Genomic organization and developmental pattern of expression of the engrailed gene from the brine shrimp Artemia

Screwed up: Spirality of segments and other iterated structures suggest an underlying principle of seriality in bilaterians

This review deals with helicomery, that is, the specific malformation of a spiral arrangement of segments and other serial structures. Helicomery was first described in annelid and arthropod body segments. However, corresponding patterns occur in arthropod appendages and other bilaterians with serially arranged body parts, such as tapeworms, nematodes, vertebrates, and probably chitons. The specifics of the spirals such as length, orientation, and handedness are described. Most spirals are dorsal and comprise only a few loops. Helicomery is formed by a shift of cells during development or in adults caused by changes in cell adhesion or mechanical impacts such as lesions. A model for the formation of helicomery is proposed, which is based on medieval church labyrinths. These complex spiral structures are derived from concentric lines by the shift of relatively few tiles. This principle of "small causes, great effect" also applies to "spiral segments," because helicomery dissolves segmental patterns and questions the concept of segments as distinct structures. The widespread occurrence of helicomery in nonhomologous serial structures might indirectly indicate an underlying principle of seriality among Bilateria.

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.

Deciphering the onychophoran 'segmentation gene cascade': Gene expression reveals limited involvement of pair rule gene orthologs in segmentation, but a highly conserved segment polarity gene network

The hallmark of the arthropods is their segmented body, although origin of segmentation, however, is unresolved. In order to shed light on the origin of segmentation we investigated orthologs of pair rule genes (PRGs) and segment polarity genes (SPGs) in a member of the closest related sister-group to the arthropods, the onychophorans. Our gene expression data analysis suggests that most of the onychophoran PRGs do not play a role in segmentation. One possible exception is the even-skipped (eve) gene that is expressed in the posterior end of the onychophoran where new segments are likely patterned, and is also expressed in segmentation-gene typical transverse stripes in at least a number of newly formed segments. Other onychophoran PRGs such as runt (run), hairy/Hes (h/Hes) and odd-skipped (odd) do not appear to have a function in segmentation at all. Onychophoran PRGs that act low in the segmentation gene cascade in insects, however, are potentially involved in segment-patterning. Most obvious is that from the expression of the pairberry (pby) gene ortholog that is expressed in a typical SPG-pattern. Since this result suggested possible conservation of the SPG-network we further investigated SPGs (and associated factors) such as Notum in the onychophoran. We find that the expression patterns of SPGs in arthropods and the onychophoran are highly conserved, suggesting a conserved SPG-network in these two clades, and indeed also in an annelid. This may suggest that the common ancestor of lophotrochozoans and ecdysozoans was already segmented utilising the same SPG-network, or that the SPG-network was recruited independently in annelids and onychophorans/arthropods.

Variation and constraint in Hox gene evolution

Despite enormous body plan variation, genes regulating embryonic development are highly conserved. Here, we probe the mechanisms that predispose ancient regulatory genes to reutilization and diversification rather than evolutionary loss. The Hox gene fushi tarazu (ftz) arose as a homeotic gene but functions as a pair-rule segmentation gene in Drosophila. ftz shows extensive variation in expression and protein coding regions but has managed to elude loss from arthropod genomes. We asked what properties prevent this loss by testing the importance of different protein motifs and partners in the developing CNS, where ftz expression is conserved. Drosophila Ftz proteins with mutated protein motifs were expressed under the control of a neurogenic-specific ftz cis-regulatory element (CRE) in a ftz mutant background rescued for segmentation defects. Ftz CNS function did not require the variable motifs that mediate differential cofactor interactions involved in homeosis or segmentation, which vary in arthropods. Rather, CNS function did require the shared DNA-binding homeodomain, which plays less of a role in Ftz segmentation activity. The Antennapedia homeodomain substituted for Ftz homeodomain function in the Drosophila CNS, but full-length Antennapedia did not rescue CNS defects. These results suggest that a core CNS function retains ftz in arthropod genomes. Acquisition of a neurogenic CRE led to ftz expression in unique CNS cells, differentiating its role from neighboring Hox genes, rendering it nonredundant. The inherent flexibility of modular CREs and protein domains allows for stepwise acquisition of new functions, explaining broad retention of regulatory genes during animal evolution.

Decoupling elongation and segmentation: notch involvement in anostracan crustacean segmentation

Repeated body segments are a key feature of arthropods. The formation of body segments occurs via distinct developmental pathways within different arthropod clades. Although some species form their segments simultaneously without any accompanying measurable growth, most arthropods add segments sequentially from the posterior of the growing embryo or larva. The use of Notch signaling is increasingly emerging as a common feature of sequential segmentation throughout the Bilateria, as inferred from both the expression of proteins required for Notch signaling and the genetic or pharmacological disruption of Notch signaling. In this study, we demonstrate that blocking Notch signaling by blocking γ-secretase activity causes a specific, repeatable effect on segmentation in two different anostracan crustaceans, Artemia franciscana and Thamnocephalus platyurus. We observe that segmentation posterior to the third or fourth trunk segment is arrested. Despite this marked effect on segment addition, other aspects of segmentation are unaffected. In the segments that develop, segment size and boundaries between segments appear normal, engrailed stripes are normal in size and alignment, and overall growth is unaffected. By demonstrating Notch involvement in crustacean segmentation, our findings expand the evidence that Notch plays a crucial role in sequential segmentation in arthropods. At the same time, our observations contribute to an emerging picture that loss-of-function Notch phenotypes differ significantly between arthropods suggesting variability in the role of Notch in the regulation of sequential segmentation. This variability in the function of Notch in arthropod segmentation confounds inferences of homology with vertebrates and lophotrochozoans.

