Association of tracheal placodes with leg primordia inDrosophilaand implications for the origin of insect tracheal systems

Thoracic and abdominal outgrowths in early pterygotes: a clue to the common ancestor of winged insects?

One of the fundamental questions in insect evolution is the origin of their wings and primary function of ancestral wing precursors. Recent phylogenomic and comparative morphological studies broadly support a terrestrial ancestor of pterygotes, but an aquatic or semiaquatic ancestor cannot be ruled out. Here new features of the branchial system of palaeodictyopteran larvae of several different instars of Katosaxoniapteron brauneri gen. et sp. nov. (Eugereonoidea) from the late Carboniferous collected at Piesberg (Germany) are described, which consist of delicate dorsolateral and lamellate caudal abdominal gills that support an aquatic or at least semiaquatic lifestyle for these insects. Moreover, the similar form and surface microstructures on the lateral abdominal outgrowths and thoracic wing pads indicate that paired serial outgrowths on segments of both tagmata presumably functioned as ancestral type of gills resembling a protopterygote model. This is consistent with the hypothesis that the wing sheaths of later stage damselfly larvae in hypoxic conditions have a respiratory role similar to abdominal tracheal gills. Hence, the primary function and driving force for the evolution of the precursors of wing pads and their abdominal homologues could be respiration.

Mechanisms of Na+ uptake from freshwater habitats in animals

Life in fresh water is osmotically and energetically challenging for living organisms, requiring increases in ion uptake from dilute environments. However, mechanisms of ion uptake from freshwater environments are still poorly understood and controversial, especially in arthropods, for which several hypothetical models have been proposed based on incomplete data. One compelling model involves the proton pump V-type H⁺ ATPase (VHA), which energizes the apical membrane, enabling the uptake of Na⁺ (and other cations) via an unknown Na⁺ transporter (referred to as the "Wieczorek Exchanger" in insects). What evidence exists for this model of ion uptake and what is this mystery exchanger or channel that cooperates with VHA? We present results from studies that explore this question in crustaceans, insects, and teleost fish. We argue that the Na⁺/H⁺ antiporter (NHA) is a likely candidate for the Wieczorek Exchanger in many crustaceans and insects; although, there is no evidence that this is the case for fish. NHA was discovered relatively recently in animals and its functions have not been well characterized. Teleost fish exhibit redundancy of Na⁺ uptake pathways at the gill level, performed by different ion transporter paralogs in diverse cell types, apparently enabling tolerance of low environmental salinity and various pH levels. We argue that much more research is needed on overall mechanisms of ion uptake from freshwater habitats, especially on NHA and other potential Wieczorek Exchangers. Such insights gained would contribute greatly to our general understanding of ionic regulation in diverse species across habitats.

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.

Abdominal serial homologues of wings in Paleozoic insects

The Late Paleozoic acquisition of wings in insects represents one of the key steps in arthropod evolution. While the origin of wings has been a contentious matter for nearly two centuries, recent evolutionary developmental studies suggest either the participation of both tergal and pleural tissues in the formation of wings¹ or wings originated from exites of the most proximal leg podite incorporated into the insect body wall.² The so-called "dual hypothesis" for wing origins finds support from studies of embryology, evo-devo, and genomics, although the degree of the presumed contribution from tergal and pleural tissues differ.^3-6 Ohde et al.,⁷ confirmed a major role for tergal tissue in the formation of the cricket wing and suggested that "wings evolved from the pre-existing lateral terga of a wingless insect ancestor." Additional work has focused on identifying partial serially homologous structures of wings on the prothorax^8,9 and abdominal segments.¹⁰ Thus, several studies have suggested that the prothoracic horns in scarab beetles,⁹ gin traps of tenebrionid and scarab beetle pupae,^11,12 or abdominal tracheal gills of mayfly larvae^1,13 evolved from serial homologues of wings. Here, we present critical information from abdominal lateral outgrowths (flaps) of Paleozoic palaeodictyopteran larvae, which show comparable structure to thoracic wings, consisting of cordate lateral outgrowths antero-basally hinged by muscle attachments. These flaps therefore most likely represent wing serial homologues. The presence of these paired outgrowths on abdominal segments I-IX in early diverging Pterygota likely corresponds to crustacean epipods^14,15 and resembles a hypothesized ancestral body plan of a "protopterygote" model.

