electrophoretic profiles of BR 2 RNA and the appearence of a symmetrical BR RNA peak in the nucleoplasm suggest the synthesis of (mainly) one RNA fraction in BR 2. The gel electrophoretic patterns of BR 1 RNA are, on the other hand, characterized by the appearance of two main fractions of high molecular weight RNA, one of which corresponds in molecular weight (about 22 X 10(6) D) to BR 2 RNA. The second RNA fraction is significantly smaller in molecular size (molecular weight: about 10 X 10(6) D) and, like the 22 X 10(6) D RNA fractions of the two Balbiani rings, resistant against heating in 8 M urea. Binding to poly (U) sepharose of a significant part of Balbiani ring RNA suggests the existence of poly (A) and/or oligo (A) sequences in the Balbiani ring RNA. -- In situ hybridization of BR RNA to the salivary gland chromosomes reveals accumulation of silver grains over the Balbiani ring regions only and demonstrates the restriction of BR DNA sequences to the corresponding Balbiani ring.

Puffing patterns during the fourth larval instar in Chironomus pallidivittatus salivary glands

The puffing pattern of polytene chromosomes in salivary glands from Chironomus pallidivittatus larvae and prepupae has been studied in glutaraldehyde-acetic acid fixed, lactic acid flattened preparations. Some observations were also made on F1 hybrid species C. pallidivittatus X C. tentans. Concerning the situation of puffing in Balbiani rings (BR), 2.783 chromosomes IV from 188 animals were scored. In standard 4th instar larvae, BR2 appears expanded, BR3 smaller but not collapsed and BRI either reduced of collapsed. During the first days following the red-head stage, which signals the beginning of the 4th instar, larvae show a large BR1; later it reduces and tends to collapse. At the end of the 4th instar, prepupae again may present an expanded BR1. On the contrary, the size of BR2 and BR3 remains unchanged from the red-head stage to the prepupa. A variable accumulation of droplets has been observed to occur in BR2 and BR1 from dated larvae and prepupae.--A characteristic pattern of puffing was found in prepupae, which consisted in the appearance of conspicuous puffs at regions I-6B, I-7B, I-7B, I-18C, III-9B and IV-4B. Puffs at I-2B, I-3B, I-9A,I-11C,II-4A, and IV-4B were observed during most of the 4th larval instar, as well as in late larvae and prepupae. Among all these puffs, those at I-7B, I-9A, I-17B, and IV-4B frequently showed variable amounts of droplets.