Conservation and divergence in molecular mechanisms of axis formation

Candidate gene screen for potential interaction partners and regulatory targets of the Hox gene labial in the spider Parasteatoda tepidariorum

The Hox gene labial (lab) governs the formation of the tritocerebral head segment in insects and spiders. However, the morphology that results from lab action is very different in the two groups. In insects, the tritocerebral segment (intercalary segment) is reduced and lacks appendages, whereas in spiders the corresponding segment (pedipalpal segment) is a proper segment including a pair of appendages (pedipalps). It is likely that this difference between lab action in insects and spiders is mediated by regulatory targets or interacting partners of lab. However, only a few such genes are known in insects and none in spiders. We have conducted a candidate gene screen in the spider Parasteatoda tepidariorum using as candidates Drosophila melanogaster genes known to (potentially) interact with lab or to be expressed in the intercalary segment. We have studied 75 P. tepidariorum genes (including previously published and duplicated genes). Only 3 of these (proboscipedia-A (pb-A) and two paralogs of extradenticle (exd)) showed differential expression between leg and pedipalp. The low success rate points to a weakness of the candidate gene approach when it is applied to lineage specific organs. The spider pedipalp has no counterpart in insects, and therefore relying on insect data apparently cannot identify larger numbers of factors implicated in its specification and formation. We argue that in these cases a de novo approach to gene discovery might be superior to the candidate gene approach.

Specification of anteroposterior axis by combinatorial signaling during Xenopus development

The specification of anteroposterior (AP) axis is a fundamental and complex patterning process that sets up the embryonic polarity and shapes a multicellular organism. This process involves the integration of distinct signaling pathways to coordinate temporal-spatial gene expression and morphogenetic movements. In the frog Xenopus, extensive embryological and molecular studies have provided major advance in understanding the mechanism implicated in AP patterning. Following fertilization, cortical rotation leads to the transport of maternal determinants to the dorsal region and creates the primary dorsoventral (DV) asymmetry. The activation of maternal Wnt/ß-catenin signaling and a high Nodal signal induces the formation of the Nieuwkoop center in the dorsal-vegetal cells, which then triggers the formation of the Spemann organizer in the overlying dorsal marginal zone. It is now well established that the Spemann organizer plays a central role in building the vertebrate body axes because it provides patterning information for both DV and AP polarities. The antagonistic interactions between signals secreted in the Spemann organizer and the opposite ventral region pattern the mesoderm along the DV axis, and this DV information is translated into AP positional values during gastrulation. The formation of anterior neural tissue requires simultaneous inhibition of zygotic Wnt and bone morphogenetic protein (BMP) signals, while an endogenous gradient of Wnt, fibroblast growth factors (FGFs), retinoic acid (RA) signaling, and collinearly expressed Hox genes patterns the trunk and posterior regions. Collectively, DV asymmetry is mostly coupled to AP polarity, and cell-cell interactions mediated essentially by the same regulatory networks operate in DV and AP patterning. For further resources related to this article, please visit the WIREs website.

Reversion of developmental mode in insects: evolution from long germband to short germband in the polyembrionic wasp Macrocentrus cingulum Brischke

Germband size in insects has played a central role in our understanding of insect patterning mechanisms and their evolution. The polarity of evolutionary change in insect patterning has been viewed so far as the unidirectional shift from the ancestral short germband patterning of basal hemimetabolous insects to the long germband patterning observed in most modern Holometabola. However, some orders of holometabolic insects display both short and long germband development, though the absence of a clear phylogenetic context does not permit definite conclusions on the polarity of change. Derived hymenoptera, that is, bees and wasps, represent a classical textbook example of long germband development. Yet, in some wasps putative short germband development has been described correlating with lifestyle changes, namely with evolution of endoparasitism and polyembryony. To address the potential reversion from long to short germband, we focused on the family Braconidae, which displays ancestral long germband development, and examined the derived polyembryonic braconid Macrocentrus cingulum. Using SEM analysis of M. cingulum embryogenesis coupled with analyses of embryonic patterning markers, we show that this wasp evolved short germband embryogenesis secondarily, in a way that is reminiscent of embryogenesis in the beetle Tribolium castaneum. This work shows that the evolution of germband size in insects is a reversible process that may correlate with other life-history traits and suggests broader implications on the mechanisms and evolvability of insect development.

