A unique Malpighian tubule architecture in Tribolium castaneum informs the evolutionary origins of systemic osmoregulation in beetles

The cryptonephridial/rectal complex: an evolutionary adaptation for water and ion conservation

Arthropods have integrated digestive and renal systems, which function to acquire and maintain homeostatically the substances they require for survival. The cryptonephridial complex (CNC) is an evolutionary novelty in which the renal organs and gut have been dramatically reorganised. Parts of the renal or Malpighian tubules (MpTs) form a close association with the surface of the rectum, and are surrounded by a novel tissue, the perinephric membrane, which acts to insulate the system from the haemolymph and thus allows tight regulation of ions and water into and out of the CNC. The CNC can reclaim water and solutes from the rectal contents and recycle these back into the haemolymph. Fluid flow in the MpTs runs counter to flow within the rectum. It is this countercurrent arrangement that underpins its powerful recycling capabilities, and represents one of the most efficient water conservation mechanisms in nature. CNCs appear to have evolved multiple times, and are present in some of the largest and most evolutionarily successful insect groups including the larvae of most Lepidoptera and in a major beetle lineage (Cucujiformia + Bostrichoidea), suggesting that the CNC is an important adaptation. Here we review the knowledge of this remarkable organ system gained over the past 200 years. We first focus on the CNCs of tenebrionid beetles, for which we have an in-depth understanding from physiological, structural and ultrastructural studies (primarily in Tenebrio molitor), which are now being extended by studies in Tribolium castaneum enabled by advances in molecular and microscopy approaches established for this species. These recent studies are beginning to illuminate CNC development, physiology and endocrine control. We then take a broader view of arthropod CNCs, phylogenetically mapping their reported occurrence to assess their distribution and likely evolutionary origins. We explore CNCs from an ecological viewpoint, put forward evidence that CNCs may primarily be adaptations for facing the challenges of larval life, and argue that their loss in many aquatic species could point to a primary function in conserving water in terrestrial species. Finally, by considering the functions of renal and digestive epithelia in insects lacking CNCs, as well as the typical architecture of these organs in relation to one another, we propose that ancestral features of these organs predispose them for the evolution of CNCs.

Something old, something new: the origins of an unusual renal cell underpinning a beetle water-conserving mechanism

Tenebrionid beetles have been highly successful in colonising environments where water is scarce, underpinned by their unique osmoregulatory adaptations. These include a cryptonephridial arrangement of their organs, in which part of their renal/Malpighian tubules are bound to the surface of the rectum. Within the cryptonephridial tubules, an unusual cell type, the leptophragmata, plays a key physiological role underpinning water conservation. Nothing was known about the developmental mechanisms or evolution of these unusual renal cells. Here, we investigate mechanisms underpinning leptophragmata development in Tribolium castaneum. We find that leptophragmata express and require the Tiptop transcription factor, similar to secondary renal cells in Drosophila melanogaster, which express Teashirt and Tiptop, despite Drosophila lacking a crypronephridial arrangement. An additional transcription factor, Dachshund, is required to establish leptophragmata identity and to distinguish them from the secondary cells in the non-cryptonephridial region of renal tubule of Tribolium. Dachshund is also expressed in a sub-population of secondary cells in Drosophila. Leptophragmata, which are unique to the beetle lineage, appear to have originated from a specific renal cell type present ancestrally and to be specified by a conserved repertoire of transcription factors.

BeetleAtlas: An Ontogenetic and Tissue-specific Transcriptomic Atlas of the Red Flour Beetle Tribolium castaneum

The red flour beetle Tribolium castaneum has emerged as a powerful model in insect functional genomics. However, a major limitation in the field is the lack of a detailed spatio-temporal view of the genetic signatures underpinning the function of distinct tissues and life stages. Here, we present an ontogenetic and tissue-specific web-based resource for Tribolium transcriptomics: BeetleAtlas (https://www.beetleatlas.org). This web application provides access to a database populated with quantitative expression data for nine adult and seven larval tissues, as well as for four embryonic stages of Tribolium. BeetleAtlas allows one to search for individual Tribolium genes to obtain values of both total gene expression and enrichment in different tissues, together with data for individual isoforms. To facilitate cross-species studies, one can also use Drosophila melanogaster gene identifiers to search for related Tribolium genes. For retrieved genes there are options to identify and display the tissue expression of related Tribolium genes or homologous Drosophila genes. Five additional search modes are available to find genes conforming to any of the following criteria: exhibiting high expression in a particular tissue; showing significant differences in expression between larva and adult; having a peak of expression at a specific stage of embryonic development; belonging to a particular functional category; and displaying a pattern of tissue expression similar to that of a query gene. We illustrate how the different feaures of BeetleAtlas can be used to illuminate our understanding of the genetic mechanisms underpinning the biology of what is the largest animal group on earth.

