From embryo to adult—beyond the conventional periodization of arthropod development

The Morphological Diversity of Antlion Larvae and Their Closest Relatives over 100 Million Years

Simple Summary

The larvae of owlflies and antlions (here shortly embraced by the term “owllions”) are ambush predators. Their mouthparts are transformed into teeth-bearing stylets and used for catching prey and sucking, which is characteristic for neuropteran larvae. Here we used the morphology of the stylets and the head capsules of a large number of extant and fossil larvae as a proxy for the morphological diversity over time. The created dataset comprises outlines of stylets and head capsules of specimens from the literature, collections, databases and the herein described and depicted 38 fossil ones. Fossils in the whole dataset come from deposits with an age of about 20, 40, and 100 million years (Miocene, Eocene, and Cretaceous, respectively). In addition to the shape analysis of the outlines from the dataset, we conducted a statistical analysis as well. Eocene and Miocene samples did not result in a clear output, but Cretaceous samples allowed for some conclusions: The morphological diversity of owllion larvae increased over time, even though some morphologies of Cretaceous larvae went extinct.

Abstract

Among lacewings (Neuroptera), representatives of the groups Ascalaphidae (owlflies) and Myrmeleontidae (antlions) are likely the most widely known ones. The exact taxonomic status of the two groups remains currently unclear, each may in fact be nested in the other group. Herein, we refer to the group including representatives of both with the neutral term “owllion”. Owllion larvae are voracious ambush hunters. They are not only known in the extant fauna, but also from the fossil record. We report here new findings of a fossil owlfly larva from Eocene Baltic amber, as well as several owlfly-like larvae from Cretaceous Kachin amber, Myanmar. Based on these fossils, combined with numerous fossil and extant specimens from the literature, collections, and databases, we compared the morphological diversity of the head and mouthpart shapes of the larvae of owllions in the extant fauna with that of owllion-like larvae from three time slices: about 100 million years ago (Cretaceous), about 40 million years ago (Eocene), and about 20 million years ago (Miocene). The comparison reveals that the samples from the Eocene and Miocene are too small for a reliable evaluation. Yet, the Cretaceous larvae allow for some conclusions: (1) the larval morphological diversity of owllion larvae increased over time, indicating a post-Cretaceous diversification; (2) certain morphologies disappeared after the Cretaceous, most likely representing ecological roles that are no longer present nowadays. In comparison, other closely related lineages, e.g., silky lacewings or split-footed lacewings, underwent more drastic losses after the Cretaceous and no subsequent diversifications.

Transcriptome Analysis Reveals the Molecular Response to Salinity Challenge in Larvae of the Giant Freshwater Prawn Macrobrachium rosenbergii

Salinity is a crucial factor influencing the growth, development, immunity, and reproduction of aquatic organisms; however, little is known about the molecular mechanism of the response to salinity challenge in larvae of the giant freshwater prawn Macrobrachium rosenbergii. Herein, larvae cultured in three treatment groups with salinities of 10, 13, and 16‰ (S10, S13, and S16) were collected, and then transcriptome analysis was conducted by RNA-seq. A total of 6,473, 3,830 and 3,584 differentially expressed genes (DEGs) were identified in the S10 vs. S13 comparison, S10 vs. S16 comparison and S13 vs. S16 comparison, respectively. These genes are involved in osmoregulation, energy metabolism, molting, and the immune response. qPCR analysis was used to detect the expression patterns of 16 DEGs to verify the accuracy of the transcriptome data. Protein–protein interaction (PPI) analysis for DEGs and microsatellite marker screening were also conducted to reveal the molecular mechanism of salinity regulation. Together, our results will provide insight into the molecular genetic basis of adaptation to salinity challenge for larvae of M. rosenbergii.

