A hemipteran insect reveals new genetic mechanisms and evolutionary insights into tracheal system development

Interlocking of co-opted developmental gene networks in Drosophila and the evolution of pre-adaptive novelty

The re-use of genes in new organs forms the base of many evolutionary novelties. A well-characterised case is the recruitment of the posterior spiracle gene network to the Drosophila male genitalia. Here we find that this network has also been co-opted to the testis mesoderm where is required for sperm liberation, providing an example of sequentially repeated developmental co-options. Associated to this co-option event, an evolutionary expression novelty appeared, the activation of the posterior segment determinant Engrailed to the anterior A8 segment controlled by common testis and spiracle regulatory elements. Enhancer deletion shows that A8 anterior Engrailed activation is not required for spiracle development but only necessary in the testis. Our study presents an example of pre-adaptive developmental novelty: the activation of the Engrailed transcription factor in the anterior compartment of the A8 segment where, despite having no specific function, opens the possibility of this developmental factor acquiring one. We propose that recently co-opted networks become interlocked, so that any change to the network because of its function in one organ, will be mirrored by other organs even if it provides no selective advantage to them.

Adaptations for gas exchange enabled the elongation of lepidopteran proboscises

The extensive biodiversification of butterflies and moths (Lepidoptera) is partly attributed to their unique mouthparts (proboscis [Pr]) that can span in length from less than 1 mm to over 280 mm in Darwin's sphinx moths. Lepidoptera, similar to other insects, are believed to inhale and exhale respiratory gases only through valve-like spiracles on their thorax and abdomen, making gas exchange through the narrow tracheae (Tr) challenging for the elongated Pr. How Lepidoptera overcome distance effects for gas transport to the Pr is an open question that is important to understanding how the Pr elongated over evolutionary time. Here, we show with scanning electron microscopy and X-ray imaging that distance effects on gas exchange are overcome by previously unreported micropores on the Pr surface and by superhydrophobic Tr that prevent water loss and entry. We find that the density of micropores decreases monotonically along the Pr length with the maxima proportional to the Pr length and that micropore diameters produce a Knudsen number at the boundary between the slip and transition flow regimes. By numerical estimation, we further show that the respiratory gas exchange for the Pr predominantly occurs via diffusion through the micropores. These adaptations are key innovations vital to Pr elongation, which likely facilitated lepidopteran biodiversification and the radiation of angiosperms by coevolutionary processes.

The tracheal immune system of insects - A blueprint for understanding epithelial immunity

The unique design of respiratory organs in multicellular organisms makes them prone to infection by pathogens. To cope with this vulnerability, highly effective local immune systems evolved that are also operative in the tracheal system of insects. Many pathogens and parasites (including viruses, bacteria, fungi, and metazoan parasites) colonize the trachea or invade the host via this route. Currently, only two modules of the tracheal immune system have been characterized in depth: 1) Immune deficiency pathway-mediated activation of antimicrobial peptide gene expression and 2) local melanization processes that protect the structure from wounding. There is an urgent need to increase our understanding of the architecture of tracheal immune systems, especially regarding those mechanisms that enable the maintenance of immune homeostasis. This need for new studies is particularly exigent for species other than Drosophila.

Adult and Larval Tracheal Systems Exhibit Different Molecular Architectures in Drosophila

Knowing the molecular makeup of an organ system is required for its in-depth understanding. We analyzed the molecular repertoire of the adult tracheal system of the fruit fly Drosophila melanogaster using transcriptome studies to advance our knowledge of the adult insect tracheal system. Comparing this to the larval tracheal system revealed several major differences that likely influence organ function. During the transition from larval to adult tracheal system, a shift in the expression of genes responsible for the formation of cuticular structure occurs. This change in transcript composition manifests in the physical properties of cuticular structures of the adult trachea. Enhanced tonic activation of the immune system is observed in the adult trachea, which encompasses the increased expression of antimicrobial peptides. In addition, modulatory processes are conspicuous, in this case mainly by the increased expression of G protein-coupled receptors in the adult trachea. Finally, all components of a peripheral circadian clock are present in the adult tracheal system, which is not the case in the larval tracheal system. Comparative analysis of driver lines targeting the adult tracheal system revealed that even the canonical tracheal driver line breathless (btl)-Gal4 is not able to target all parts of the adult tracheal system. Here, we have uncovered a specific transcriptome pattern of the adult tracheal system and provide this dataset as a basis for further analyses of the adult insect tracheal system.

Pleopodal lung development in a terrestrial isopod, Porcellio scaber (Oniscidea)

During evolution, various lineages of arthropods colonized land and independently acquired air-breathing organs. Some taxa of oniscidean isopods (Crustacea, Isopoda, Oniscidea) are the most successful crustacean lineages on land and possess organs called "lungs" or "pseudotrachea" for air-breathing in their abdominal appendages, i.e., in pleopods. Although these lungs are important for adapting to the terrestrial environment, their developmental process has not yet been elucidated. In the present study, we investigated the process of lung development in Porcellio scaber, the common rough woodlouse with pleopodal lungs in the first two pairs of pleopods. The lungs in the second pleopods developed at the manca 1 stage (immediately after hatching) and became functional at the manca 2 stage. In the first pleopods, which appear at the manca 3 stage, the lungs were gradually developed during the manca 3 stage and became functional in post-manca juveniles. In the second pleopods, epithelial invaginations led to lung development. These results suggest that some novel developmental mechanisms with epithelial invaginations and cuticle formation were acquired during terrestrialization, resulting in the development of functional lungs in the terrestrial isopod lineages.

