Ecdysone-dependent feedback regulation of prothoracicotropic hormone controls the timing of developmental maturation

A dynamical model of growth and maturation in Drosophila

The decision to stop growing and mature into an adult is a critical point in development that determines adult body size, impacting multiple aspects of an adult's biology. In many animals, growth cessation is a consequence of hormone release that appears to be tied to the attainment of a particular body size or condition. Nevertheless, the size-sensing mechanism animals use to initiate hormone synthesis is poorly understood. Here, we develop a simple mathematical model of growth cessation in Drosophila melanogaster, which is ostensibly triggered by the attainment of a critical weight (CW) early in the last instar. Attainment of CW is correlated with the synthesis of the steroid hormone ecdysone, which causes a larva to stop growing, pupate, and metamorphose into the adult form. Our model suggests that, contrary to expectation, the size-sensing mechanism that initiates metamorphosis occurs before the larva reaches CW; that is, the critical-weight phenomenon is a downstream consequence of an earlier size-dependent developmental decision, not a decision point itself. Further, this size-sensing mechanism does not require a direct assessment of body size but emerges from the interactions between body size, ecdysone, and nutritional signaling. Because many aspects of our model are evolutionarily conserved among all animals, the model may provide a general framework for understanding how animals commit to maturing from their juvenile to adult form.

One genome, multiple phenotypes: decoding the evolution and mechanisms of environmentally induced developmental plasticity in insects

Plasticity in developmental processes gives rise to remarkable environmentally induced phenotypes. Some of the most striking and well-studied examples of developmental plasticity are seen in insects. For example, beetle horn size responds to nutritional state, butterfly eyespots are enlarged in response to temperature and humidity, and environmental cues also give rise to the queen and worker castes of eusocial insects. These phenotypes arise from essentially identical genomes in response to an environmental cue during development. Developmental plasticity is taxonomically widespread, affects individual fitness, and may act as a rapid-response mechanism allowing individuals to adapt to changing environments. Despite the importance and prevalence of developmental plasticity, there remains scant mechanistic understanding of how it works or evolves. In this review, we use key examples to discuss what is known about developmental plasticity in insects and identify fundamental gaps in the current knowledge. We highlight the importance of working towards a fully integrated understanding of developmental plasticity in a diverse range of species. Furthermore, we advocate for the use of comparative studies in an evo-devo framework to address how developmental plasticity works and how it evolves.

Circadian clock regulates developmental time through ecdysone and juvenile hormones in Bombyx mori

The circadian clock plays an integral role in hormone biosynthesis and secretion. However, how the circadian clock precisely coordinates hormonal homeostasis to maintain normal animal development remains unclear. Here, we show that knocking out the core clock gene Cryptochrome 1 (Cry1) significantly delays the developmental time in Bombyx mori. This study focuses on the ecdysone and juvenile hormone signalling pathways of fifth instar larvae with the longest developmental time delay. We found that the mutant reduced prothoracicotropic hormone synthesis in the brain, and could not produce sufficient ecdysone in the prothoracic gland, resulting in a delayed peak of 20-hydroxyecdysone titre in the hemolymph of fifth instar larvae, prolonging developmental time. Moreover, further investigation revealed that the mutant enhanced juvenile hormone biosynthesis and signalling pathway and that this higher juvenile hormone titre also resulted in prolonged developmental time in fifth instar larvae. Our results provide insights into the molecular mechanisms by which the circadian clock regulates animal development by maintaining hormonal homeostasis.

Ecdysone acts through cortex glia to regulate sleep in Drosophila

Steroid hormones are attractive candidates for transmitting long-range signals to affect behavior. These lipid-soluble molecules derived from dietary cholesterol easily penetrate the brain and act through nuclear hormone receptors (NHRs) that function as transcription factors. To determine the extent to which NHRs affect sleep:wake cycles, we knocked down each of the 18 highly conserved NHRs found in Drosophila adults and report that the ecdysone receptor (EcR) and its direct downstream NHR Eip75B (E75) act in glia to regulate the rhythm and amount of sleep. Given that ecdysone synthesis genes have little to no expression in the fly brain, ecdysone appears to act as a long-distance signal and our data suggest that it enters the brain more at night. Anti-EcR staining localizes to the cortex glia in the brain and functional screening of glial subtypes revealed that EcR functions in adult cortex glia to affect sleep. Cortex glia are implicated in lipid metabolism, which appears to be relevant for actions of ecdysone as ecdysone treatment mobilizes lipid droplets (LDs), and knockdown of glial EcR results in more LDs. In addition, sleep-promoting effects of exogenous ecdysone are diminished in lsd-2 mutant flies, which are lean and deficient in lipid accumulation. We propose that ecdysone is a systemic secreted factor that modulates sleep by stimulating lipid metabolism in cortex glia.