New extreme morphologies as exemplified by 100 million-year-old lacewing larvae

Larvae of the group Holometabola (beetles, wasps, flies, moths and others) differ significantly in their morphology from their corresponding adults. In most larvae, appendages and other structures protruding from the body (antennae, palps, legs, trunk processes) appear less elongate than in their corresponding adults, providing the impression that these larvae are restricted to a certain degree in developing more elongate structures. We provide here numerous counterexamples of larvae of lacewings (Neuroptera). These include different forms of elongated antennae, mandibles, maxillae, labial palps, legs, trunk processes and neck regions. Most of these examples are larvae preserved in different types of 100 million-year-old amber. The longest neck region was found in an extant specimen. All these examples demonstrate that certain branches of Neuroptera indeed had larval forms that possessed strongly elongated structures. Hence there is no principal constraint that hinders holometabolan larvae to develop such structures.

RNA interference shows that Spook, the precursor gene of 20-hydroxyecdysone (20E), regulates the molting of Macrobrachium nipponense

The aim of this study was to explore the function of the Mn-Spook gene, which was found in the ovary transcriptome of the Oriental river prawn (Macrobrachium nipponense). The Spook gene, which is the precursor gene of 20-hydroxyecdysone (20E), plays an important role in the process of molting in many arthropods, but its function in M. nipponense is unclear. We cloned the full-length Mn-Spook gene from the ovary of M. nipponense and found that it had the same conserved domains as the P450 gene of the Halloween family of genes. The Mn-Spook gene was highly expressed in ovary and gill tissue during the breeding period. During ovarian development, Mn-spook gene expression was highest at the nearly-ripe stage, and it also was highly expressed in the zoea developmental stage. Cellular localization analysis showed that Mn-Spook signals accumulated in the cytoplasmic membrane and nucleus of oocytes. Finally, we used RNA interference to evaluate the function of the Mn-Spook gene. Compared with the control group, in vivo injection of Mn-Spook dsRNA effectively downregulated the expression of Mn-Spook and the content of 20E. The molting frequency of M. nipponense in the experimental group also was significantly inhibited. These results demonstrated that the Mn-Spook gene played an important role in the molting process of M. nipponense.

Morphological changes during the post-embryonic ontogeny of mesothelan spiders and aspects of character evolution in early spiders

Most morphological studies focus on adult specimens, or if developmental studies are pursued, especially in Euarthropoda, they focus on embryonic development. Araneae (spiders) is one of these groups, in consequence with understudied post-embryonic development. Here we present aspects of the post-embryonic stages of different species of Mesothelae, sister group to the remaining spiders (when fossil species are not taken into account). We used different imaging methods and measured different external morphological structures to detect possible ontogenetic changes. One structure exhibiting post-embryonic changes is the chelicera. Here the significant change occurs between the last immature stage and the adult, yet only in males. For the spinnerets, we could not detect ontogenetic changes, but instead a high variability in length and width, probably due to their lack of pivot joints between the elements. The strongest morphological change during ontogeny occurred on the sternum, which begins with a rather roundish shape in the first stage and changes to being fairly elongate in shape in the last immature stages and the adult. This specific sternum shape only occurs in adults of mesothelan spiders, while opisthothelan spiders have a broader sternum also in the adult. We discuss our results in an evolutionary context, also taking into account recent finds of fossil spiders.

The Development of Arthropod Segmentation Across the Embryonic/Post-embryonic Divide – An Evolutionary Perspective

In many arthropods, the appearance of new segments and their differentiation are not completed by the end of embryogenesis but continue, in different form and degree, well after hatching, in some cases up to the last post-embryonic molt. Focusing on the segmentation process currently described as post-embryonic segment addition (or, anamorphosis), we revise here the current knowledge and discuss it in an evolutionary framework which involves data from fossils, comparative morphology of extant taxa and gene expression. We advise that for a better understanding of the developmental changes underlying the evolution of arthropod segmentation, some key concepts should be applied in a critical way. These include the notion of the segment as a body block and the idea that hatching represents a well-defined divide, shared by all arthropods, between two contrasting developmental phases, embryonic vs. post-embryonic. This eventually reveals the complexity of the developmental processes occurring across hatching, which can evolve in different directions and with a different pace, creating the observed vagueness of the embryonic/post-embryonic divide.

