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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.ydbio.2010.04.019 Byline: Monica Garcia-Solache, Johannes Jaeger, Michael Akam Keywords: Evolution; Segment determination; Diptera; Nematocera; Pattern formation; Gene regulation; Even-skipped Abstract: The segmentation gene hierarchy of Drosophila melanogaster represents one of the best understood of the gene networks that generate pattern during embryogenesis. Some components of this network are ancient, while other parts of the network have evolved within the higher Diptera. To further understand the evolution of this gene network, we are studying the role of gap genes in a representative of a basally diverging dipteran lineage, the moth midge Clogmia albipunctata. We have isolated orthologues of all of the Drosophila trunk gap genes from Clogmia, and determined their domains of expression during the blastoderm stage of development, in relation to one another, and in relation to the expression of even-skipped (Calb-eve), a component of the pair-rule system that is directly regulated by the gap genes in Drosophila. We find that hunchback (Calb-hb), Kruppel (Calb-Kr), knirps (Calb-knl), giant (Calb-gt) and tailless (Calb-tll) are all expressed in patterns consistent with a gap segmentation role during blastoderm formation, but huckebein (Calb-hkb) is not. In the anterior half of the embryo, the relative positions of the gap gene expression domains in relation to one another, and in relation to the eve stripes, are rather well conserved. In the posterior half of the embryo, there are significant differences. Posteriorly, Calb-gt is expressed only transiently and very weakly, in a domain that overlaps entirely with that of Calb-knl. At late blastoderm stages, none of the candidate genes we have tested is expressed in the region between the posterior Calb-knl domain and Calb-tll. It is likely that the regulation of Calb-eve expression in this posterior region depends on combinations of gap gene factors that differ from those utilised for the same stripes in Drosophila. Both the gap and the pair-rule patterns of gene expression are dynamic in Clogmia, as they are in Drosophila, shifting anteriorly as blastoderm development proceeds. Author Affiliation: Laboratory for Development and Evolution, University Museum of Zoology and Department of Zoology, Downing Street, Cambridge CB2 3EJ, UK Article History: Received 22 November 2009; Revised 19 April 2010; Accepted 21 April 2010

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.ydbio.2008.02.038 Byline: Ariel D. Chipman, Michael Akam Keywords: Segmentation; Notch-signalling; Arthropod; Myriapod; Evolution; Pattern formation Abstract: The centipede Strigamia maritima forms all of its segments during embryogenesis. Trunk segments form sequentially from an apparently undifferentiated disk of cells at the posterior of the germ band. We have previously described periodic patterns of gene expression in this posterior disc that precede overt differentiation of segments, and suggested that a segmentation oscillator may be operating in the posterior disc. We now show that genes of the Notch signalling pathway, including the ligand Delta, and homologues of the Drosophila pair-rule genes even-skipped and hairy, show periodic expression in the posterior disc, consistent with their involvement in, or regulation by, such an oscillator. These genes are expressed in a pattern of apparently expanding concentric rings around the proctodeum, which become stripes at the base of the germ band where segments are emerging. In this transition zone, these primary stripes define a double segment periodicity: segmental stripes of engrailed expression, which mark the posterior of each segment, arise at two different phases of the primary pattern. Delta and even-skipped are also activated in secondary stripes that intercalate between primary stripes in this region, further defining the single segment repeat. These data, together with observations that Notch mediated signalling is required for segment pattern formation in other arthropods, suggest that the ancestral arthropod segmentation cascade may have involved a segmentation oscillator that utilised Notch signalling. Author Affiliation: University Museum of Zoology and Department of Zoology, Downing St., Cambridge CB2 3EJ, UK Article History: Received 1 January 2008; Revised 19 February 2008; Accepted 19 February 2008