Surprising flexibility in a conserved Hox transcription factor over 550 million years of evolution

Although metazoan body plans are remarkably diverse, the structure and function of many embryonic regulatory genes are conserved because large changes would be detrimental to development. However, the fushi tarazu (ftz) gene has changed dramatically during arthropod evolution from Hox-like to a pair-rule segmentation gene in Drosophila. Changes in both expression and protein sequence contributed to this new function: ftz expression switched from Hox-like to stripes and changes in Ftz cofactor interaction motifs led to loss of homeotic and gain of segmentation potential. Here, we reconstructed ftz changes in a rigorous phylogenetic context. We found that ftz did not simply switch from Hox-like to segmentation function; rather, ftz is remarkably labile, having undergone multiple changes in sequence and expression. The segmentation LXXLL motif was stably acquired in holometabolous insects after the appearance of striped expression in early insect lineages. The homeotic YPWM motif independently degenerated multiple times. These "degen-YPWMs" showed varying degrees of homeotic potential when expressed in Drosophila, suggesting variable loss of Hox function in different arthropods. Finally, the intensity of ftz Hox-like expression decreased to marginal levels in some crustaceans. We propose that decreased expression levels permitted ftz variants to arise and persist in populations without disadvantaging organismal development. This process, in turn, allowed evolutionary transitions in protein function, as weakly expressed "hopeful gene variants" were coopted into alternative developmental pathways. Our findings show that variation of a pleiotropic transcription factor is more extensive than previously imagined, suggesting that evolutionary plasticity may be widespread among regulatory genes.

Parasegmental appendage allocation in annelids and arthropods and the homology of parapodia and arthropodia

The new animal phylogeny disrupts the traditional taxon Articulata (uniting arthropods and annelids) and thus calls into question the homology of the body segments and appendages in the two groups. Recent work in the annelid Platynereis dumerilii has shown that although the set of genes involved in body segmentation is similar in the two groups, the body units of annelids correspond to arthropod parasegments not segments. This challenges traditional ideas about the homology of "segmental" organs in annelids and arthropods, including their appendages. Here I use the expression of engrailed, wingless and Distal-less in the arthropod Artemia franciscana to identify the parasegment boundary and the appendage primordia. I show that the early body organization including the appendage primordia is parasegmental and thus identical to the annelid organization and by deriving the different adult appendages from a common ground plan I suggest that annelid and arthropod appendages are homologous structures despite their different positions in the adult animals. This also has implications for the new animal phylogeny, because it suggests that Urprotostomia was not only parasegmented but also had parasegmental appendages similar to extant annelids, and that limb-less forms in the Protostomia are derived from limb-bearing forms.

Expression patterns of engrailed and dpp in the gastropod Lymnaea stagnalis

We isolated the full-length cDNAs of engrailed and dpp-BMP2/4 orthologues from the pond snail Lymnaea stagnalis and examined their expression patterns during development by the whole mount in situ hybridization. At the gastrula and trochophore stages, engrailed is expressed in the peripheral ectoderm of the presumptive and invaginating shell gland, corroborating its role in the shell formation that is widely conserved among molluscs. At the same stages, dpp-BMP2/4 is expressed in the right-hand side ectoderm of the shell gland and in the invaginating stomodaeum. Unlike in the gastropod Patella vulgata, our results suggested that dpp-BMP2/4 has a role in the shell formation, rather than in the regional specification and that it could be involved in the specification pathway of the left-right asymmetry of the developing shell in L. stagnalis.

Segmental expression of Pax3/7 and engrailed homologs in tardigrade development

How morphological diversity arises through evolution of gene sequence is a major question in biology. In Drosophila, the genetic basis for body patterning and morphological segmentation has been studied intensively. It is clear that some of the genes in the Drosophila segmentation program are functioning similarly in certain other taxa, although many questions remain about when these gene functions arose and which taxa use these genes similarly to establish diverse body plans. Tardigrades are an outgroup to arthropods in the Ecdysozoa and, as such, can provide insight into how gene functions have evolved among the arthropods and their close relatives. We developed immunostaining methods for tardigrade embryos, and we used cross-reactive antibodies to investigate the expression of homologs of the pair-rule gene paired (Pax3/7) and the segment polarity gene engrailed in the tardigrade Hypsibius dujardini. We find that in H. dujardini embryos, Pax3/7 protein localizes not in a pair-rule pattern but in a segmentally iterated pattern, after the segments are established, in regions of the embryo where neurons later arise. Engrailed protein localizes in the posterior ectoderm of each segment before ectodermal segmentation is apparent. Together with previous results from others, our data support the conclusions that the pair-rule function of Pax3/7 is specific to the arthropods, that some of the ancient functions of Pax3/7 and Engrailed in ancestral bilaterians may have been in neurogenesis, and that Engrailed may have a function in establishing morphological boundaries between segments that is conserved at least among the Panarthropoda.