Voltage-gated ion channels as novel regulators of epithelial ion transport in the osmoregulatory organs of insects

Voltage-gated ion channels (VGICs) respond to changes in membrane potential (Vm) and typically exhibit fast kinetic properties. They play an important role in signal detection and propagation in excitable tissues. In contrast, the role of VGICs in non-excitable tissues like epithelia is less studied and less clear. Studies in epithelia of vertebrates and invertebrates demonstrate wide expression of VGICs in epithelia of animals. Recently, VGICs have emerged as regulators of ion transport in the Malpighian tubules (MTs) and other osmoregulatory organs of insects. This mini-review aims to concisely summarize which VGICs have been implicated in the regulation of ion transport in the osmoregulatory epithelia of insects to date, and highlight select groups for further study. We have also speculated on the roles VGICs may potentially play in regulating processes connected directly to ion transport in insects (e.g., acid-base balance, desiccation, thermal tolerance). This review is not meant to be exhaustive but should rather serve as a thought-provoking collection of select existing highlights on VGICs, and to emphasize how understudied this mechanism of ion transport regulation is in insect epithelia.

Genomic and Transcriptomic Insights into the Genetic Basis of Foam Secretion in Rice Spittlebug Callitettix versicolor

Many animal species produce protective foams, the majority of which exhibit evolutionary adaptability. Although the function and composition of foams have been widely studied, the genetic basis of foam secretion remains unknown. Unlike most species that produce foam under specific situations, spittlebugs continuously secrete foams throughout all nymphal stages. Here, we capitalize on the rice spittlebug (Callitettix versicolor) to explore the genetic basis of foam secretion through genomic and transcriptomic approaches. Our comparative genomic analysis for C. versicolor and eight other insect species reveals 606 species-specific gene families and 66 expanded gene families, associated with carbohydrate and lipid metabolism. These functions are in accordance with the composition of foams secreted by spittlebugs. Transcriptomic analyses of malpighian tubules across developmental stages detected 3192 differentially expressed genes. Enrichment analysis of these genes highlights functions also revealed by our comparative genomic analysis and aligns with previous histochemical and morphological observations of foam secretion. This consistency suggests the important roles of these candidate genes in foam production. Our study not only provides novel insights into the genetic basis of foam secretion in rice spittlebugs but also contributes valuable knowledge for future evolutionary studies of spittlebugs and the development of pest control strategies for C. versicolor.

Mechanisms of Systemic Osmoregulation in Insects

Water is essential to life. Terrestrial insects lose water by evaporation from the body surface and respiratory surfaces, as well as in the excretory products, posing a challenge made more acute by their high surface-to-volume ratio. These losses must be kept to a minimum and be offset by water gained from other sources. By contrast, insects such as the blood-sucking bug Rhodnius prolixus consume up to 10 times their body weight in a single blood meal, necessitating rapid expulsion of excess water and ions. How do insects manage their ion and water budgets? A century of study has revealed a great deal about the organ systems that insects use to maintain their ion and water balance and their regulation. Traditionally, a taxonomically wide range of species were studied, whereas more recent research has focused on model organisms to leverage the power of the molecular genetic approach. Key advances in new technologies have become available for a wider range of species in the past decade. We document how these approaches have already begun to inform our understanding of the diversity and conservation of insect systemic osmoregulation. We advocate that these technologies be combined with traditional approaches to study a broader range of nonmodel species to gain a comprehensive overview of the mechanism underpinning systemic osmoregulation in the most species-rich group of animals on earth, the insects.

Managing fuels and fluids: Network integration of osmoregulatory and metabolic hormonal circuits in the polymodal control of homeostasis in insects

Osmoregulation in insects is an essential process whereby changes in hemolymph osmotic pressure induce the release of diuretic or antidiuretic hormones to recruit individual osmoregulatory responses in a manner that optimizes overall homeostasis. However, the mechanisms by which different osmoregulatory circuits interact with other homeostatic networks to implement the correct homeostatic program remain largely unexplored. Surprisingly, recent advances in insect genetics have revealed several important metabolic functions are regulated by classic osmoregulatory pathways, suggesting that internal cues related to osmotic and metabolic perturbations are integrated by the same hormonal networks. Here, we review our current knowledge on the network mechanisms that underpin systemic osmoregulation and discuss the remarkable parallels between the hormonal networks that regulate body fluid balance and those involved in energy homeostasis to provide a framework for understanding the polymodal optimization of homeostasis in insects.