Life after the mother's hug: Late post-embryonic development of Cryptops parisi (Chilopoda: Scolopendromorpha: Cryptopidae)

Here we describe in detail the late post-embryonic development of the common European scolopendromorph centipede Cryptops parisi. Canonical variate analyses of two groups of external morphological characters, viz., cephalic capsule characters (head length, length of the anterior and posterior paramedian cephalic sutures) and coxopleuron surface characters (number of pores in the coxal pore-field, number of setae on the posterior coxopleuron edge, their number on the coxal pore-field, and their number posterior to the coxal pore-field) were conducted on a large sample of specimens collected from two localities in Serbia. Ten free-living stages are recognized: three pre-adult stages (adolescens I, II, and III) and seven adult stages (one maturus junior stage, four maturus, and two maturus senior stages). The fourth late post-embryonic stage is the first mature stage in both sexes. Sexual dimorphism in the aforementioned characters was not observed. Morphological variation of coxopleuron characters was more informative for the discrimination of developmental stages in Cryptops than the morphological variation of cephalic capsule characters.

Expression profiling of winged- and wingless-destined pea aphid embryos implicates insulin/insulin growth factor signaling in morph differences

Developmental plasticity allows the matching of adult phenotypes to different environments. Although considerable effort has gone into understanding the evolution and ecology of plasticity, less is known about its developmental genetic basis. We focused on the pea aphid wing polyphenism, in which high- or low-density environments cause viviparous aphid mothers to produce winged or wingless offspring, respectively. Maternally provided ecdysone signals to embryos to be winged or wingless, but it is unknown how embryos respond to that signal. We used transcriptional profiling to investigate the gene expression state of winged-destined (WD) and wingless-destined (WLD) embryos at two developmental stages. We found that embryos differed in a small number of genes, and that gene sets were enriched for the insulin-signaling portion of the FoxO pathway. To look for a global signature of insulin signaling, we examined the size and stage of WD and WLD embryos but found no differences. These data suggest the hypothesis that FoxO signaling is important for morph development in a tissue-specific manner. We posit that maternally supplied ecdysone affects embryonic FoxO signaling, which ultimately plays a role in alternative morph development. Our study is one of an increasing number that implicate insulin signaling in the generation of alternative environmentally induced morphologies.

Beetle larvae with unusually large terminal ends and a fossil that beats them all (Scraptiidae, Coleoptera)

Larvae, and especially fossil larvae, are challenging to deal with from a purely taxonomic view. Often one cannot determine which species the larvae belong to. Yet, larvae can still contribute to various scientific questions. Especially morphological traits of a fossil larva can be highly informative for reconstructing character evolution. Also the occurrence of specific larval types and larval characters in time and the disappearance of such forms can well be reconstructed also without being able to narrow down the phylogenetic relationship of a larva very far. Here, we report two new beetle larvae preserved in Baltic amber which are identified as representatives of Scraptiidae, based on an enlarged terminal end ('9th abdomen segment'); this is only the third record of such larvae. In comparison to modern forms, the terminal ends of the two new fossil larvae is even larger in relation to the remaining body than in any known larva. Unfortunately, our knowledge of such larvae in the modern fauna is very limited. Still, one of the two already known fossil larvae of Scraptiidae also has a very long terminal end, but not as long as those of the two new fossils. These three fossil larvae therefore seem to possess a specific morphology not known from the modern fauna. This might either mean that they (1) represent a now extinct larval morphology, a phenomenon well known in other euarthropodan lineages, or that (2) these forms represent a part of the larval phase not known from modern day species as they have not been described yet; such cases occur in closely related lineages. In any case, the fossils expand the known diversity of larval morphologies.