Pattern association and consolidation emerges from connectivity properties between cortex and hippocampus

The basic structure of the cortico-hippocampal system is highly conserved across mammalian species. Comparatively few hippocampal neurons can represent and address a multitude of cortical patterns, establish associations between cortical patterns and consolidate these associations in the cortex. In this study, we investigate how elementary anatomical properties in the cortex-hippocampus loop along with synaptic plasticity contribute to these functions. Specifically, we focus on the high degree of connectivity between cortex and hippocampus leading to converging and diverging forward and backward projections and heterogenous synaptic transmission delays that result from the detached location of the hippocampus and its multiple loops. We found that in a model incorporating these concepts, each cortical pattern can evoke a unique spatio-temporal spiking pattern in hippocampal neurons. This hippocampal response facilitates a reliable disambiguation of learned associations and a bridging of a time interval larger than the time window of spike-timing dependent plasticity in the cortex. Moreover, we found that repeated retrieval of a stored association leads to a compression of the interval between cue presentation and retrieval of the associated pattern from the cortex. Neither a high degree of connectivity nor heterogenous synaptic delays alone is sufficient for this behavior. We conclude that basic anatomical properties between cortex and hippocampus implement mechanisms for representing and consolidating temporal information. Since our model reveals the observed functions for a range of parameters, we suggest that these functions are robust to evolutionary changes consistent with the preserved function of the hippocampal loop across different species.

Noncanonical expression of caudal during early embryogenesis in the pea aphid Acyrthosiphon pisum: maternal cad-driven posterior development is not conserved

Previously we identified anterior localization of hunchback (Aphb) mRNA in oocytes and early embryos of the parthenogenetic and viviparous pea aphid Acyrthosiphon pisum, suggesting that the breaking of anterior asymmetry in the oocytes leads to the formation of the anterior axis in embryos. In order to study posterior development in the asexual pea aphid, we cloned and analysed the developmental expression of caudal (Apcad), a posterior gene highly conserved in many animal phyla. We found that transcripts of Apcad were not detected in germaria, oocytes and embryos prior to the formation of the blastoderm in the asexual (viviparous) pea aphid. This unusual expression pattern differs from that of the existing insect models, including long- and short-germ insects, where maternal cad mRNA is passed to the early embryos and forms a posterior-anterior gradient. The first detectable Apcad expression occurred in the newly formed primordial germ cells and their adjacent blastodermal cells during late blastulation. From gastrulation onward, and as in other insects, Apcad mRNA is restricted to the posteriormost region of the germ band. Similarly, in the sexual (oviparous) oocytes we were able to identify anterior localization of Aphb mRNA but posterior localization of Apcad was not detected. This suggests that cad-driven posterior development is not conserved during early embryogenesis in asexual and sexual pea aphids.

Notch and PKC are involved in formation of the lateral region of the dorso-ventral axis in Drosophila embryos

The Notch gene encodes an evolutionarily conserved cell surface receptor that generates regulatory signals based on interactions between neighboring cells. In Drosophila embryos it is normally expressed at a low level due to strong negative regulation. When this negative regulation is abrogated neurogenesis in the ventral region is suppressed, the development of lateral epidermis is severely disrupted, and the dorsal aminoserosa is expanded. Of these phenotypes only the anti-neurogenic phenotype could be linked to excess canonical Notch signaling. The other phenotypes were linked to high levels of Notch protein expression at the surface of cells in the lateral regions indicating that a non-canonical Notch signaling activity normally functions in these regions. Results of our studies reported here provide evidence. They show that Notch activities are inextricably linked to that of Pkc98E, the homolog of mammalian PKCδ. Notch and Pkc98E up-regulate the levels of the phosphorylated form of IκBCactus, a negative regulator of Toll signaling, and Mothers against dpp (MAD), an effector of Dpp signaling. Our data suggest that in the lateral regions of the Drosophila embryos Notch activity, in conjunction with Pkc98E activity, is used to form the slopes of the opposing gradients of Toll and Dpp signaling that specify cell fates along the dorso-ventral axis.