Mammalian puberty: a fly perspective

Mammalian puberty and Drosophila metamorphosis, despite their evolutionary distance, exhibit similar design principles and conservation of molecular components. In this Viewpoint, we review recent advances in this area and the similarities between both processes in terms of the signaling pathways and neuroendocrine circuits involved. We argue that the detection and uptake of peripheral fat by Drosophila prothoracic endocrine cells induces endomembrane remodeling and ribosomal maturation, leading to the acquisition of high biosynthetic and secretory capacity. The absence of this fat-neuroendocrine interorgan communication leads to giant, obese, non-pupating larvae. Importantly, human leptin is capable of signaling the pupariation process in Drosophila, and its expression prevents obesity and triggers maturation in mutants that do not pupate. This implies that insect metamorphosis can be used to address issues related to the biology of leptin and puberty.

Role of bioactive xenobiotics towards reproductive potential of Odontotermes longignathus through in silico study: An amalgamation of ecoinformatics and ecotechnological insights of termite mounds from a tropical forest, India

The present research study has evaluated the roles of different naturally occurring compounds in termite mounds of Odontotermes longignathus (GenBank Id: MZ542727.1) which facilitate to promote higher population growth of termites and subsequent biodegradation. The study has also monitored the change in physicochemical parameters along with the trend of biodegradation of complex organic carbon-based compounds like lignin, polysaccharides etc. and nitrogenous compounds from two different types of termite mounds such as developing (T1) and developed (T2) mounds. The GC MS profiling of mound samples have revealed the occurrence of different humic acids like organic materials in both T1 and T2 mound samples. Both the termite mounds have demonstrated a high population density as T1 (23.67 ± 1.56) individuals and T2 (43.51 ± 2.36) individuals per 0.1 kg of mound materials. Such observations have prompted to undertake molecular docking experiments which revealed that different molecules interact at low binding affinity with hormone receptors involved in moulting, spermatogenesis and oogenesis of termite like Adamantane carboxylate (EcR: -7.6 Kcal/mol; BTB-KLHL10: -6.2 Kcal/mol; USP-LBD: -7.3 Kcal/mol; VgR: -6.8 Kcal/mol), Benzene dicarboxylic acid (EcR: -5.5 Kcal/mol; BTB-KLHL10: -5.1 Kcal/mol; USP-LBD: -5.4 Kcal/mol; VgR: -5.6 Kcal/mol), Hexadecanol (EcR: -6.0 Kcal/mol; BTB-KLHL10: -4.4 Kcal/mol; USP-LBD: -6.9 Kcal/mol; VgR: -6.0 Kcal/mol), oxirane (EcR: -5.3 Kcal/mol; BTB-KLHL10: -4.9 Kcal/mol; USP-LBD: -5.2 Kcal/mol; VgR: -5.3 Kcal/mol) and tocopherol (EcR: -8.0 Kcal/mol; BTB-KLHL10: -5.4 Kcal/mol; USP-LBD: -7.6 Kcal/mol; VgR: -7.0 Kcal/mol). Such spontaneous ligand binding phenomenon coupled with high population density of termites have established the significance of different bioactive xenobiotics in achieving high reproductive potential of termites which in turn facilitate the process of biodegradation and enhance the nutrient enrichment in the soils of tropical deciduous forest.