What crustaceans can tell us about the evolution of insect wings and other morphologically novel structures

Acquisition of novel structures often has a profound impact on the adaptation of organisms. The wing of insects is one such example, facilitating their massive success and enabling them to become the dominant clade on this planet. However, its evolutionary origin as well as the mechanisms underpinning its evolution remain elusive. Studies in crustaceans, a wingless sister group of insects, have played a pivotal role in the wing origin debate. Three recent investigations into the genes related to insect wings and legs in crustaceans provided intriguing insights into how and where insect wings evolved. Interestingly, each study proposes a distinct mechanism as a key process underlying insect wing evolution. Here, I discuss what we can learn about the evolution of insect wings and morphological novelty in general by synthesizing the outcomes of these studies.

Two sets of candidate crustacean wing homologues and their implication for the origin of insect wings

The origin of insect wings is a biological mystery that has fascinated scientists for centuries. Identification of tissues homologous to insect wings from lineages outside of Insecta will provide pivotal information to resolve this conundrum. Here, through expression and clustered regularly interspaced short palindromic repeats (CRISPR)-CRISPR-associated protein 9 (Cas9) functional analyses in Parhyale, we show that a gene network similar to the insect wing gene network (preWGN) operates both in the crustacean terga and in the proximal leg segments, suggesting that the evolution of a preWGN precedes the emergence of insect wings, and that from an evo-devo perspective, both of these tissues qualify as potential crustacean wing homologues. Combining these results with recent wing origin studies in insects, we discuss the possibility that both tissues are crustacean wing homologues, which supports a dual evolutionary origin of insect wings (that is, novelty through a merger of two distinct tissues). These outcomes have a crucial impact on the course of the intellectual battle between the two historically competing wing origin hypotheses.

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

The diversity in the organization of the tracheal system is one of the drivers of insect evolutionary success; however, the genetic mechanisms responsible are yet to be elucidated. Here, we highlight the advantages of utilizing hemimetabolous insects, such as the milkweed bug Oncopeltus fasciatus, in which the final adult tracheal patterning can be directly inferred by examining its blueprint in embryos. By reporting the expression patterns, functions, and Hox gene regulation of trachealess (trh), ventral veinless (vvl), and cut (ct), key genes involved in tracheal development, this study provides important insights. First, Hox genes function as activators, modifiers, and suppressors of trh expression, which in turn results in a difference between the thoracic and abdominal tracheal organization. Second, spiracle morphogenesis requires the input of both trh and ct, where ct is positively regulated by trh As Hox genes regulate trh, we can now mechanistically explain the previous observations of their effects on spiracle formation. Third, the default state of vvl expression in the thorax, in the absence of Hox gene expression, features three lateral cell clusters connected to ducts. Fourth, the exocrine scent glands express vvl and are regulated by Hox genes. These results extend previous findings [Sánchez-Higueras et al., 2014], suggesting that the exocrine glands, similar to the endocrine, develop from the same primordia that give rise to the trachea. The presence of such versatile primordia in the miracrustacean ancestor could account for the similar gene networks found in the glandular and respiratory organs of both insects and crustaceans.

Chelicerates and the Conquest of Land: A View of Arachnid Origins Through an Evo-Devo Spyglass

The internal phylogeny of Chelicerata and the attendant evolutionary scenario of arachnid terrestrialization have a long and contentious history. Previous studies of developmental gene expression data have suggested that respiratory systems of spiders, crustaceans, and insects are all serially homologous structures derived from the epipods (outer appendage rami) of the arthropod ancestor, corresponding to an ancestral gill. A separate body of evidence has suggested that the respiratory systems of arachnids are modified, inverted telopods (inner rami, or legs). Here I review these dissonant homology statements and compare the developmental genetic basis for respiratory system development in insects and arachnids. I show that the respiratory primordia of arachnids are not positionally homologous to those of insects. I further demonstrate that candidate genes critical to tracheal fate specification in Drosophila melanogaster are expressed very differently in arachnid exemplars. Taken together, these data suggest that mechanisms of respiratory system development are not derived from homologous structures or mechanisms in insects and arachnids, and that different terrestrial arthropod lineages have solved the challenge of aerial respiration using different developmental mechanisms.