Progenitor cell capacity of NeuroD1-expressing globose basal cells in the mouse olfactory epithelium

The basic helix-loop-helix transcription factor NeuroD1 is expressed in embryonic and adult mouse olfactory epithelium (OE), as well as during epithelial regeneration, suggesting that it plays an important role in olfactory neurogenesis. We characterized NEUROD1-expressing progenitors, determined their progeny in the adult OE, and identified a subtle phenotype in ΔNeuroD1-knockout mice. All olfactory sensory neurons (OSNs) derive from a NeuroD1-expressing progenitor as shown by recombination-mediated lineage tracing, as do other sensory receptors of the nose, including vomeronasal, nasal septal, and Grunenberg ganglion neurons. NEUROD1-expressing cells are found among the globose basal cell population: they are actively proliferating and frequently coexpress Neurog1, but not the transit amplifying cell marker MASH1, nor the neuronal marker NCAM. As a consequence, NEUROD1-expressing globose basal cells are best classified as immediate neuronal precursors. In adolescent ΔNeuroD1-LacZ knock-in null mice the OE displays subtle abnormalities, as compared to wildtype and heterozygous littermates. In some areas of the OE, mature neurons are absent, or sparse, although those same areas retain immature OSNs and LacZ-expressing progenitors, albeit both of these populations are smaller than expected. Our results support the conclusion that most, if not all, nasal chemosensory neurons derive from NeuroD1-expressing globose basal cells of the immediate neuronal precursor variety. Moreover, elimination of NeuroD1 by gene knockout, while it does not disrupt initial OSN differentiation, does compromise the integrity of parts of the olfactory epithelium by altering proliferation, neuronal differentiation, or neuronal survival there.

Mechanistic framework for establishment, maintenance, and alteration of cell polarity in plants

Cell polarity establishment, maintenance, and alteration are central to the developmental and response programs of nearly all organisms and are often implicated in abnormalities ranging from patterning defects to cancer. By residing at the distinct plasma membrane domains polar cargoes mark the identities of those domains, and execute localized functions. Polar cargoes are recruited to the specialized membrane domains by directional secretion and/or directional endocytic recycling. In plants, auxin efflux carrier PIN proteins display polar localizations in various cell types and play major roles in directional cell-to-cell transport of signaling molecule auxin that is vital for plant patterning and response programs. Recent advanced microscopy studies applied to single cells in intact plants reveal subcellular PIN dynamics. They uncover the PIN polarity generation mechanism and identified important roles of AGC kinases for polar PIN localization. AGC kinase family members PINOID, WAG1, and WAG2, belonging to the AGC-3 subclass predominantly influence the polar localization of PINs. The emerging mechanism for AGC-3 kinases action suggests that kinases phosphorylate PINs mainly at the plasma membrane after initial symmetric PIN secretion for eventual PIN internalization and PIN sorting into distinct ARF-GEF-regulated polar recycling pathways. Thus phosphorylation status directs PIN translocation to different cell sides. Based on these findings a mechanistic framework evolves that suggests existence of cell side-specific recycling pathways in plants and implicates AGC3 kinases for differential PIN recruitment among them for eventual PIN polarity establishment, maintenance, and alteration.

Apparent role of Tribolium orthodenticle in anteroposterior blastoderm patterning largely reflects novel functions in dorsoventral axis formation and cell survival

In the short-germ beetle Tribolium castaneum, the head gap gene orthodenticle (Tc-otd) has been proposed to functionally substitute for bicoid, the anterior morphogen unique to higher dipterans. In this study we reanalyzed the function of Tc-otd. We obtained a similar range of cuticle phenotypes as in previously described RNAi experiments; however, we noticed unexpected effects on blastodermal cell fates. First, we found that Tc-otd is essential for dorsoventral patterning. RNAi depletion results in lateralized embryos, a fate map change that by itself can explain the observed loss of the anterior head, which is a ventral anlage in Tribolium. We find that this effect is due to diminished expression of short gastrulation (sog), a gene essential for establishment of the Decapentaplegic (Dpp) gradient in this species. Second, we found that gnathal segment primordia in Tc-otd RNAi embryos are shifted anteriorly but otherwise appear patterned normally. This anteroposterior (AP) fate map shift might largely be due to diminished zen-1 expression and is not responsible for the severe segmentation defects observed in some Tc-otd RNAi embryos. As neither Tc-sog nor Tc-zen-1 probably requires Otd gradient-mediated positional information, we posit that the blastoderm function of Tc-Otd depends on its initial homogeneous maternal expression and that this maternal factor does not provide significant positional information for Tribolium blastoderm embryos.