Specification of the endocrine primordia controlling insect moulting and metamorphosis by the JAK/STAT signalling pathway

The corpora allata and the prothoracic glands control moulting and metamorphosis in insects. These endocrine glands are specified in the maxillary and labial segments at positions homologous to those forming the trachea in more posterior segments. Glands and trachea can be homeotically transformed into each other suggesting that all three evolved from a metamerically repeated organ that diverged to form glands in the head and respiratory organs in the trunk. While much is known about tracheal specification, there is limited information about corpora allata and prothorathic gland specification. Here we show that the expression of a key regulator of early gland development, the snail gene, is controlled by the Dfd and Scr Hox genes and by the Hedgehog and Wnt signalling pathways that induce localised transcription of upd, the ligand of the JAK/STAT signalling pathway, which lies at the heart of gland specification. Our results show that the same upstream regulators are required for the early gland and tracheal primordia specification, reinforcing the hypothesis that they originated from a segmentally repeated organ present in an ancient arthropod.

The Daphnia carapace and other novel structures evolved via the cryptic persistence of serial homologs

Understanding how novel structures arise is a central question in evolution. Novel structures are often defined as structures that are not derived from (homologous to) any structure in the ancestor.¹ The carapace of the crustacean Daphnia magna is a bivalved "cape" of exoskeleton. Shiga et al.² proposed that the carapace of crustaceans like Daphnia and many other plate-like outgrowths in arthropods are novel structures that arose through the repeated co-option of genes like vestigial that also pattern insect wings.^2-4 To determine whether the Daphnia carapace is a novel structure, we compare previous functional work² with the expression of genes known to pattern the proximal leg region (pannier, araucan, and vestigial)^5,6 between Daphnia, Parhyale, and Tribolium. Our results suggest that the Daphnia carapace did not arise by co-option but instead derived from an exite (lateral leg lobe) that emerges from an ancestral proximal leg segment that was incorporated into the Daphnia body wall. The Daphnia carapace, therefore, appears to be homologous to the Parhyale tergal plate and the insect wing.⁵ Remarkably, the vestigial-positive tissue that gives rise to the Daphnia carapace appears to be present in Parhyale⁷ and Tribolium as a small, inconspicuous protrusion. Thus, rather than a novel structure resulting from gene co-option, the Daphnia carapace appears to have arisen from a shared, ancestral tissue (morphogenetic field) that persists in a cryptic state in other arthropod lineages. Cryptic persistence of unrecognized serial homologs may thus be a general solution for the origin of novel structures.

Anterior Hox Genes and the Process of Cephalization

During evolution, bilateral animals have experienced a progressive process of cephalization with the anterior concentration of nervous tissue, sensory organs and the appearance of dedicated feeding structures surrounding the mouth. Cephalization has been achieved by the specialization of the unsegmented anterior end of the body (the acron) and the sequential recruitment to the head of adjacent anterior segments. Here we review the key developmental contribution of Hox1-5 genes to the formation of cephalic structures in vertebrates and arthropods and discuss how this evolved. The appearance of Hox cephalic genes preceded the evolution of a highly specialized head in both groups, indicating that Hox gene involvement in the control of cephalic structures was acquired independently during the evolution of vertebrates and invertebrates to regulate the genes required for head innovation.

Functional analysis of sense organ specification in the Tribolium castaneum larva reveals divergent mechanisms in insects

Abstract

Insects and other arthropods utilise external sensory structures for mechanosensory, olfactory, and gustatory reception. These sense organs have characteristic shapes related to their function, and in many cases are distributed in a fixed pattern so that they are identifiable individually. In Drosophila melanogaster, the identity of sense organs is regulated by specific combinations of transcription factors. In other arthropods, however, sense organ subtypes cannot be linked to the same code of gene expression. This raises the questions of how sense organ diversity has evolved and whether the principles underlying subtype identity in D. melanogaster are representative of other insects. Here, we provide evidence that such principles cannot be generalised, and suggest that sensory organ diversification followed the recruitment of sensory genes to distinct sensory organ specification mechanism.

Results

We analysed sense organ development in a nondipteran insect, the flour beetle Tribolium castaneum, by gene expression and RNA interference studies. We show that in contrast to D. melanogaster, T. castaneum sense organs cannot be categorised based on the expression or their requirement for individual or combinations of conserved sense organ transcription factors such as cut and pox neuro, or members of the Achaete-Scute (Tc ASH, Tc asense), Atonal (Tc atonal, Tc cato, Tc amos), and neurogenin families (Tc tap). Rather, our observations support an evolutionary scenario whereby these sensory genes are required for the specification of sense organ precursors and the development and differentiation of sensory cell types in diverse external sensilla which do not fall into specific morphological and functional classes.

Conclusions

Based on our findings and past research, we present an evolutionary scenario suggesting that sense organ subtype identity has evolved by recruitment of a flexible sensory gene network to the different sense organ specification processes. A dominant role of these genes in subtype identity has evolved as a secondary effect of the function of these genes in individual or subsets of sense organs, probably modulated by positional cues.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-00948-y.