Embryological evidence substantiates the subcoxal theory on the origin of pleuron in insects

The lateral body plate pleuron is a significant structure in insects that contributes to the development and elaboration of wings and limbs (appendages). Although the pleuron is thought to originate from the proximal-most appendicular segment, the subcoxa, details remain unclear, and the morphological boundary between the dorsal body plate tergum and appendage (BTA) has not been clearly specified. Employing low-vacuum scanning electron microscopy (SEM) and the nano-suit method for SEM, we followed, in detail, the development of the thoracic segments of the two-spotted cricket Gryllus bimaculatus and succeeded in clearly defining the BTA. This study demonstrates the subcoxal origin of the pleuron, suggests the tergal origin of spiracles, and reveals that the wing proper originates exclusively from the tergum, whereas the wing hinge and direct muscles may be appendicular in origin, suggesting the dual origin (i.e., tergal plus appendicular origin) of wings.

Origin and diversification of wings: Insights from a neopteran insect

Winged insects underwent an unparalleled evolutionary radiation, but mechanisms underlying the origin and diversification of wings in basal insects are sparsely known compared with more derived holometabolous insects. In the neopteran species Oncopeltus fasciatus, we manipulated wing specification genes and used RNA-seq to obtain both functional and genomic perspectives. Combined with previous studies, our results suggest the following key steps in wing origin and diversification. First, a set of dorsally derived outgrowths evolved along a number of body segments including the first thoracic segment (T1). Homeotic genes were subsequently co-opted to suppress growth of some dorsal flaps in the thorax and abdomen. In T1 this suppression was accomplished by Sex combs reduced, that when experimentally removed, results in an ectopic T1 flap similar to prothoracic winglets present in fossil hemipteroids and other early insects. Global gene-expression differences in ectopic T1 vs. T2/T3 wings suggest that the transition from flaps to wings required ventrally originating cells, homologous with those in ancestral arthropod gill flaps/epipods, to migrate dorsally and fuse with the dorsal flap tissue thereby bringing new functional gene networks; these presumably enabled the T2/T3 wing's increased size and functionality. Third, "fused" wings became both the wing blade and surrounding regions of the dorsal thorax cuticle, providing tissue for subsequent modifications including wing folding and the fit of folded wings. Finally, Ultrabithorax was co-opted to uncouple the morphology of T2 and T3 wings and to act as a general modifier of hindwings, which in turn governed the subsequent diversification of lineage-specific wing forms.

Multipotent versus differentiated cell fate selection in the developing Drosophila airways

Developmental potentials of cells are tightly controlled at multiple levels. The embryonic Drosophila airway tree is roughly subdivided into two types of cells with distinct developmental potentials: a proximally located group of multipotent adult precursor cells (P-fate) and a distally located population of more differentiated cells (D-fate). We show that the GATA-family transcription factor (TF) Grain promotes the P-fate and the POU-homeobox TF Ventral veinless (Vvl/Drifter/U-turned) stimulates the D-fate. Hedgehog and receptor tyrosine kinase (RTK) signaling cooperate with Vvl to drive the D-fate at the expense of the P-fate while negative regulators of either of these signaling pathways ensure P-fate specification. Local concentrations of Decapentaplegic/BMP, Wingless/Wnt, and Hedgehog signals differentially regulate the expression of D-factors and P-factors to transform an equipotent primordial field into a concentric pattern of radially different morphogenetic potentials, which gradually gives rise to the distal-proximal organization of distinct cell types in the mature airway.

DOI:http://dx.doi.org/10.7554/eLife.09646.001

Many organs are composed of tubes of different sizes, shapes and patterns that transport vital substances from one site to another. In the fruit fly species Drosophila melanogaster, oxygen is transported by a tubular network, which divides into finer tubes that allow the oxygen to reach every part of the body.

Different parts of the fruit fly’s airways develop from different groups of tracheal precursor cells. P-fate cells form the most 'proximal' tubes (which are found next to the outer layer of the fly). These cells are 'multipotent' stem cells, and have the ability to specialize into many different types of cells during metamorphosis. The more 'distal' branches that emerge from the proximal tubes develop from D-fate cells. These are cells that generally acquire a narrower range of cell identities.