Knockdown of spalt function by RNAi causes de-repression of Hox genes and homeotic transformations in the crustacean Artemia franciscana

Hox genes play a central role in the specification of distinct segmental identities in the body of arthropods. The specificity of Hox genes depends on their restricted expression domains, their interaction with specific cofactors and selectivity for particular target genes. spalt genes are associated with the function of Hox genes in diverse species, but the nature of this association varies: in some cases, spalt collaborates with Hox genes to specify segmental identities, in others, it regulates Hox gene expression or acts as their target. Here we study the role of spalt in the branchiopod crustacean Artemia franciscana. We find that Artemia spalt is expressed in the pre-segmental 'growth zone' and in stripes in each of the trunk (thoracic, genital and post-genital) segments that emerge from this zone. Using RNA interference (RNAi), we show that knocking down the expression of spalt has pleiotropic effects, which include thoracic to genital (T-->G), genital to thoracic (G-->T) and post-genital to thoracic (PG-->T) homeotic transformations. These transformations are associated with a stochastic de-repression of Hox genes in the corresponding segments of RNAi-treated animals (AbdB for T-->G and Ubx/AbdA for G-->T and PG-->T transformations). We discuss a possible role of spalt in the maintenance of Hox gene repression in Artemia and in other animals.

Expression of 'segmentation' genes during larval and juvenile development in the polychaetes Capitella sp. I and H. elegans

Polychaete annelids and arthropods are both segmented protostome invertebrates. To investigate whether the segmented body plan of these two phyla share a common molecular ground pattern, we report the developmental expression of orthologues of the arthropod segment polarity genes engrailed (en), hedgehog (hh), and wingless (wg/Wnt1) in larval and juvenile stages of the polychaete annelid Capitella sp. I and en in a second polychaete, Hydroides elegans. Temporally, neither Wnt1 nor hh are detected in the segmented region of the larval body until after morphological segmentation is apparent. Expression of CapI-Wnt1 is limited to a ring of ectoderm marking the future anus during larval segmentation. CapI-hh is expressed in a ring of the hindgut internal to that of CapI-Wnt1, as well as in a subset of ventral nerve cord neurons, anterior gut tissue, and mesoderm. In both H. elegans and Capitella sp. I, en is expressed in a spatially and temporally dynamic manner in segmentally iterated structures as well as a population of cells that migrate internally from ectoderm to mesoderm, possibly representing a population of ecto-mesodermal precursors. Significantly, the expression patterns we report for wg, en, and hh orthologues in Capitella sp. I and for en in larval development of H. elegans are not comparable to the highly conserved ectodermal segment polarity pattern observed in arthropods at any life history stage, consistent with distinct origins of segmentation between annelids and arthropods.

Pax3/7 genes reveal conservation and divergence in the arthropod segmentation hierarchy

Several features of Pax3/7 gene expression are shared among distantly related insects, including pair-rule, segment polarity, and neural patterns. Recent data from arachnids imply that roles in segmentation and neurogenesis are likely to be played by Pax3/7 genes in all arthropods. To further investigate Pax3/7 genes in non-insect arthropods, we isolated two monoclonal antibodies that recognize the products of Pax3/7 genes in a wide range of taxa, allowing us to quickly survey Pax3/7 expression in all four major arthropod groups. Epitope analysis reveals that these antibodies react to a small subset of Paired-class homeodomains, which includes the products of all known Pax3/7 genes. Using these antibodies, we find that Pax3/7 genes in crustaceans are expressed in an early broad and, in one case, dynamic domain followed by segmental stripes, while myriapods and chelicerates exhibit segmental stripes that form early in the posterior-most part of the germ band. This suggests that Pax3/7 genes acquired their role in segmentation deep within, or perhaps prior to, the arthropod lineage. However, we do not detect evidence of pair-rule patterning in either myriapods or chelicerates, suggesting that the early pair-rule expression pattern of Pax3/7 genes in insects may have been acquired within the crustacean-hexapod lineage.

Expression of hunchback during trunk segmentation in the branchiopod crustacean Artemia franciscana

Comparative studies have shown that some aspects of segmentation are widely conserved among arthropods. Yet, it is still unclear whether the molecular prepatterns that are required for segmentation in Drosophila are likely to be similarly conserved in other arthropod groups. Homologues of the Drosophila gap genes, like hunchback, show regionally restricted expression patterns during the early phases of segmentation in diverse insects, but their expression patterns in other arthropod groups are not yet known. Here, we report the cloning of a hunchback orthologue from the crustacean Artemia franciscana and its expression during the formation of trunk segments. Artemia hunchback is expressed in a series of segmental stripes that correspond to individual thoracic/trunk, genital, and postgenital segments. However, this expression is not associated with the segmenting ectoderm but is restricted to mesodermal cells that associate with the ectoderm in a regular metameric pattern. All cells in the early segmental mesoderm appear to express hunchback. Later, mesodermal expression fades, and a complex expression pattern appears in the central nervous system (CNS), which is comparable to hunchback expression in the CNS of insects. No regionally restricted expression, reminiscent of gap gene expression, is observed during trunk segmentation. These patterns suggest that the expression patterns of hunchback in the mesoderm and in the CNS are likely to be ancient and conserved among crustaceans and insects. In contrast, we find no evidence for a conserved role of hunchback in axial patterning in the trunk ectoderm.