The receptor tyrosine kinase torso regulates ecdysone homeostasis to control developmental timing in Bombyx mori

Insect growth and development are precisely controlled by hormone homeostasis. The prothoracicotropic hormone (PTTH) receptor, Torso, is a member of the receptor tyrosine kinase family in insects. Activation of Torso by PTTH triggers biosynthesis and release of the steroid hormone in the prothoracic gland (PG). Although numbers of genes functioning in steroid hormone synthesis and metabolism have been identified in insects, the PTTH transduction pathway via its receptor Torso is poorly understood. In the current study, we describe a loss-of-function analysis of Torso in the silkworm, Bombyx mori, by targeted gene disruption using the transgenic CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats/RNA-guided Cas9 nucleases) system. Depletion of B. mori Torso (BmTorso) did not eventually affect larval ecdysis and metamorphosis processes. Instead, BmTorso deficiency resulted in significant extension of developing time during larval and pupal stages with increased pupa and cocoon sizes. The ecdysteriod titers in the hemolymph of BmTorso mutants sharpy declined. Transcriptional levels of genes involved in ecdysone biosynthesis and ecdysteroid signaling pathways were significantly reduced in BmTorso-deficient animals. Additionally, RNA-Seq analysis revealed that genes involved in the longevity pathway and protein processing in the endoplasmic reticulum pathway were affected after BmTorso deletion. These results indicate that Torso is critical for maintaining steroid hormone homeostasis in insects.

Function and Evolution of Nuclear Receptors in Environmental-Dependent Postembryonic Development

Nuclear receptors (NRs) fulfill key roles in the coordination of postembryonal developmental transitions in animal species. They control the metamorphosis and sexual maturation in virtually all animals and by that the two main environmental-dependent developmental decision points. Sexual maturation and metamorphosis are controlled by steroid receptors and thyroid receptors, respectively in vertebrates, while both processes are orchestrated by the ecdysone receptor (EcR) in insects. The regulation of these processes depends on environmental factors like nutrition, temperature, or photoperiods and by that NRs form evolutionary conserved mediators of phenotypic plasticity. While the mechanism of action for metamorphosis and sexual maturation are well studied in model organisms, the evolution of these systems is not entirely understood and requires further investigation. We here review the current knowledge of NR involvement in metamorphosis and sexual maturation across the animal tree of life with special attention to environmental integration and evolution of the signaling mechanism. Furthermore, we compare commonalities and differences of the different signaling systems. Finally, we identify key gaps in our knowledge of NR evolution, which, if sufficiently investigated, would lead to an importantly improved understanding of the evolution of complex signaling systems, the evolution of life history decision points, and, ultimately, speciation events in the metazoan kingdom.

Integrating animal development: How hormones and metabolism regulate developmental transitions and brain formation

Our current knowledge on how individual tissues or organs are formed during animal development is considerable. However, the development of each organ does not occur in isolation and thus their formation needs to be done in a coordinated manner. This coordination is regulated by hormones, systemic signals that instruct the simultaneous development of all organs and direct tissue specific developmental programs. In addition, multi- and individual-organ development requires the integration of the nutritional state of the animal, since this affects nutrient availability necessary for the progression of development and growth. Variations in the nutritional state of the animal are normal during development, as the sources and access to nutrients greatly differ depending on the animal stage. Furthermore, adversities of the external environment also exert major alterations in extrinsic nutritional conditions. Thus, both in normal and malnutrition circumstances, the animal needs to trigger metabolic changes to maintain energy homeostasis and sustain growth and development. This metabolic flexibility is mediated by hormones, that drive both developmental encoded metabolic transitions throughout development and adaptation responses according to the nutritional state of the animal. This review aims to provide a comprehensive summary of the current knowledge of how endocrine regulation coordinates multi-organ development by orchestrating metabolic transitions and how it integrates metabolic adaptation responses to starvation. We also focus on the particular case of brain development, as it is extremely sensitive to hormonally induced metabolic changes. Finally, we discuss how brain development is prioritized over the development of other organs, as its growth can be spared from nutrient deprivation.

Timing the Juvenile-Adult Neurohormonal Transition: Functions and Evolution

Puberty and metamorphosis are two major developmental transitions linked to the reproductive maturation. In mammals and vertebrates, the central brain acts as a gatekeeper, timing the developmental transition through the activation of a neuroendocrine circuitry. In addition to reproduction, these neuroendocrine axes and the sustaining genetic network play additional roles in metabolism, sleep and behavior. Although neurohormonal axes regulating juvenile-adult transition have been classically considered the result of convergent evolution (i.e., analogous) between mammals and insects, recent findings challenge this idea, suggesting that at least some neuroendocrine circuits might be present in the common bilaterian ancestor Urbilateria. The initial signaling pathways that trigger the transition in different species appear to be of a single evolutionary origin and, consequently, many of the resulting functions are conserved with a few other molecular players being co-opted during evolution.