By performing a genetic analysis of fruit fly embryos, Matsuda et al. have now identified several proteins and signaling molecules that control whether tracheal precursor cells become D-fate or P-fate cells. For example, several signaling pathways work with a protein called Ventral veinless to cause D-fate cells to develop instead of P-fate cells. However, molecules that prevent signaling occurring via these pathways help P-fate cells to form. Different amounts of the molecules that either promote or hinder these signaling processes are present in different parts of the fly embryo; this helps the airways of the fly to develop in the correct pattern.

This work provides a comprehensive view of how cell types with different developmental potentials are positioned in a complex tubular network. This sets a basis for future studies addressing how the respiratory organs – and indeed the entire organism – are sustained.

DOI:http://dx.doi.org/10.7554/eLife.09646.002

Evolutionary mechanisms of habitat invasions, using the copepod Eurytemora affinis as a model system

The study of the copepod Eurytemora affinis has provided unprecedented insights into mechanisms of invasive success. In this invited review, I summarize a subset of work from my laboratory to highlight key insights gained from studying E. affinis as a model system. Invasive species with brackish origins are overrepresented in freshwater habitats. The copepod E. affinis is an example of such a brackish invader, and has invaded freshwater habitats multiple times independently in recent years. These invasions were accompanied by the evolution of physiological tolerance and plasticity, increased body fluid regulation, and evolutionary shifts in ion transporter (V‐type H⁺ATPase, Na⁺, K⁺‐ATPase) activity and expression. These evolutionary changes occurred in parallel across independent invasions in nature and in laboratory selection experiments. Selection appears to act on standing genetic variation during invasions, and maintenance of this variation is likely facilitated through ‘beneficial reversal of dominance’ in salinity tolerance across habitats. Expression of critical ion transporters is localized in newly discovered Crusalis leg organs. Increased freshwater tolerance is accompanied by costs to development time and greater requirements for food. High‐food concentration increases low‐salinity tolerance, allowing saline populations to invade freshwater habitats. Mechanisms observed here likely have relevance for other taxa undergoing fundamental niche expansions.

Development: a deep breath for endocrine organ evolution

Developmental biologists have made surprising discoveries on the evolutionary origins of cell types, organs and body plans. Now, an elegant study in Drosophila raises interesting questions about the origin of two major endocrine organs of insects.

Common origin of insect trachea and endocrine organs from a segmentally repeated precursor

Segmented organisms have serially repeated structures [1] that become specialized in some segments [2]. We show here that the Drosophila corpora allata, prothoracic glands, and trachea have a homologous origin and can convert into each other. The tracheal epithelial tubes develop from ten trunk placodes [3, 4], and homologous ectodermal cells in the maxilla and labium form the corpora allata and the prothoracic glands. The early endocrine and trachea gene networks are similar, with STAT and Hox genes inducing their activation. The initial invagination of the trachea and the endocrine primordia is identical, but activation of Snail in the glands induces an epithelial-mesenchymal transition (EMT), after which the corpora allata and prothoracic gland primordia coalesce and migrate dorsally, joining the corpora cardiaca to form the ring gland. We propose that the arthropod ectodermal endocrine glands and respiratory organs arose through an extreme process of divergent evolution from a metameric repeated structure.

Mechanisms of tentacle morphogenesis in the sea anemone Nematostella vectensis

Evolution of the capacity to form secondary outgrowths from the principal embryonic axes was a crucial innovation that potentiated the diversification of animal body plans. Precisely how such outgrowths develop in early-branching metazoan species remains poorly understood. Here we demonstrate that three fundamental processes contribute to embryonic tentacle development in the cnidarian Nematostella vectensis. First, a pseudostratified ectodermal placode forms at the oral pole of developing larvae and is transcriptionally patterned into four tentacle buds. Subsequently, Notch signaling-dependent changes in apicobasal epithelial thickness drive elongation of these primordia. In parallel, oriented cell rearrangements revealed by clonal analysis correlate with shaping of the elongating tentacles. Taken together, our results define the mechanism of embryonic appendage development in an early-branching metazoan, and thereby provide a novel foundation for understanding the diversification of body plans during animal evolution.