Segments and parasegments in arthropods: a functional perspective

I review how both the parasegmental and segmental frames are used in constructing the body plan of the arthropods. The parasegment is the primary genetic unit, as shown by Hox gene expression, and the parasegmental design is maintained in the nerve cord. It is, however, not maintained in the epidermis, where the cuticle grooves are segmental, and in the musculature, which is segmental in organisation. This frame shift is reflected in the sensory and motor nerve connections between the ganglia and the periphery. I suggest that the need for movement in an organism equipped with a hard exoskeleton was the functional constraint that shaped this apparently complex mode of development.

Playing by pair-rules?

Although in Drosophila pair-rule genes play crucial roles in the genetic hierarchy that subdivides the embryo into segments, the extent to which pair-rule patterning is utilized by different arthropods and other segmented phyla is unknown. Recent data of Dearden et al.1 and Henry et al.,2 however, hint that a pair-rule mechanism might play a role in the segmentation process of basal arthropods and vertebrates.

Expression of the Artemia trachealess gene in the salt gland and epipod

The Drosophila trachealess gene encodes a basic-helix-loop-helix-PAS transcription factor that controls the formation of the trachea and salivary duct. An ortholog of trachealess was identified in the brine shrimp, Artemia franciscana, and was shown to be highly conserved by sequence identity. Expression of Artemia trachealess was observed at two sites during development: the naupliar salt gland and the juvenile thoracic epipod. These two organs function at their respective times of development in osmoregulation, an important aspect of brine shrimp physiology. This extends the range of putative functions of trachealess to include formation of osmoregulatory, respiratory, and ductile organs.

Expression of Patella vulgata orthologs of engrailed and dpp-BMP2/4 in adjacent domains during molluscan shell development suggests a conserved compartment boundary mechanism

The engrailed gene is well known from its role in segmentation and central nervous system development in a variety of species. In molluscs, however, engrailed is involved in shell formation. So far, it seemed that engrailed had been co-opted uniquely for this particular process in molluscs. Here, we show that, in the gastropod mollusc Patella vulgata, an engrailed ortholog is expressed in the edge of the embryonic shell and in the anlage of the apical sensory organ. Surprisingly, a dpp-BMP2/4 ortholog is expressed in cells of the ectoderm surrounding, but not overlapping, the engrailed-expressing shell-forming cells. It is also expressed in the anlage of the eyes. Earlier it was shown that a compartment boundary exists between the cells of the embryonic shell and the adjacent ectoderm. We conclude that engrailed and dpp are most likely involved in setting up a compartment boundary between these cells, very similar to the situation in, for example, the developing wing imaginal disc in Drosophila. We suggest that engrailed became involved in shell formation because of its ancestral role, which is to set up compartment boundaries between embryonic domains.

Formation of the head-trunk boundary in the animal body plan: an evolutionary perspective

Gene expression analyses and anatomical studies suggest that the body plans of protostomes and deuterostomes are phylogenetically related. In the central nervous system (CNS), arthropods and vertebrates (as well as their closest related phyla the urochordates and cephalochordates) share a nerve cord with rostral specification: the cerebral neuromeres in Drosophila, cerebral sensory vesicle of ascidians and lancelets and the large brain of craniates. Homologous genes, in particular of the otd/Otx and Hox families, are at play in these species to specify the anterior and posterior CNS territories, respectively. In contrast, homologies in the establishment of boundary regions like those separating head and trunk structures in arthropods or mid- and hindbrain domains in chordates are still unclear. We compare in these species the formation, properties and molecular characteristics of these boundaries during embryonic development. We also discuss recent findings suggesting that insects and vertebrates might have co-opted factors of related families to control the formation of these boundary regions, the evolution of which would then appear dramatically different from that of the anterior and posterior CNS domains.

Short, long, and beyond: molecular and embryological approaches to insect segmentation

Over the past dozen years, studies comparing the expression of orthologues of the Drosophila segmentation genes among various insects have served to broaden our view of the ways in which insects make segments. The molecular data suggest that, although the overall genetic mechanisms of segmentation during embryogenesis have been conserved, the details of this process vary both within and between various insect orders. Here we summarize comparative gene expression data relevant to segmentation with an emphasis on understanding the extent of molecular patterning prior to gastrulation. These results are discussed in embryological context with an eye toward understanding the evolution of segmentation within insects.

Developmental modularity and the evolutionary diversification of arthropod limbs

Segmentation is one of the most salient characteristics of arthropods, and differentiation of segments along the body axis is the basis of arthropod diversification. This article evaluates whether the evolution of segmentation involves the differentiation of already independent units, i.e., do segments evolve as modules? Because arthropod segmental differentiation is commonly equated with differential character of appendages, we analyze appendages by comparing similarities and differences in their development. The comparison of arthropod limbs, even between species, is a comparison of serially repeated structures. Arthropod limbs are not only reiterated along the body axis, but limbs themselves can be viewed as being composed of reiterated parts. The interpretation of such reiterated structures from an evolutionary viewpoint is far from obvious. One common view is that serial repetition is evidence of a modular organization, i.e., repeated structures with a common fundamental identity that develop semi-autonomously and are free to diversify independently. In this article, we evaluate arthropod limbs from a developmental perspective and ask: are all arthropod limbs patterned using a similar set of mechanisms which would reflect that they all share a generic coordinate patterning system? Using Drosophila as a basis for comparison, we find that appendage primordia, positioned along the body using segmental patterning coordinates, do indeed have elements of common identity. However, we do not find evidence of a single coordinate system shared either between limbs or among limb branches. Data concerning the other diagnostic of developmental modularity--semi-autonomy of development--are not currently available for sufficient taxa. Nonetheless, some data comparing patterns of morphogenesis provide evidence that limbs cannot always be temporally or spatially decoupled from the development of their neighbors, suggesting that segment modularity is a derived character.