Segmentation and limb formation during naupliar development of Tigriopus californicus (Copepoda, Harpacticoida)

Naupliar development in copepods includes the generation of usually five pairs of post-mandibular segments. Since copepod nauplii show no outer body articulation, the only indication of larval segmentation is the expression of limb buds. Yet, in copepods the timing and sequence of limb bud expression in larval development varies to a large degree. In harpacticoid nauplii for instance, the 1^st maxillae are formed at an early naupliar stage. By contrast, the four remaining pairs of limb buds frequently appear simultaneously with the last naupliar stage. The complete process of larval segment formation takes place under the body surface and has never been described in detail. To broaden our knowledge of early segmentation in copepods, we here describe the segmentation of the harpacticoid nauplius Tigriopus californicus by analysing the expression of the segment marker Engrailed and uncover the sequential addition of seven post-mandibular segments. The stripe formation and arrangement of labelled cells corresponds largely to those of other crustaceans studied in this respect. Together with a morphological approach using histology, SEM, and 3D-reconstructions based on CLSM we solve the so far controversial identity of the external limb buds in the final naupliar stage. In contrast to previous studies, we can show that all limb pairs from the 1^st maxillae to the 3^rd thoracopods are formed. Yet, the anlage of the maxilliped (1^st thoracopod) remains hidden underneath the cuticle being never externally expressed in the nauplius.

Stage- and thermal-specific genetic architecture for preadult viability in natural populations of Drosophila melanogaster

Studying the processes affecting variation for preadult viability is essential to understand the evolutionary trajectories followed by natural populations. This task requires focusing on the complex nature of the phenotype-genotype relationship by taking into account usually neglected aspects of the phenotype and recognizing the modularity between different ontogenetic stages. Here, we describe phenotypic variability for viability during the larval and pupal stages in lines derived from three natural populations of Drosophila melanogaster, as well as the variability for phenotypic plasticity and canalization at two different rearing temperatures. The results indicate that the three populations present significant phenotypic differences for preadult viability. Furthermore, distinct aspects of the phenotype (means, plasticity, canalization, plasticity of canalization) are affected by different genetic bases underlying changes in viability in a stage- and environment-specific manner. These findings explain the generalized maintenance of genetic variability for this fitness trait.

Pronymphs, hatching, and proboscis assembly in leafhoppers and froghoppers (Hemiptera, Cicadellidae and Aphrophoridae)

Pharate 1st instar nymphs enclosed in the embryonic cuticle, referred to as pronymphs, were studied in a froghopper Aphrophora pectoralis Mats. (Aphrophoridae) and the leafhoppers Oncopsis flavicollis (L.), Populicerus populi (L.), Alebra wahlbergi (Boh.), Igutettix oculatus (Lindb.), and Scenergates viridis (Vilb.) (Cicadellidae). The species vary in the relative length of the pronymphal antennae and details of sculpturing of the cephalic region. No egg bursting structures were observed, except small denticles on the crown region of S. viridis pronymphs. Rudimentary mandibular and maxillary stylets of a pronymph are external, short, tubular appendages containing tips of the corresponding nymphal stylets, whose more basal parts develop inside of the head. Casting off of the embryonic cuticle results in the nymphal stylets being passively pulled out and assuming a close-set parallel orientation. Once the sheaths of unsclerotized cuticle secreted by the peripodial epithelium and enveloping each developing stylet have been cast off with the exuviae, the bare stylets become squeezed and interlocked into a functional bundle. The roles of the maxillary plates, clypeus, labrum, and labium in the stylet bundle assembly are discussed. The process repeats after each molt.

The ride of the parasite: a 100-million-year old mantis lacewing larva captured while mounting its spider host

BACKGROUND

Adult mantis lacewings, neuropteran holometabolan insects of the group Mantispidae, possess anterior walking legs transformed into prey-catching grasping appendages reminiscent of those of praying mantises. While adult mantis lacewings are hence active "wait-and-catch" predators, the larvae of many mantis lacewings have a quite different biology: first-stage larvae seek out female spiders, mount them, and either wait until the spider has produced an egg sac or, in some cases, choose a female already bearing an egg sac. The larva then enters the egg sac and feeds on the eggs. While first stage larvae are highly mobile with comparably long legs and a certain degree of dorso-ventral flattening ("campodeiform"), larval stages two and three are almost immobile, grub-like, and simply remain within the egg sac. Fossils of mantis lacewings are relatively rare, fossils of larval mantis lacewings are even rarer; only a single larva sitting on a juvenile spider has been described from ca. 50 million year old Baltic amber.