Evolution of air breathing: oxygen homeostasis and the transitions from water to land and sky

Life originated in anoxia, but many organisms came to depend upon oxygen for survival, independently evolving diverse respiratory systems for acquiring oxygen from the environment. Ambient oxygen tension (PO2) fluctuated through the ages in correlation with biodiversity and body size, enabling organisms to migrate from water to land and air and sometimes in the opposite direction. Habitat expansion compels the use of different gas exchangers, for example, skin, gills, tracheae, lungs, and their intermediate stages, that may coexist within the same species; coexistence may be temporally disjunct (e.g., larval gills vs. adult lungs) or simultaneous (e.g., skin, gills, and lungs in some salamanders). Disparate systems exhibit similar directions of adaptation: toward larger diffusion interfaces, thinner barriers, finer dynamic regulation, and reduced cost of breathing. Efficient respiratory gas exchange, coupled to downstream convective and diffusive resistances, comprise the "oxygen cascade"-step-down of PO2 that balances supply against toxicity. Here, we review the origin of oxygen homeostasis, a primal selection factor for all respiratory systems, which in turn function as gatekeepers of the cascade. Within an organism's lifespan, the respiratory apparatus adapts in various ways to upregulate oxygen uptake in hypoxia and restrict uptake in hyperoxia. In an evolutionary context, certain species also become adapted to environmental conditions or habitual organismic demands. We, therefore, survey the comparative anatomy and physiology of respiratory systems from invertebrates to vertebrates, water to air breathers, and terrestrial to aerial inhabitants. Through the evolutionary directions and variety of gas exchangers, their shared features and individual compromises may be appreciated.

Reanalysis and experimental evidence indicate that the earliest trace fossil of a winged insect was a surface-skimming neopteran

A recent description and analysis of an imprint fossil from the Carboniferous concluded that it was made by a mayfly landing in sediment at the edge of water. Here, I reanalyze that trace fossil and supply experimental evidence regarding wing traces and behavior. The thorax of the trace maker lacked structures characteristic of mayflies, but closely matches a modern neopteran insect family (Taeniopterygidae, Plecoptera) little changed from Early Permian fossils. Edges of the folded wings of live Taeniopteryx leave marks on sediment closely matching marks in the trace fossil. Faint marks lateral to and beyond the reach of meso- and metathoracic legs match the location where wings of surface-skimming Taeniopteryx stoneflies lightly touch the sediment when these insects skim onto wet ground at shorelines. Dimensions of the thorax of the trace indicate relatively weak flight ability compared to fossils from the Early Permian, making doubtful the hypothesis that the trace maker was flight capable. Ultimately, this fossil best fits a scenario in which a neopteran insect skimmed across the surface of water, then folded its wings. Surface skimming as a precursor to the evolution of flight in insects is supported by this fossil evidence of skimming behavior in a Carboniferous insect.

Characterization of the biochemical properties and biological function of the formin homology domains of Drosophila DAAM

We characterized the properties of Drosophila melanogaster DAAM-FH2 and DAAM-FH1-FH2 fragments and their interactions with actin and profilin by using various biophysical methods and in vivo experiments. The results show that although the DAAM-FH2 fragment does not have any conspicuous effect on actin assembly in vivo, in cells expressing the DAAM-FH1-FH2 fragment, a profilin-dependent increase in the formation of actin structures is observed. The trachea-specific expression of DAAM-FH1-FH2 also induces phenotypic effects, leading to the collapse of the tracheal tube and lethality in the larval stages. In vitro, both DAAM fragments catalyze actin nucleation but severely decrease both the elongation and depolymerization rate of the filaments. Profilin acts as a molecular switch in DAAM function. DAAM-FH1-FH2, remaining bound to barbed ends, drives processive assembly of profilin-actin, whereas DAAM-FH2 forms an abortive complex with barbed ends that does not support profilin-actin assembly. Both DAAM fragments also bind to the sides of the actin filaments and induce actin bundling. These observations show that the D. melanogaster DAAM formin represents an extreme class of barbed end regulators gated by profilin.