The spatial and temporal expression of Ch-en, the engrailed gene in the polychaete Chaetopterus, does not support a role in body axis segmentation

We are interested in understanding whether the annelids and arthropods shared a common segmented ancestor and have approached this question by characterizing the expression pattern of the segment polarity gene engrailed (en) in a basal annelid, the polychaete Chaetopterus. We have isolated an en gene, Ch-en, from a Chaetopterus cDNA library. Genomic Southern blotting suggests that this is the only en class gene in this animal. The predicted protein sequence of the 1.2-kb cDNA clone contains all five domains characteristic of en proteins in other taxa, including the en class homeobox. Whole-mount in situ hybridization reveals that Ch-en is expressed throughout larval life in a complex spatial and temporal pattern. The Ch-en transcript is initially detected in a small number of neurons associated with the apical organ and in the posterior portion of the prototrochophore. At later stages, Ch-en is expressed in distinct patterns in the three segmented body regions (A, B, and C) of Chaetopterus. In all segments, Ch-en is expressed in a small set of segmentally iterated cells in the CNS. In the A region, Ch-en is also expressed in a small group of mesodermal cells at the base of the chaetal sacs. In the B region, Ch-en is initially expressed broadly in the mesoderm that then resolves into one band/segment coincident with morphological segmentation. The mesodermal expression in the B region is located in the anterior region of each segment, as defined by the position of ganglia in the ventral nerve cord, and is involved in the morphogenesis of segment-specific feeding structures late in larval life. We observe banded mesodermal and ectodermal staining in an anterior-posterior sequence in the C region. We do not observe a segment polarity pattern of expression of Ch-en in the ectoderm, as is observed in arthropods.

Phenotypic and dynamical transitions in model genetic networks. II. Application to the evolution of segmentation mechanisms

Knowledge of the genetic control of segmentation in Drosophila has made insect segmentation a paradigmatic case in the study of the evolution of developmental mechanisms. In Drosophila, the patterns of expression of segmentation genes are established simultaneously in all segments by a complex set of interactions between transcriptional factors that diffuse in a syncytium occupying the whole embryo. Such mechanisms cannot act in short germ-band insects where segments appear sequentially from a cellularized posterior proliferative zone. Here, we compare mechanisms of segmentation in different organisms and discuss how the transition between the different types of segmentation can be explained by small and progressive changes in the underlying gene networks. The recent discovery of a temporal oscillation in expression during somitogenesis of vertebrate homologs of the pair-rule gene hairy enhances the plausibility of an earlier proposal that the evolutionary origin of both the short- and long germ-band modes of segmentation was an oscillatory genetic network (Newman 1993). An implication of this scenario is that the self-organizing, pattern-forming system embodied in an oscillatory network operating in the context of a syncytium (i.e., a reaction-diffusion system)-which is hypothesized to have originated the simultaneous mode of segmentation-must have been replaced by the genetic hierarchy seen in modern-day Drosophila over the course of evolution. As demonstrated by the simulations in the accompanying article, the tendency for "emergent" genetic networks, associated with self-organizing processes, to be replaced through natural selection with hierarchical networks is discussed in relation to the evolution of segmentation.

Characterization of a novel heterodimeric cathepsin L-like protease and cDNA encoding the catalytic subunit of the protease in embryos of Artemia franciscana

Embryos and larvae of the brine shrimp, Artemia franciscana, contain a novel cathepsin L-like cysteine protease (ACP) composed of 28.5- and 31.5-kDa subunits. Both subunits of the ACP are glycosylated, and seven isoforms of the protease were identified by isoelectric focusing with pI values ranging from 4.6 to 6.2. Several clones containing sequences coding for the 28.5-kDa subunit of the ACP were isolated from an Artemia embryo cDNA library in lambda ZAP II. One clone of 1229 bp, with an open reading frame of 1014 bp, was sequenced and found to contain 50-65% amino acid sequence identity with several members of the cathepsin L subfamily of cysteine proteases. The mature protein predicted from this sequence consisted of 217 amino acids with a mass of 23.5 kDa prior to post-translational modifications. The mature protein showed 68.6% amino acid sequence identity with human cathepsin L and 73.9% identity with cathepsin L-like proteases from Sarcophaga. peregrina and Drosophila melanogaster. The full-length cDNA clone analyzed in this study (pCP-3b) was renamed AFCATL1 (A. franciscana Cathepsin L1) and the sequence has been deposited in the Genbank database, accession number AF147207. Northern blot analyses identified a single transcript of about 1.4 kb in both embryos and young larvae of Artemia. Southern blot analyses of Artemia genomic DNA treated with various restriction endonucleases indicated a single gene for the ACP. The catalytic subunit of the ACP was tightly associated with a 31.5-kDa protein, which may localize the protease to nonlysosomal sites in embryos and larvae.Key words: cathepsin L, proteases, embryos, development, Artemia.