The 'Marx' of Groucho on development and disease

Groucho proteins are abundant and broadly expressed nuclear factors that lack intrinsic DNA-binding activity but can interact with a variety of DNA-binding proteins. The recruitment of Groucho to specific gene regulatory sequences results in transcriptional repression. In both invertebrates and vertebrates, Groucho family members act as important regulators of several signaling mechanisms, including the Notch, Wingless/Wnt and Dpp/BMP/TGF-beta signaling pathways. Recent studies of embryonic development in several species point to an important role for Groucho in the regulation of multiple patterning and differentiation events. Moreover, a deregulated expression of human Groucho family members is correlated with several neoplastic conditions. Here we focus on the functions of Groucho proteins during body patterning and their implication in tumorigenesis.

AcaudalmRNA gradient controls posterior development in the waspNasonia

One of the earliest steps of embryonic development is the establishment of polarity along the anteroposterior axis. Extensive studies of Drosophila embryonic development have elucidated mechanisms for establishing polarity, while studies with other model systems have found that many of these molecular components are conserved through evolution. One exception is Bicoid, the master organizer of anterior development in Drosophila and higher dipterans, which is not conserved. Thus, the study of anteroposterior patterning in insects that lack Bicoid can provide insight into the evolution of the diversity of body plan patterning networks. To this end, we have established the long germ parasitic wasp Nasonia vitripennis as a model for comparative studies with Drosophila.Here we report that, in Nasonia, a gradient of localized caudal mRNA directs posterior patterning, whereas, in Drosophila, the gradient of maternal Caudal protein is established through translational repression by Bicoid of homogeneous caudalmRNA. Loss of caudal function in Nasonia results in severe segmentation defects. We show that Nasonia caudal is an activator of gap gene expression that acts far towards the anterior of the embryo, placing it atop a cascade of early patterning. By contrast, activation of gap genes in flies relies on redundant functions of Bicoid and Caudal, leading to a lack of dramatic action on gap gene expression: caudal instead plays a limited role as an activator of pair-rule gene expression. These studies,together with studies in short germ insects, suggest that caudal is an ancestral master organizer of patterning, and that its role has been reduced in higher dipterans such as Drosophila.

Orthodenticle and empty spiracles genes are expressed in a segmental pattern in chelicerates

Members of the orthodenticle (otd/Otx) and empty spiracles (ems/Emx) gene families are head gap genes that encode homeodomain-containing DNA-binding proteins. Although numerous studies show their central role in developmental processes in brain specification, a surprisingly high number of other developmental processes have been shown to involve their expression. In this paper, we report the identification and expression of ems and otd in two chelicerate species: a scorpion, Euscorpius flavicaudis (Chactidae, Scorpiona, Arachnida, Euchelicerata) and a spider, Tegenaria saeva (Aranea, Arachnida, Euchelicerata). We show that both ems and otd are expressed not only in an anterior head domain but also along the entire anterior-posterior axis during embryonic development. The expression patterns for both genes are typically segmental and concern neurectodermal territories. During patterning of the opisthosoma, ems and otd are expressed in the lateral ectoderm just anterior to the limb bud primordia giving rise to respiratory organs and spinnerets (spider). This common pattern found in two divergent species thus appears to be a conserved character of chelicerates. These results are discussed in terms of evolutionary origin of respiratory organs and/or functional pathway recruitment.

The roles of wingless and decapentaplegic in axis and appendage development in the red flour beetle, Tribolium castaneum

Axis patterning and appendage development have been well studied in Drosophila melanogaster, a species in which both limb and segment morphogenesis are derived. In Drosophila, positional information from genes important in anteroposterior and dorsoventral axis formation, including wingless (wg) and decapentaplegic (dpp), is required for allocating and patterning the appendage primordia. We used RNA interference to characterize the functions of wg and dpp in the red flour beetle, Tribolium castaneum, which retains more ancestral modes of limb and segment morphogenesis. We also characterized the expression of potential targets of the WG and DPP signaling pathways in these embryos. Tribolium embryos in which dpp had been downregulated had defects in the dorsalmost body wall, but did not appear to have been globally repatterned and had normal appendages. Downregulation of wg led to the loss of segment boundaries, gnathal and thoracic appendages, and lateral head lobes, and to changes in the expression of dpp, Distal-less, and Engrailed. The functions of wg varied along both the anteroposterior and dorsoventral axes of the embryo. Phylogenetic comparisons indicate that the role of WNT signaling in segment boundary formation is evolutionarily old, but that its role in appendage allocation originated in the common ancestor of holometabolous insects.