RESULTS

Here we describe a second occurrence of a larval mantis lacewing from significantly older Burmese amber, about 100 million years old. The specimen is preserved in a position right at the leg of a spider, similar to modern-day larvae that are about to mount their prospective host. The claws of the larva can be seen to grab around the leg of the spider.

CONCLUSIONS

We discuss how reliable these fossils are as indicators of palaeo-parasitism, and in which aspects the behaviour of mantis lacewing larvae in general indeed represents parasitism. While the specimen appears to be about to board the spider, it may not necessarily represent a parasite in the strict sense. Evaluating the actual ecological role of a fossil heavily depends on comparison to modern forms, and not all modern-day larvae of Mantispidae are parasites. We therefore provide a closer look into the known feeding habits of modern mantis lacewing larvae.

"Intermetamorphic" developmental stages in 150 million-year-old achelatan lobsters--The case of the species tenera Oppel, 1862

We re-investigated the fossil species tenera Oppel, 1862, an achelatan lobster (traditionally named Palinurina tenera) found in 150 million years old limestones of southern Germany. All known specimens attributed to this species show a mixture of characters, which in modern forms occur either in larvae or post-larval juveniles. Hence these specimens provide insight into a phase in ontogeny that is no longer present in the developmental sequence of any modern achelatan lobster, as the latter ones skip this phase and replace it by a drastic metamorphosis. Comparable cases have been described earlier, yet did only comprise single stages or two successive ones at most. In the here described case four developmental stages are preserved. The reconstructed ontogeny of tenera therefore represents the currently best known sequence of an early achelatan lobster that covers this specific intermediate phase. The largest known stage most likely still represents an immature of a yet undiscovered adult. These findings support the interpretation that early achelatan lobsters developed in a more gradual ontogenetic sequence than modern forms. It furthermore demonstrates that it was even more gradual than anticipated previously.

How to stay on mummy's back: Morphological and functional changes of the pretarsus in arachnid postembryonic stages

A specific type of maternal care occurs in several groups of Arachnida: mothers carry their offspring on their back (pulli-carrying behaviour). In scorpions, whip scorpions and whip spiders it is the prenymphal stage that settles on the mother. The prenymph is not yet fully developed for a free life and very limited in its mobility, but its feet are equipped with special adhesive organs (arolia) that become lost at the nymphal stage. Here we study the morphology, ultrastructure and mechanical function of the arolia. In scorpions (Scorpiones) the contact area between arolia and substrate and thus adhesion of the pad is controlled by the antagonistic work of hydrostatic pressure and muscular retraction. Arolia of whip scorpions (Thelyphonida) do not require muscular action for strong attachment. Arrays of long, branching fibres in the mesocuticle lead to high compliancy of the pad. In whip spiders (Amblypygi) the prenymphal pretarsus is already equipped with sclerites and claws. Its arolium is retained in nymphs and adults in some taxa, but acquires a more complex structure. These results contribute to our knowledge on the postembryonic development of arachnids and to the understanding of attachment pad evolution among arthropods. Some of the described developmental, structural, and mechanical phenomena are not known from other animals and might be of potential interest for further biomimetic developments.

Developmental constraints in cave beetles

In insects, whilst variations in life cycles are common, the basic patterns typical for particular groups remain generally conserved. One of the more extreme modifications is found in some subterranean beetles of the tribe Leptodirini, in which the number of larval instars is reduced from the ancestral three to two and ultimately one, which is not active and does not feed. We analysed all available data on the duration and size of the different developmental stages and compared them in a phylogenetic context. The total duration of development was found to be strongly conserved, irrespective of geographical location, habitat type, number of instars and feeding behaviour of the larvae, with a single alteration of the developmental pattern in a clade of cave species in southeast France. We also found a strong correlation of the size of the first instar larva with adult size, again regardless of geographical location, ecology and type of life cycle. Both results suggest the presence of deeply conserved constraints in the timing and energy requirements of larval development. Past focus on more apparent changes, such as the number of larval instars, may mask more deeply conserved ontogenetic patterns in developmental timing.