An efficient approach to isolate STAT regulated enhancers uncovers STAT92E fundamental role in Drosophila tracheal development

The ventral veinless (vvl) and trachealess (trh) genes are determinants of the Drosophila trachea. Early in development both genes are independently activated in the tracheal primordia by signals that are ill defined. Mutants blocking JAK/STAT signaling at any level do not form a tracheal tree suggesting that STAT92E may be an upstream transcriptional activator of the early trachea determinants. To test this hypothesis we have searched for STAT92E responsive enhancers activating the expression of vvl and trh in the tracheal primordia. We show that STAT92E regulated enhancers can be rapidly and efficiently isolated by focusing the analysis on genomic regions with clusters of putative STAT binding sites where at least some of them are phylogenetically conserved. Detailed analysis of a vvl early tracheal enhancer shows that non-conserved sites collaborate with conserved sites for enhancer activation. We find that STAT92E regulated enhancers can be located as far 60 kb from the promoters. Our results indicate that vvl and trh are independently activated by STAT92E which is the most important transcription factor required for trachea specification.

The embryonic development of the malacostracan crustacean Porcellio scaber (Isopoda, Oniscidea)

To examine the evolution of development and put it into a phylogenetic context, it is important to have, in addition to a model organism like Drosophila, more insights into the huge diversity of arthropod morphologies. In recent years, the malacostracan crustacean Porcellio scaber Latreille, 1804 has become a popular animal for studies in evolutionary and developmental biology, but a detailed and complete description of its embryonic development is still lacking. Therefore, the embryonic development of the woodlouse P. scaber is described in a series of discrete stages easily identified by examination of living animals and the widely used technique of nuclei staining on fixed specimens. It starts with the first cleavage of the zygote and ends with a hatched manca that eventually leaves the mother's brood pouch. Classical methods like normal light microscopy, scanning electron microscopy and fluorescence microscopy are used, in addition to confocal LCM and computer-aided 3D reconstruction in order to visualise important processes during ontogeny. The purpose of these studies is to offer an easy way to define the different degrees of development for future comparative analyses of embryonic development amongst crustaceans in particular, as well as between different arthropod groups. In addition, several aspects of Porcellio embryonic development, such as the mouth formation, limb differentiations and modifications or the formation of the digestive tract, make this species particularly interesting for future studies in evolutionary and developmental biology.

Delayed onset of midline netrin expression in Artemia franciscana coincides with commissural axon growth and provides evidence for homology of midline cells in distantly related arthropods

Although many similarities in arthropod central nervous systems (CNS) development exist, differences in midline cell formation and ventral nerve cord axonogenesis have been noted in arthropods. It is possible that changes in the expression of axon guidance molecules such as Netrin, which functions during commissural axon guidance in Drosophila and many other organisms, may parallel these differences. In this investigation, we analyze this hypothesis by examining Netrin accumulation during development of the brine shrimp Artemia franciscana, a branchiopod crustacean. An Artemia franciscana netrin (afrnet) orthologue was cloned. An antibody to the afrNet protein was generated and used to examine the pattern of afrNet accumulation during Artemia development. Despite differences between Drosophila and Artemia nerve cord development, examination of afrNet accumulation suggests that this protein functions to regulate commissure formation during Artemia CNS development. However, detection of afrNet at the midline and on commissural axons occurs at a relatively later time point in Artemia as compared with Drosophila. Detection of afrNet in a subset of midline cells that closely resemble Netrin-expressing cells at the Drosophila midline provides evidence for homology of midline cells in arthropods. Expression of Netrins in many other tissues is comparable, suggesting that Netrin proteins may play many conserved roles during arthropod development.

The emergence of shape: notions from the study of the Drosophila tracheal system

The generation of bodies and body parts with specific shapes and sizes has been a longstanding issue in biology. Morphogenesis in general and organogenesis in particular are complex events that involve global changes in cell populations in terms of their proliferation, migration, differentiation and shape. Recent studies have begun to address how these synchronized changes are controlled by the genes that specify cell fate and by the ability of cells to respond to extracellular cues. In particular, a notable shift in this research has occurred owing to the ability to address these issues in the context of the whole organism. For such studies, the Drosophila tracheal system has proven to be a particularly appropriate model. Here, my aim is to highlight some ideas that have arisen through our studies, and those from other groups, of Drosophila tracheal development. Rather than providing an objective review of the features of tracheal development, I intend to discuss some selected notions that I think are relevant to the question of shape generation.