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.ydbio.2005.12.028 Byline: Kristen A. Panfilio (a), Paul Z. Liu (b), Michael Akam (a), Thomas C. Kaufman (b) Keywords: Hemiptera; Extraembryonic membrane; Serosa; Amnion; Katatrepsis; Blastokinesis; zen; Hox3; RNA interference; Evolution of gene function Abstract: Unlike most Hox cluster genes, with their canonical role in anterior-posterior patterning of the embryo, the Hox3 orthologue of insects has diverged. Here, we investigate the zen orthologue in Oncopeltus fasciatus (Hemiptera:Heteroptera). As in other insects, the Of-zen gene is expressed extraembryonically, and RNA interference (RNAi) experiments demonstrate that it is functionally required in this domain for the proper occurrence of katatrepsis, the phase of embryonic movements by which the embryo emerges from the yolk and adjusts its orientation within the egg. After RNAi knockdown of Of-zen, katatrepsis does not occur, causing embryos to complete development inside out. However, not all aspects of expression and function are conserved compared to grasshopper, beetle, and fly orthologues. Of-zen is not expressed in the extraembryonic tissue until relatively late, suggesting it is not involved in tissue specification. Within the extraembryonic domain, Of-zen is expressed in the outer serosal membrane, but unlike orthologues, it is not detectable in the inner extraembryonic membrane, the amnion. Thus, the role of zen in the interaction of serosa, amnion, and embryo may differ between species. Of-zen is also expressed in the blastoderm, although this early expression shows no apparent correlation with defects seen by RNAi knockdown. Author Affiliation: (a) University Museum of Zoology, Department of Zoology, Downing Street, Cambridge CB2 3EJ, UK (b) Department of Biology, Indiana University, 1001 East Third Street, Bloomington, IN 47405, USA Article History: Received 27 October 2005; Accepted 1 December 2005

Vasa is a widely conserved germline marker, both in vertebrates and invertebrates. We identify a vasa orthologue, Sgvasa, and use it to study germline development in the grasshopper Schistocerca gregaria, a species in which no germ plasm has been identified. In adults, Sgvasa is specifically expressed in the ovary and testis. It is expressed at high levels during early oogenesis, but no detectable vasa RNA and little Vasa protein are present in mature unlaid eggs. None appears to be localized to any defined region of the egg cortex, suggesting that germline specification may not depend on maternal germ plasm expressing vasa. Vasa protein is expressed in most cleavage energids as they reach the egg surface and persists at high levels in most cells aggregating to form the embryonic primordium. However, after gastrulation, Vasa protein persists only in extraembryonic membranes and in cells at the outer margin of the late heart-stage embryo. In the embryo, it then become restricted to cells at the dorsal margin of the forming abdomen. In older embryos, these Vasa-positive cells move toward the midline; Vasa protein accumulates asymmetrically in their cytoplasm, a pattern closely resembling that of germ cells in late embryonic gonads. Thus, we suggest that the Vasa-stained cells in the abdominal margin are germ cells, as proposed by Nelson (1934), and not cardioblasts, as has been proposed by others. Key Words: arthropod; Orthoptera; germ cell; gonad; vasa; embryogenesis; ovary; testis; antibody.

Flexible joints separate the rigid sections of the insect leg, allowing them to move. In Drosophila, the initial patterning of these joints is apparent in the larval imaginal discs from which the adult legs will develop. Here, we describe the later patterning and morphogenesis of the joints, which occurs after pupariation (AP). In the tibial/tarsal joint, the apodeme insertion site provides a fixed marker for the boundary between proximal and distal joint territories (the P/D boundary}. Cells on either side of this boundary behave differently during morphogenesis. Morphogenesis begins with the apical constriction of distal joint cells, about 24 h AP. Distal cells then become columnar, causing distal tissue nearest the P/D boundary to fold into the leg. In the last stage of joint morphogenesis, the proximal joint cells closest to the P/D boundary align and elongate to form a 'palisade' (a row of columnar cells) over the distal joint cells. The proximal and distal joint territories are characterised by the differential organisation of cytoskeletal and extracellular matrix proteins, and by the differential expression of enhancer trap lines and other gene markers. These markers also define a number of more localised territories within the pupal joint. Key Words: arthropod limb; leg imaginal discs; metamorphosis; morphogenesis; cell shape; joint territories; tibia; tarsus; SEM; GAL4 enhancer trap; plasmid rescue.