Changes in ecdysteroid levels and expression patterns of ecdysteroid-responsive factors and neuropeptide hormones during the embryogenesis of the blue crab, Callinectes sapidus

Embryogenesis requires the involvement and coordination of multiple networks of various genes, according to a timeline governing development. Crustacean embryogenesis usually includes the first molt, a process that is known to be positively controlled by ecdysteroids. We determined the amounts of ecdysteroids, as well as other related factors: the ecdysone receptor (CasEcR), the retinoid X receptor (CasRXR), the molt-inhibiting hormone (CasMIH), and crustacean hyperglycemic hormone (CasCHH) during the ovarian and embryonic developments of Callinectes sapidus. In summary, the ovaries at stages 1-4 have expression levels of maternal CasEcR and CasRXR 10-50 times higher than levels seen in embryos at the yolk stage. This large difference in the amount of the these factors in C. sapidus ovaries suggests that these maternal ecdysteroid-responsive factors may be utilized at the initiation of embryogenesis. During embryogenesis, the changes in total ecdysteroids and levels of CasEcR and CasRXR expression are similar to those observed in juvenile molts. The full-length cDNA sequence of the C. sapidus BTB domain protein (CasBTBDP) initially isolated from Y-organ cDNA, contains only Broad-Complex, Tramtrack, and Bric a brac (BTB) domains. The levels of CasBTBDP are kept constant throughout embryogenesis. The expression profiles of CasMIH and CasCHH are similar to the titers of ecdysteroids. However, the timing of their appearance is followed by increases in CasEcRs and CasRXRs, implying that the expressions of these neuropeptides may be influenced by ecdysteroids. Moreover, the ecdysteroid profile during embryogenesis may track directly with the timing of organogenesis of Y-organs and their activity. Our work reports, for first time, the observed expression and changes of ecdysteroid-responsive factors, along with CasCHH and CasMIH, during embryogenesis in the crustacean C. sapidus.

Prenatal chemosensory learning by the predatory mite Neoseiulus californicus

Background

Prenatal or embryonic learning, behavioral change following experience made prior to birth, may have significant consequences for postnatal foraging behavior in a wide variety of animals, including mammals, birds, fish, amphibians, and molluscs. However, prenatal learning has not been previously shown in arthropods such as insects, spiders and mites.

Methodology/Principal Findings

We examined prenatal chemosensory learning in the plant-inhabiting predatory mite Neoseiulus californicus. We exposed these predators in the embryonic stage to two flavors (vanillin or anisaldehyde) or no flavor (neutral) by feeding their mothers on spider mite prey enriched with these flavors or not enriched with any flavor (neutral). After the predators reached the protonymphal stage, we assessed their prey choice through residence and feeding preferences in experiments, in which they were offered spider mites matching the maternal diet (neutral, vanillin or anisaldehyde spider mites) and non-matching spider mites. Predator protonymphs preferentially resided in the vicinity of spider mites matching the maternal diet irrespective of the type of maternal diet and choice situation. Across treatments, the protonymphs preferentially fed on spider mites matching the maternal diet. Prey and predator sizes did not differ among neutral, vanillin and anisaldehyde treatments, excluding the hypothesis that size-assortative predation influenced the outcome of the experiments.

Conclusions/Significance

Our study reports the first example of prenatal learning in arthropods.