A major role for zygotichunchbackin patterning theNasoniaembryo

Developmental genetic analysis has shown that embryos of the parasitoid wasp Nasonia vitripennis depend more on zygotic gene products to direct axial patterning than do Drosophila embryos. In Drosophila, anterior axial patterning is largely established by bicoid, a rapidly evolving maternal-effect gene, working with hunchback, which is expressed both maternally and zygotically. Here,we focus on a comparative analysis of Nasonia hunchback function and expression. We find that a lesion in Nasonia hunchback is responsible for the severe zygotic headless mutant phenotype, in which most head structures and the thorax are deleted, as are the three most posterior abdominal segments. This defines a major role for zygotic Nasonia hunchback in anterior patterning, more extensive than the functions described for hunchback in Drosophila or Tribolium. Despite the major zygotic role of Nasonia hunchback, we find that it is strongly expressed maternally, as well as zygotically. NasoniaHunchback embryonic expression appears to be generally conserved; however, the mRNA expression differs from that of Drosophila hunchback in the early blastoderm. We also find that the maternal hunchback message decays at an earlier developmental stage in Nasonia than in Drosophila, which could reduce the relative influence of maternal products in Nasonia embryos. Finally, we extend the comparisons of Nasonia and Drosophila hunchback mutant phenotypes, and propose that the more severe Nasonia hunchback mutant phenotype may be a consequence of differences in functionally overlapping regulatory circuitry.

Nanos (nos) genes of the vector mosquitoes, Anopheles gambiae, Anopheles stephensi and Aedes aegypti

A number of genetics-based strategies for the control of vector-borne diseases require the development of genetic drive systems for introgressing antipathogen effector genes into wild populations of insects. Modified transposons whose mobilization is controlled by the DNA elements of developmentally regulated genes offer a potential solution for introducing effector genes into mosquitoes. Such elements could exhibit sex-, stage- and species-specific transposition, thus mitigating some of the concerns associated with autonomous transposition. Hybridizations in situ show that the transcription products of the nanos orthologous genes of Anopheles gambiae (Anga nos), An. stephensi (Anst nos) and Aedes aegypti (Aeae nos) accumulate in developing oocytes in adult females and localize to the posterior pole in early embryos. These features make nos genes promising candidates for donating control sequences to modified transposons.

Depletion of Bmp2, Bmp4, Bmp7 and Spemann organizer signals induces massive brain formation in Xenopus embryos

To address the patterning function of the Bmp2, Bmp4 and Bmp7 growth factors, we designed antisense morpholino oligomers (MO) that block their activity in Xenopus laevis. Bmp4 knockdown was sufficient to rescue the ventralizing effects caused by loss of Chordin activity. Double Bmp4 and Bmp7 knockdown inhibited tail development. Triple Bmp2/Bmp4/Bmp7 depletion further compromised trunk development but did not eliminate dorsoventral patterning. Unexpectedly, we found that blocking Spemann organizer formation by UV treatment or beta-Catenin depletion caused BMP inhibition to have much more potent effects, abolishing all ventral development and resulting in embryos having radial central nervous system (CNS) structures. Surprisingly, dorsal signaling molecules such as Chordin, Noggin, Xnr6 and Cerberus were not re-expressed in these embryos. We conclude that BMP inhibition is sufficient for neural induction in vivo, and that in the absence of ventral BMPs, Spemann organizer signals are not required for brain formation.

Differential expression of caudal and dorsal genes in the teloblast lineages of the oligochaete annelid Tubifex tubifex