The red flour beetle T. castaneum: elaborate genetic toolkit and unbiased large scale RNAi screening to study insect biology and evolution

The red flour beetle Tribolium castaneum has emerged as an important insect model system for a variety of topics. With respect to studying gene function, it is second only to the vinegar fly D. melanogaster. The RNAi response in T. castaneum is exceptionally strong and systemic, and it appears to target all cell types and processes. Uniquely for emerging model organisms, T. castaneum offers the opportunity of performing time- and cost-efficient large-scale RNAi screening, based on commercially available dsRNAs targeting all genes, which are simply injected into the body cavity. Well established transgenic and genome editing approaches are met by ease of husbandry and a relatively short generation time. Consequently, a number of transgenic tools like UAS/Gal4, Cre/Lox, imaging lines and enhancer trap lines are already available. T. castaneum has been a genetic experimental system for decades and now has become a workhorse for molecular and reverse genetics as well as in vivo imaging. Many aspects of development and general biology are more insect-typical in this beetle compared to D. melanogaster. Thus, studying beetle orthologs of well-described fly genes has allowed macro-evolutionary comparisons in developmental processes such as axis formation, body segmentation, and appendage, head and brain development. Transgenic approaches have opened new ways for in vivo imaging. Moreover, this emerging model system is the first choice for research on processes that are not represented in the fly, or are difficult to study there, e.g. extraembryonic tissues, cryptonephridial organs, stink gland function, or dsRNA-based pesticides.

Blending physiology and RNAseq to provide new insights into regulation of epithelial transport: switching between ion secretion and reabsorption

This Review addresses the means by which epithelia change the direction of vectorial ion transport. Recent studies have revealed that insect Malpighian (renal) tubules can switch from secreting to reabsorbing K+. When the gut of larval lepidopterans is empty (during the moult cycle) or when the larvae are reared on K+-deficient diet, the distal ileac plexus segment of the tubule secretes K+ from the haemolymph into the tubule lumen. By contrast, in larvae reared on K+-rich diet, ions and fluid are reabsorbed from the rectal lumen into the perinephric space surrounding the cryptonephridial tubules of the rectal complex. Ions and fluid are then transported from the perinephric space into the lumen of the cryptonephridial tubules, thus supplying the free segments of the tubule downstream. Under these conditions, some of the K+ and water in the tubule lumen is reabsorbed across the cells of the distal ileac plexus, allowing for expansion of haemolymph volume in the rapidly growing larvae, as well as recycling of K+ and base equivalents. RNA sequencing data reveal large-scale changes in gene transcription that are associated with the switch between ion secretion and ion reabsorption by the distal ileac plexus. An unexpected finding is the presence of voltage-gated, ligand-gated and mechanosensitive ion channels, normally seen in excitable cells, in Malpighian tubules. Transcriptomic surveys indicate that these types of channels are also present in multiple other types of vertebrate and invertebrate epithelia, suggesting that they may play novel roles in epithelial cell signalling and regulation of epithelial ion transport.

A nutrient-responsive hormonal circuit mediates an inter-tissue program regulating metabolic homeostasis in adult Drosophila

Animals maintain metabolic homeostasis by modulating the activity of specialized organs that adjust internal metabolism to external conditions. However, the hormonal signals coordinating these functions are incompletely characterized. Here we show that six neurosecretory cells in the Drosophila central nervous system respond to circulating nutrient levels by releasing Capa hormones, homologs of mammalian neuromedin U, which activate the Capa receptor (CapaR) in peripheral tissues to control energy homeostasis. Loss of Capa/CapaR signaling causes intestinal hypomotility and impaired nutrient absorption, which gradually deplete internal nutrient stores and reduce organismal lifespan. Conversely, increased Capa/CapaR activity increases fluid and waste excretion. Furthermore, Capa/CapaR inhibits the release of glucagon-like adipokinetic hormone from the corpora cardiaca, which restricts energy mobilization from adipose tissue to avoid harmful hyperglycemia. Our results suggest that the Capa/CapaR circuit occupies a central node in a homeostatic program that facilitates the digestion and absorption of nutrients and regulates systemic energy balance.