Nervous system development in Spinicaudata and Cyclestherida (Crustacea, Branchiopoda)--comparing two different modes of indirect development by using an event pairing approach

Cladocera are the ecologically most important group within the Branchiopoda. They are unquestionably branchiopods but their evolutionary origin remains unclear. One favored explanation of their origin is that they evolved from a reproductive larva of a clam shrimp-like ancestor. To reveal a transformation and identify (potential) changes in chronology (heterochrony), we investigated and compared the development of representatives of two clam shrimp taxa, one of the Spinicaudata (Leptestheria dahalacensis) and one of the Cyclestherida (Cyclestheria hislopi), the sister group of Cladocera. Both taxa develop indirectly although the exact modes are quite different. The development of the nervous system, labeled and analyzed using immunohistochemical techniques and confocal microscopy, and that of the external morphology, scanned with an electron microscope, was investigated. L. dahalacensis hatch as a free-swimming nauplius and the nervous system and external morphology develop gradually. In C. hislopi, on the other hand, several internal and external structures develop before the hatching of a nonswimming embryo-like larva which is still carried in a dorsal brood pouch. The development in L. dahalacensis is directed from anterior to posterior, whereas in C. hislopi a more synchronous anterior and posterior differentiation is present. A comparison of both developmental sequences gives us the first indications of the evolutionary transformation which the Cladocera may have undergone from a clam shrimp-like ancestor.

Gene expression during postembryonic segmentation in the centipede Lithobius peregrinus (Chilopoda, Lithobiomorpha)

Postembryonic segmentation (anamorphosis) is widespread among arthropods, but only partially known as for its developmental mechanics and control. Studies on developmental genetics of segmentation in anamorphic arthropods are mostly limited to the germ band stage, during early phases of embryonic development. This work presents the first data on the postembryonic expression of a segmentation gene in a myriapod. Using real-time PCR, we analyzed engrailed expression patterns during the anamorphic stages of the centipede Lithobius peregrinus. A variation pattern in en RNA level during anamorphosis suggests that gene expression is precisely modulated during this period of development and that engrailed is mainly expressed in the posterior part of the body, in the newly differentiating segments of each stage. As anamorphosis is possibly the primitive segmentation mode in arthropods, the postembryonic en expression pattern documented here provides evidence for a conservation of en role in ontogeny, across the embryonic/postembryonic boundary, as well as in phylogeny, across the same boundary, but in the opposite direction, from primitive postembryonic expression to the more derived expression in clades with exclusively embryonic segmentation.

Developmental plasticity and the evolution of animal complex life cycles

Metazoan life cycles can be complex in different ways. A number of diverse phenotypes and reproductive events can sequentially occur along the cycle, and at certain stages a variety of developmental and reproductive options can be available to the animal, the choice among which depends on a combination of organismal and environmental conditions. We hypothesize that a diversity of phenotypes arranged in developmental sequence throughout an animal's life cycle may have evolved by genetic assimilation of alternative phenotypes originally triggered by environmental cues. This is supported by similarities between the developmental mechanisms mediating phenotype change and alternative phenotype determination during ontogeny and the common ecological condition that favour both forms of phenotypic variation. The comparison of transcription profiles from different developmental stages throughout a complex life cycle with those from alternative phenotypes in closely related polyphenic animals is expected to offer critical evidence upon which to evaluate our hypothesis.

Idealization in evolutionary developmental investigation: a tension between phenotypic plasticity and normal stages

Idealization is a reasoning strategy that biologists use to describe, model and explain that purposefully departs from features known to be present in nature. Similar to other strategies of scientific reasoning, idealization combines distinctive strengths alongside of latent weaknesses. The study of ontogeny in model organisms is usually executed by establishing a set of normal stages for embryonic development, which enables researchers in different laboratory contexts to have standardized comparisons of experimental results. Normal stages are a form of idealization because they intentionally ignore known variation in development, including variation associated with phenotypic plasticity (e.g. via strict control of environmental variables). This is a tension between the phenomenon of plasticity and the practice of staging that has consequences for evolutionary developmental investigation because variation is conceptually removed as a part of rendering model organisms experimentally tractable. Two compensatory tactics for mitigating these consequences are discussed: employing a diversity of model organisms and adopting alternative periodizations.