An amphioxus Emx homeobox gene reveals duplication during vertebrate evolution

Members of the Emx homeobox gene class are expressed during embryogenesis in the brain and/or other head structures of phylogenetically diverse phyla. Here, we describe sequence, genomic structure, and molecular phylogenetic analysis of a cephalochordate (amphioxus) Emx class gene termed AmphiEmxA. The genomic structure of AmphiEmxA is very similar to that of vertebrate Emx genes, with two conserved intron sites. The Drosophila homolog empty spiracles (ems) has just one intron, which may be shared with chordates; the other has been secondarily lost in this Drosophila gene and in a cnidarian Emx-related gene. We identify a highly conserved peptide motif close to the amino terminus of Emx proteins, demonstrate its similarity to a sequence found in a variety of transcription factors, and argue that it arose through convergent evolution in homeobox and forkhead genes. Finally, our molecular phylogenetic analysis strongly supports the presence of a single Emx gene in the ancestor of chordates and gene duplication along the vertebrate lineage.

High DNA sequence variability at the alpha 1 Na/K-ATPase locus of Artemia franciscana (brine shrimp): polymorphism in a gene for salt-resistance in a salt-resistant organism

We previously reported that the Na/K-ATPase alpha 1 subunit coding gene showed signs of being a very polymorphic locus in Artemia franciscana. This species is adapted to highly saline waters, and the Na/K-ATPase alpha 1 isoform presumably plays a key role in this adaptation. Therefore, we were interested in further study of the alpha 1 Na/K-ATPase polymorphisms to examine whether they might be due to an adaptation to salt resistance driven by natural selection. Using coding sequences from 10 genomic clones and 3 cDNAs, we observed that most substitutions are in synonymous positions (88.8%). The 12 nonsynonymous substitutions code for conservative amino acid replacements with an apparent scattered distribution across functional domains of the protein. Interspecific comparison between these sequences and two genomic clones from Artemia parthenogenetica containing 1,122 bp of the alpha 1 Na/K-ATPase locus coding sequence showed independence of the synonymous/nonsynonymous ratio in the comparison within A. franciscana and between A. franciscana and A. parthenogenetica, which fits the neutral model of evolution. Since there were no previous studies on DNA polymorphism for other A. franciscana genes, we also studied variability at the Actin 302 locus for comparison. Both loci were amplified by reverse transcription-polymerase chain reaction, and 20 sequences were obtained for each. This study shows that the amplified region of the alpha 1 Na/K-ATPase gene is 3.5 times as polymorphic as the Actin 302 gene and 2.9 times as heterozygotic. Interestingly, under a model of neutral evolution, the data observed would be expected with a probability of approximately 0.05, suggesting an excess of intraspecific variation of alpha 1 Na/K-ATPase with respect to Actin 302. Restriction fragment length polymorphism studies show similar patterns of polymorphism along the approximately 41-kb span of the alpha 1 Na/K-ATPase locus. Most of the nucleotide differences are linked in a few haplotypes, although recombination events are also inferred from the data. We propose a possible explanation for the high polymorphic levels at the alpha 1 Na/K-ATPase locus which invokes positive selection acting tightly to the locus in transiently isolated or semi-isolated subpopulations.

APH-1, a POU homeobox gene expressed in the salt gland of the crustacean Artemia franciscana

We characterized the first POU-homeoprotein in a crustacean (designated APH-1 for Artemia POU-Homeoprotein, EMBL Y15070). The amino acid sequence of the APH-1 POU-domain is identical, except for two residues, to that of the two class III POU proteins Cf1-a (Drosophila) and POU-M1 (Bombyx mori). Southern blot analysis suggests that crustaceans have only one class III POU gene. RT-PCR and whole-mount in situ hybridization show that APH-1 mRNA is present in larvae specifically in the salt gland, an organ which is involved in osmoregulation, and disappears in the adult.

PCR cloning and expression of the molt-inhibiting hormone gene for the crab (Charybdis feriatus)

A PCR-based genomic DNA walking technique was used to clone the gene for the molt-inhibiting hormone of the crab, Charybdis feriatus. Several overlapping genomic clones were isolated, and the MIH gene for the crab was reconstructed. DNA sequence determination of the overlapping clone reveals that the MIH gene spans 4.3kb and consists of three exons and two introns. Exons 1 and 2 carry a coding sequence for the signal peptide, and exons 2 and 3 consist of coding sequence for the mature peptide. The exon-intron boundary of the crab MIH gene also follows the 'GT-AG rule' for the splice donor and acceptor. The deduced amino acid sequence of MIH shows the highest overall similarity to those of the crabs, Callinectes sapidus and Carcinus maenas, and the gonad-inhibiting hormone (GIH) of the lobster. The putative polyadenylation signal is approximately 1.0kb 3' downstream of the termination codon (TGA). Genomic Southern blot analysis indicates that few genomic fragments were hybridized to the cDNA probe. The 5' flanking region contains a putative promoter with several putative cis elements similar to some vertebrate neuropeptide genes. The 530-bp flanking region was subcloned separately to two promoterless reporter plasmids carrying either the Green Fluorescent Protein gene (GFP) or the Choramphenicol Acetyltransferase gene (CAT). The DNA constructs were transfected into insect cells (Sf21) and mouse pituitary cells (GH4ZR7), respectively. Green fluorescent protein was detected in some of the transfected insect cells, and expression of the CAT was detected in cells transfected with DNA constructs containing the crab promoter. By RT-PCR, MIH transcripts can be detected in the eyestalk of shrimp in intermolt, early premolt, late premolt stages and females that brood their eggs. It can also be found in the brain, but not in the ovary, hepatopancreas, muscle and epidermis. During early larval development, MIH mRNA can be detected in the pre-hatched and the newly hatched larvae. Unlike the adult, the expression of the MIH in the larvae is exclusively in the brain.