We have cloned homologues (designated Ttu-cdx and Ttu-dl) of caudal and dorsal genes from the oligochaete annelid Tubifex tubifex. In situ hybridization revealed that Ttu-cdx begins to be expressed around the time of completion of ectodermal teloblastogenesis (i.e., the onset of gastrulation). At this time, Ttu-cdx expression is detected in a few cells that are located in the anteriormost part of the mesodermal germ bands (GBs). As development proceeds and the GBs elongate, the domain of Ttu-cdx-expressing cells in the GBs extends posteriorly. During this process, M teloblasts and primary blast cells remained negative for Ttu-cdx expression. This Ttu-cdx expression pattern is in sharp contrast to the posterior caudal expression in other organisms. Expression of Ttu-dl is detected from the one-cell stage through to the gastrula stage. Although Ttu-dl is expressed in most blastomeres, there are regional differences in Ttu-dl expression levels. During early cleavage, a large amount of Ttu-dl mRNA, which is supplied maternally, is inherited by D-cell line micromeres 2d and 4d; the remaining micromeres and macromeres inherit relatively small amounts of Ttu-dl transcripts. Another regional difference in Ttu-dl expression levels is seen in the ectodermal GB that is comprised of four bast-cell bandlets. The ventralmost bandlet (N lineage) exhibits the highest level of Ttu-dl expression with the lowest level in the two middle bandlets (O and P lineages) and an intermediate level in the dorsalmost bandlet (Q lineage).

Dorsal-ventral patterning and neural induction in Xenopus embryos

We review the current status of research in dorsal-ventral (D-V) patterning in vertebrates. Emphasis is placed on recent work on Xenopus, which provides a paradigm for vertebrate development based on a rich heritage of experimental embryology. D-V patterning starts much earlier than previously thought, under the influence of a dorsal nuclear -Catenin signal. At mid-blastula two signaling centers are present on the dorsal side: The prospective neuroectoderm expresses bone morphogenetic protein (BMP) antagonists, and the future dorsal endoderm secretes Nodal-related mesoderm-inducing factors. When dorsal mesoderm is formed at gastrula, a cocktail of growth factor antagonists is secreted by the Spemann organizer and further patterns the embryo. A ventral gastrula signaling center opposes the actions of the dorsal organizer, and another set of secreted antagonists is produced ventrally under the control of BMP4. The early dorsal -Catenin signal inhibits BMP expression at the transcriptional level and promotes expression of secreted BMP antagonists in the prospective central nervous system (CNS). In the absence of mesoderm, expression of Chordin and Noggin in ectoderm is required for anterior CNS formation. FGF (fibroblast growth factor) and IGF (insulin-like growth factor) signals are also potent neural inducers. Neural induction by anti-BMPs such as Chordin requires mitogen-activated protein kinase (MAPK) activation mediated by FGF and IGF. These multiple signals can be integrated at the level of Smad1. Phosphorylation by BMP receptor stimulates Smad1 transcriptional activity, whereas phosphorylation by MAPK has the opposite effect. Neural tissue is formed only at very low levels of activity of BMP-transducing Smads, which require the combination of both low BMP levels and high MAPK signals. Many of the molecular players that regulate D-V patterning via regulation of BMP signaling have been conserved between Drosophila and the vertebrates.

Symmetry Breaking and the Evolution of Development

Because of its simplicity, the binary-switch nature of left-right asymmetry permits meaningful comparisons among many different organisms. Phylogenetic analyses of asymmetry variation, inheritance, and molecular mechanisms reveal unexpected insights into how development evolves. First, directional asymmetry, an evolutionary novelty, arose from nonheritable origins almost as often as from mutations, implying that genetic assimilation ("phenotype precedes genotype") is a common mode of evolution. Second, the molecular pathway directing hearts leftward-the nodal cascade-varies considerably among vertebrates (homology of form does not require homology of development) and was possibly co-opted from a preexisting asymmetrical chordate organ system. Finally, declining frequencies of spontaneous asymmetry reversal throughout vertebrate evolution suggest that heart development has become more canalized.

The origin of dorsoventral polarity in Drosophila

In Drosophila dorsoventral (DV) polarity arises during oogenesis when the oocyte nucleus moves from a central posterior to an asymmetrical anterior position. Nuclear movement is a symmetry-breaking step and establishes orthogonality between the anteroposterior and the DV axes. The asymmetrically anchored nucleus defines a cortical region within the oocyte which accumulates high levels of gurken messenger RNA (mRNA) and protein. Gurken is an ovarian-specific member of the transforming growth factor-alpha (TGF-alpha) family of secreted ligands. Secreted Gurken forms a concentration gradient that results in a dorsal-to-ventral gradient of EGF receptor activation in the follicle cells surrounding the oocyte. This leads to concentration-dependent activation or repression of target genes of the EGF pathway in the follicular epithelium. One outcome of this process is the restriction of pipe expression to a ventral domain that comprises 40% of the egg circumference. Pipe presumably modifies extracellular matrix components that are secreted by the follicle cells and are present at the ventral side of embryo after egg deposition. Here, they activate a proteolytic cascade that generates a gradient of the diffusible ligand, Spätzle. Spätzle activates the Toll receptor at the surface of the embryo that stimulates the nuclear uptake of the transcription factor Dorsal. This leads to a nuclear concentration gradient of Dorsal that specifies the cell types along the DV axis of the embryo.