The origins of larval forms: what the data indicate, and what they don't

What is a larva, if it is not what survives of an ancestor's adult, compressed into a transient pre-reproductive phase, as suggested by Haeckel's largely disreputed model of evolution by recapitulation? A recently published article hypothesizes that larva and adult of holometabolous insects are developmental expressions of two different genomes coexisting in the same animal as a result of an ancient hybridization event between an onychophoran and a primitive insect with eventless post-embryonic development. More likely, however, larvae originated from late embryonic or early post-embryonic stages of ancestors with direct development. Evolutionary novelties would thus be intercalary rather than terminal, with respect to the ancestor's ontogenetic schedule. This scenario, supported by current research on holometabolous insects and marine invertebrates with complex life cycles, offers a serious alternative to the traditional scenario ('what is early in ontogeny is also early in phylogeny') underlying the current perception of the evolution of genetic regulatory networks.

Saltational evolution of trunk segment number in centipedes

Saltational changes in segment numbers have likely occurred in arthropod evolution, especially if mechanisms of segment formation involve a multiplicative phase, as recently suggested in the evo-devo literature. Here we provide for the first time evidence of major phenotypic saltation in the evolution of segment number in a lineage of centipedes, with a newly discovered species of scolopender having segment numbers duplicated with respect to its closest relatives, and to all the remaining 700+ species of Scolopendromorpha known to date.

Typology reconfigured: from the metaphysics of essentialism to the epistemology of representation

The goal of this paper is to encourage a reconfiguration of the discussion about typology in biology away from the metaphysics of essentialism and toward the epistemology of classifying natural phenomena for the purposes of empirical inquiry. First, I briefly review arguments concerning 'typological thinking', essentialism, species, and natural kinds, highlighting their predominantly metaphysical nature. Second, I use a distinction between the aims, strategies, and tactics of science to suggest how a shift from metaphysics to epistemology might be accomplished. Typological thinking can be understood as a scientific tactic that involves representing natural phenomena using idealizations and approximations, which facilitates explanation, investigation, and theorizing via abstraction and generalization. Third, a variety of typologies from different areas of biology are introduced to emphasize the diversity of this representational reasoning. One particular example is used to examine how there can be epistemological conflict between typology and evolutionary analysis. This demonstrates that alternative strategies of typological thinking arise due to the divergent explanatory goals of researchers working in different disciplines with disparate methodologies. I conclude with several research questions that emerge from an epistemological reconfiguration of typology.

Life history and development--a framework for understanding developmental plasticity in lower termites

Termites (Isoptera) are the phylogenetically oldest social insects, but in scientific research they have always stood in the shadow of the social Hymenoptera. Both groups of social insects evolved complex societies independently and hence, their different ancestry provided them with different life-history preadaptations for social evolution. Termites, the 'social cockroaches', have a hemimetabolous mode of development and both sexes are diploid, while the social Hymenoptera belong to the holometabolous insects and have a haplodiploid mode of sex determination. Despite this apparent disparity it is interesting to ask whether termites and social Hymenoptera share common principles in their individual and social ontogenies and how these are related to the evolution of their respective social life histories. Such a comparison has, however, been much hampered by the developmental complexity of the termite caste system, as well as by an idiosyncratic terminology, which makes it difficult for non-termitologists to access the literature. Here, we provide a conceptual guide to termite terminology based on the highly flexible caste system of the "lower termites". We summarise what is known about ultimate causes and underlying proximate mechanisms in the evolution and maintenance of termite sociality, and we try to embed the results and their discussion into general evolutionary theory and developmental biology. Finally, we speculate about fundamental factors that might have facilitated the unique evolution of complex societies in a diploid hemimetabolous insect taxon. This review also aims at a better integration of termites into general discussions on evolutionary and developmental biology, and it shows that the ecology of termites and their astounding phenotypic plasticity have a large yet still little explored potential to provide insights into elementary evo-devo questions.