Research has been conducted on the bristle pattern difference control by the Hox gene Ultrabithorax. Results indicate that this gene blocks bristle development of the third thoracic segment at different points in sensory organ development.

It has been possible to identify Hox genes from two previously unsampled phyla, a priapulid (Priapulus caudatus) and a lophophorate (the brachiopod Lingula anatina). An extensive Hox gene data set has also been collected from a polychaete annelid (Nereis virens), along with partial data from a gastropod mollusc (the limpet Patella vulgata). The stem lophotrochozoan had a least 10 Hox genes, while the stem ecdysozoan had at least eight. The common protostome ancestor also had at least eight Hox genes, with the common bilaterian ancestor having at least seven.

The conserved organization of the Hox genes throughout the animal kingdom has become one of the major paradigms of evolutionary developmental biology. We have examined the organization of the Hox genes of the grasshopper, Schistocerca gregaria. We find that the grasshopper Hox cluster is over 700 kb long, and is not split into equivalents of the Antennapedia complex and the bithorax complex of Drosophila melanogaster. SgDax and probably also Sgzen, the grasshopper homologues of fushi-tarazu (ftz) and Zerknullt (zen), respectively, are also in the cluster, showing that the non-homeotic Antp-class genes, 'accessory genes,' are an ancient feature of insect Hox clusters.

The expression of four Hox genes in the branchiopod crustacean Artemia franciscana have been compared with Hox gene patterns from insects to investigate the macroevolutionary changes in body architecture. Though crustaceans and insects share a common origin of segmentation, the specialized trunk segments may have independent origins. The three trunk genes in Artemia, Antp, Ubx, and abdA, have largely overlapping domains in the uniform thoracic region. In insects these genes form distinct segment types in thorax and abdomen.

Abstract Background The heads of annelids (earthworms, polychaetes, and others) and arthropods (insects, myriapods, spiders, and others) and the arthropod-related onychophorans (velvet worms) show similar brain architecture and for this reason have long been considered homologous. However, this view is challenged by the 'new phylogeny' placing arthropods and annelids into distinct superphyla, Ecdysozoa and Lophotrochozoa, together with many other phyla lacking elaborate heads or brains. To compare the organisation of annelid and arthropod heads and brains at the molecular level, we investigated head regionalisation genes in various groups. Regionalisation genes subdivide developing animals into molecular regions and can be used to align head regions between remote animal phyla. Results We find that in the marine annelid Platynereis dumerilii, expression of the homeobox gene six3 defines the apical region of the larval body, peripherally overlapping the equatorial otx+ expression. The six3+ and otx+ regions thus define the developing head in anterior-to-posterior sequence. In another annelid, the earthworm Pristina, as well as in the onychophoran Euperipatoides, the centipede Strigamia and the insects Tribolium and Drosophila, a six3/optix+ region likewise demarcates the tip of the developing animal, followed by a more posterior otx/otd+ region. Identification of six3+ head neuroectoderm in Drosophila reveals that this region gives rise to median neurosecretory brain parts, as is also the case in annelids. In insects, onychophorans and Platynereis, the otx+ region instead harbours the eye anlagen, which thus occupy a more posterior position. Conclusions These observations indicate that the annelid, onychophoran and arthropod head develops from a conserved anterior-posterior sequence of six3+ and otx+ regions. The six3+ anterior pole of the arthropod head and brain accordingly lies in an anterior-median embryonic region and, in consequence, the optic lobes do not represent the tip of the neuraxis. These results support the hypothesis that the last common ancestor of annelids and arthropods already possessed neurosecretory centres in the most anterior region of the brain. In light of its broad evolutionary conservation in protostomes and, as previously shown, in deuterostomes, the six3-otx head patterning system may be universal to bilaterian animals.

The Juan March Foundation sponsored a meeting from Nov 3 to 5, 1997 on evolutionary developmental biology. Some of the issues that were discussed at the event focused on the evolution of the body patterns and parts of animals and their correlation with cell physiology. Other topics focused on the genetic aspects of changes associated with evolutionary development biology and the relationship between the nature of linkage between topography and developmental operations.