A genetic and molecular analysis of an invectedDominant mutation in Drosophila melanogaster

The invected gene of Drosophila melanogaster is a homeobox-containing gene that is closely related to engrailed. A dominant gain of function allele, invectedDominant, was derived from mutagenesis of a dominant allele of vestigial, In(2R)vgW. A careful analysis of the phenotype of invectedDominant shows that it is associated with a transformation of the anterior compartment of the wing to a posterior fate. This transformation is normally limited to the wing blade itself and does not involve the remaining tissues derived from the wing imaginal disc, including the wing hinge and dorsal thorax of the fly. The ectopic expression of invected protein associated with invectedDominant correlates spatially with the normal expression pattern of vestigial in the wing imaginal disc, suggesting that control elements of vestigial are driving ectopic invected expression. This was confirmed by sequence analysis that shows that the dominant vestigial activity was eliminated by a deletion that removes the 3' portion of the vestigial coding region. This leaves a gene fusion wherein the vestigial enhancer elements are still juxtaposed immediately 5' to the invected transcriptional start site, but with the vg sequences harboring an additional lesion. Unlike recessive invected alleles, the invectedDominant allele produces an observable phenotype, and as such should prove useful in determining the role of invected in patterning the wing imaginal disc. Genetic analysis has shown that mutations of polyhomeotic, a gene involved in regulating engrailed expression, cause a reproducible alteration in the invectedDominant phenotype. Finally, the invectedDominant allele should prove valuable for identifying and characterizing genes that are activated within the posterior compartment. A screen using various lacZ lines that are asymmetrically expressed in an anterior-posterior manner in the wing imaginal disc isolated one line that shows posterior-specific expression within the transformed anterior compartment.

Insect evolution: how did insect wings originate?

Recent developmental studies aimed at elucidating the evolutionary origin of insect wings highlight the difficulties of identifying homology between dramatically different structures.

Sequence and embryonic expression of the amphioxus engrailed gene (AmphiEn): the metameric pattern of transcription resembles that of its segment-polarity homolog in Drosophila

Vertebrate segmentation has been proposed as an evolutionary inheritance either from some metameric protostome or from a more closely related deuterostome. To address this question, we studied the developmental expression of AmphiEn, the engrailed gene of amphioxus, the closest living invertebrate relative of the vertebrates. In neurula embryos of amphioxus, AmphiEn is expressed along the anteroposterior axis as metameric stripes, each located in the posterior part of a nascent or newly formed segment. This pattern resembles the expression stripes of the segment-polarity gene engrailed, which has a key role in establishing and maintaining the metameres in embryos of Drosophila and other metameric protostomes. Later, amphioxus embryos express AmphiEn in non-metameric patterns - transiently in the embryonic ectoderm and dorsal nerve cord. Nerve cord expression occurs in a few cells approximately midway along the rostrocaudal axis and also in a conspicuous group of anterior cells in the cerebral vesicle at a level previously identified as corresponding to the vertebrate diencephalon. Compared to vertebrate engrailed expression at the midbrain/hindbrain boundary, AmphiEn expression in the cerebral vesicle is relatively late. Thus, it is uncertain whether the cerebral vesicle expression marks the rostral end of the amphioxus hindbrain; if it does, then amphioxus may have little or no homolog of the vertebrate midbrain. The segmental expression of AmphiEn in forming somites suggests that the functions of engrailed homologs in establishing and maintaining a metameric body plan may have arisen only once during animal evolution. If so, the protostomes and deuterostomes probably shared a common segmented ancestor.

Insect—crustacean relationships: insights from comparative developmental and molecular studies

The phylogenetic relationships between the major arthropod groups are still far from being resolved. Phylogenetic analyses have usually relied on detailed morphological comparisons which are confounded by the extensive occurrence of convergence. We examine the available morphological evidence in the light of recent comparative developmental and molecular studies and suggest ways in which genetic-developmental information could help assess homology and overcome the problem of convergence. On the basis of such considerations we support the common origin of crustaceans and insects from a crustaceanlike mandibulate ancestor. Focusing on the specific relationships between crustaceans, myriapods and insects, we suggest that insects could emerge from this crustacean-like ancestor independently from myriapods, and after the major crustacean radiations.