The genes orthodenticle and hunchback substitute for bicoid in the beetle Tribolium

In Drosophila, the morphogen Bicoid organizes anterior patterning in a concentration-dependent manner by activating the transcription of target genes such as orthodenticle (otd) and hunchback (hb), and by repressing the translation of caudal. Homologues of the bicoid gene have not been isolated in any organism apart from the higher Dipterans. In fact, head and thorax formation in other insects is poorly understood. To elucidate this process in a short-germband insect, I analysed the function of the conserved genes orthodenticle-1 (otd-1) and hb in the flour beetle Tribolium castaneum. Here I show that, in contrast to Drosophila, Tribolium otd-1 messenger RNA is maternally inherited by the embryo. Reduction of Tribolium otd-1 levels by RNA interference (RNAi) results in headless embryos. This shows that otd-1 is required for anterior patterning in Tribolium. As in Drosophila, Tribolium hb specifies posterior gnathal and thoracic segments. The head, thorax and the anterior abdomen fail to develop in otd-1/hb double-RNAi embryos. This phenotype is similar to that of strong bicoid mutants in Drosophila. I propose that otd-1 and hb are part of an ancestral anterior patterning system.

A taxonomy for artificial embryogeny

A major challenge for evolutionary computation is to evolve phenotypes such as neural networks, sensory systems, or motor controllers at the same level of complexity as found in biological organisms. In order to meet this challenge, many researchers are proposing indirect encodings, that is, evolutionary mechanisms where the same genes are used multiple times in the process of building a phenotype. Such gene reuse allows compact representations of very complex phenotypes. Development is a natural choice for implementing indirect encodings, if only because nature itself uses this very process. Motivated by the development of embryos in nature, we define artificial embryogeny (AE) as the subdiscipline of evolutionary computation (EC) in which phenotypes undergo a developmental phase. An increasing number of AE systems are currently being developed, and a need has arisen for a principled approach to comparing and contrasting, and ultimately building, such systems. Thus, in this paper, we develop a principled taxonomy for AE. This taxonomy provides a unified context for long-term research in AE, so that implementation decisions can be compared and contrasted along known dimensions in the design space of embryogenic systems. It also allows predicting how the settings of various AE parameters affect the capacity to efficiently evolve complex phenotypes.

Role of Tea1p, Tea3p and Pom1p in the determination of cell ends inSchizosaccharomyces pombe

Schizosaccharomyces pombe cells are rod-shaped and grow along a single axis from their two ends. Microtubules extend from the cell centre terminating at the cell ends. The ERM(ezrin/radixin/moesin)-like proteins Tea1p and Tea3p, and the Dyrk-like kinase Pom1p are cell end markers involved in the regulation of growth and microtubular dynamics at the cell ends. We have analysed the relative contribution of these three proteins to the determination of cell ends as sites both for cell growth and for microtubular termination. Pom1Delta, in combination with Tea1Delta or Tea3Delta, has the greatest difficulty in relocalizing actin to the cell ends following actin depolymerization and generates the most defective growth pattern. Tea1Delta, in combination with Pom1Delta or Tea3Delta, displays the highest number of microtubules bending round the cell ends. Tea1DeltaPom1Delta, which has the most defective growth pattern and microtubules, also displays the highest number of branched cells. We show that Tea1p, Tea3p and Pom1p all contribute, to different extents, to the determination of cell ends, as sites for both cell growth and microtubular termination. We also show that the fission yeast cell relies on both the positioning of landmarks and a properly organized microtubule cytoskeleton to direct cell growth.

Cell movements during gastrulation: snail dependent and independent pathways

The morphogenetic process of gastrulation requires multiple inputs and intricate coordination. Genetic analyses demonstrate critical roles of vertebrate and invertebrate Snail proteins in this process. Together with other regulatory molecules including Wnt and BMP, the Snail pathways specify cell fate and reorganize cellular machineries to coordinate morphological changes and cell movements during gastrulation.