Hox genes and the diversification of insect and crustacean body plans

Crustaceans and insects share a common origin of segmentation, but the specialization of trunk segments appears to have arisen independently in insects and various crustacean subgroups. Such macroevolutionary changes in body architecture may be investigated by comparative studies of conserved genetic markers. The Hox genes are well suited for this purpose, as they determine positional identity along the body axis in a wide range of animals. Here we examine the expression of four Hox genes in the branchiopod crustacean Artemia franciscana, and compare this with Hox expression patterns from insects. In Artemia the three 'trunk' genes Antp, Ubx and abdA are expressed in largely overlapping domains in the uniform thoracic region, whereas in insects they specify distinct segment types within the thorax and abdomen. Our comparisons suggest a multistep process for the diversification of these Hox gene functions, involving early differences in tissue specificity and the later acquisition of a role in defining segmental differences within the trunk. We propose that the branchiopod thorax may be homologous to the entire pregenital (thoracic and abdominal) region of the insect trunk.

Regionalization and segmentation of the leech

Regionalization and segmentation of the leech body plan have been examined by numerous approaches over the years. A wealth of knowledge has accumulated regarding the normally invariant cell lineages of the leech and the degree of developmental plasticity that is possible in each cell line in early development and in neurogenesis. Homologues of genes that control regionalization and segmentation in Drosophila have been cloned from the leech and the expression patterns reveal conserved features with those in Drosophila and other organisms. Possible developmental functions of the en-class proteins in spatial and temporal modes of segment formation are discussed in light of leech and Drosophila development. Annelida and Arthropoda cell lineages of engrailed-class gene expression are compared in leech blast cell clones and crustacean parasegments. In addition, future directions for molecular analysis of segmentation of the leech are summarized.

Homologues of the engrailed gene from five molluscan classes

We used the polymerase chain reaction (PCR) to amplify, clone, and sequence 10 engrailed homeodomains from 8 species in the five major molluscan classes, including the serially organized chiton (Polyplacophora) lineage. The Drosophila melanogaster gene engrailed (en) is one of several genes involved in embryonic segment polarity determination. Studies of engrailed sequence and expression in molluscs are of interest due to questions regarding the evolution and homology of segmentation in these taxa. Nucleotide and deduced amino acid sequence comparisons reflect evolutionary conservation within helices of the en homeodomain and ancient divergences in the region 3' to the homeodomain.

Insect segmentation. A glance posterior

Segmentation gene expression patterns can be radically different in some short-germ and long-germ insects, but other types of short/intermediate-germ insects may use Drosophila-like segmentation mechanisms.

Mesendoderm cell and archenteron formation in isolated blastomeres from the shrimp Sicyonia ingentis

The fate map of 2- and 4-cell-stage Sicyonia ingentis embryos was determined by microinjection of lysyl-rhodamine-dextran into single blastomeres. Microinjected embryos were cultured to the limb bud stage, when the body plan of the nauplius larva was evident. The animal blastomere, AB, gave rise to anterior ectoderm, while the vegetal blastomere, CD, gave rise to posterior structures, including the invagination site during gastrulation. The A blastomere gave rise to mirror-image patterns of dorsal-lateral ectoderm, while the B blastomere gave rise to anterior, ventral ectoderm. The C blastomere gave rise to posterior, dorsal-lateral ectoderm, complementary to the A pattern, as well as some naupliar mesoderm. The D blastomere gave rise to mesendoderm, naupliar mesoderm, and some posterior ectoderm. To study the specification of the early blastomeres, they were microsurgically separated and cultured in isolation. Two mesendoderm cells formed in 1/2, 1/4, 1/8, and 1/16 blastomeres in embryos dissociated at the 2-, 4-, 8-, and 16-cell stages, respectively. CD and D blastomeres could be distinguished by their larger size and gave rise to the mesendoderm cells. Archenteron formation and elongation of the embryo occurred in CD but not in AB isolates. Isolated blastomeres were recombined in various ways to determine whether their state of commitment could be altered in different cellular environments. Duplicated mesendoderm cells and archenterons formed in CD + CD recombinations, while AB + AB recombinations formed blastulae but did not produce mesendoderm cells and did not invaginate. The normal number of mesendoderm cells and a single archenteron formed in D + AB recombinations, while C + AB recombinations remained as blastulae and did not form mesendoderm cells. The results suggest that the mesendoderm cells are autonomously specified, possibly by cytoplasmic localization at the vegetal pole. The mesendoderm may also function as a signaling region to organize other developmental events.

Embryonic expression of the single Tribolium engrailed homolog

We have cloned and sequenced the single Tribolium homolog of the Drosophila engrailed gene. The predicted protein contains a homeobox and several domains conserved among all engrailed genes identified to date. In addition it contains several features specific to the invected homologs of Bombyx and Drosophila, indicating that these features most likely were present in the ancestral gene in the common ancestor of holometobolous insects. We used the cross-reacting monoclonal antibody, 4D9, to follow the expression of the Engrailed protein during segmentation in Tribolium embryos. As in other insects, Engrailed accumulates in the nuclei of cells along the posterior margin of each segment. The first Engrailed stripe appears as the embryonic rudiment condenses. Then as the rudiment elongates into a germ band, Engrailed stripes appear in an anterior to posterior progression, just prior to morphological evidence of the formation of each segment. As in Drosophila (a long germ insect), expression of engrailed in Tribolium (classified as a short germ insect) is preceded by the expression of several homologous segmentation genes, suggesting that similar genetic regulatory mechanisms are shared by diverse developmental types.