- Email: [email protected]
Nematode Gastrulation: Having a BLASTocoel!
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know where to look. With a molecular suspect now identified, surveillance of these nearmembrane foci for Ca2+ influx may provide a breakthrough in the study of these ephemeral coupling domains. Evanescent wave imaging has a solid pedigree for this task [14], as the unparalled optical resolution of this method can detect Ca2+ signals resulting from the activity of single Ca2+ channels at the cell surface [18]. Finally, we should remember that STIM1 was originally cloned from a chromosomal region linked to several cancers [19]. STIM1 overexpression induces growth arrest in certain cells, leading to its characterization as a potential tumor growth suppressor [20]. Unraveling the pathophysiological linkage between STIM1, SOCE and cell proliferation will be a further area of research catalyzed by these provocative new data. References 1. Putney, J.W., Jr. (1986). A model for receptor-regulated calcium entry. Cell Calcium 7, 1–12. 2. Venkatachalam, K., van Rossum, D.B., Patterson, R.L., Ma, H.-T., and Gill, D.J. (2002). The cellular and molecular basis of store-operated calcium entry. Nat. Cell Biol. 4, E263–E272. 3. Berridge, M.J. (2004). Conformational coupling: A physiological calcium entry mechanism. Sci. STKE 243, pe33.
4. Bolotina, V.M. (2004). Store-operated channels: diversity and activation mechanisms. Sci. STKE 243, pe34. 5. Parekh, A.B., and Putney, J.W., Jr. (2005). Store-operated calcium channels. Physiol. Rev. 85, 757–810. 6. Rosado, J.A., Redondo, P.C., Sage, S.O., Pariente, J.A., and Salido, G.M. (2005). Store-operated Ca2+ entry: vesicle fusion or reversible trafficking and de novo conformational coupling? J. Cell. Physiol., in press. 7. Roos, J., DiGregorio, P.J., Yeromin, A.V., Ohlsen, K., Lioudyno, M., Zhang, S., Safrina, O., Kozak, J.A., Wagner, S.L., Cahalan, M.D., et al. (2005). STIM1, an essential and conserved component of store-operated Ca2+ channel function. J. Cell Biol. 169, 435–445. 8. Liou, J., Kim, L.M., Heo, W.D., Jones, J.T., Myers, J.W., Ferrell, J.E., Jr., and Meyer, T. (2005). STIM is a Ca2+ sensor essential for Ca2+ store depletion triggered Ca2+ influx. Curr. Biol. 15, this issue. 9. Williams, R.T., Manji, S.S.M., Parker, N.J., Hancock, M.S., Van Stekelenburg, L., Eid, J.-P., Senior, P.V., Kazenwadel, J.S., Shandala, T., Saint, R., et al. (2001). Identification and characterization of the STIM (stromal interaction molecule) gene family: coding for a novel class of transmembrane proteins. Biochem. J. 357, 673–685. 10. Putney, J.W., Jr. (2005). Capacitative calcium entry: sensing the calcium stores. J. Cell Biol. 169, 381–382. 11. Manji, S.S.M., Parker, N.J., Williams, R.T., van Stekelenburg, L., Person, R.B., Dziadek, M., and Smith, P.J. (2000). STIM1: a novel phosphoprotein located at the cell surface. Biochim. Biophys. Acta 1481, 147–155. 12. Williams, R.T., Senior, P.V., van Stekelenburg, L., Layton, J.E., Simth, P.J., and Dziadek, M.A. (2002). Stromal interaction molecule 1 (STIM1), a transmembrane proteins with growth supressor activity, contains an extracellular SAM domain modified by N-
Nematode Gastrulation: Having a BLASTocoel! During gastrulation of the nematode worm Caenorhabditis elegans, individual cells ingress into a solid ball of cells. Gastrulation in a basal nematode, in contrast, has now been found to occur by invagination into a blastocoel, revealing an unanticipated embryological affinity between nematodes and all other triploblastic metazoans. Pradeep M. Joshi and Joel H. Rothman The reorganization of an amorphous ball of cells during embryogenesis into distinct germ layers with unique developmental potentials results in the formation of the gut and other internal organs. This process is called gastrulation, from the Greek word ‘gaster’, or belly. In 1872, Haeckel defined a gastrula as a hollow diploblastic embryonic stage
common to all metazoans, wherein the inner layer, the endoderm, delimits a gastric cavity connected to the exterior by an opening interpreted to be the mouth [1]. The gastrula was the basis for Haeckel’s gastraea theory in which he proposed that all metazoa evolved from a primitive organism, a ‘gastraea’, retaining the gastrula stage of development throughout their evolutionary history [1]. This hypothesis was crystallized in Haeckel’s famous claim that
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linked glycosylation. Biochim. Biophys. Acta 1596, 131–137. Oritani, K., and Kincade, P.W. (1996). Identification of stromal cell products that interact with pre-B cells. J. Cell Biol. 134, 771–782. Axelrod, D. (2001). Selective imaging of surface fluorescence with very high aperture microscope objectives. J. Biomed. Opt. 6, 6–13. Berridge, M.J. (1995). Capacitative calcium entry. Biochem. J. 312, 1–11. Park, M.K., Petersen, O.H., and Tepikin, A.V. (2000). The endoplasmic reticulum as one continuous Ca2+ pool: visualization of rapid Ca2+ movements and equilibration. EMBO J. 19, 5729–5739. Tateishi, Y., Hattori, M., Nakayama, T., Iwai, M., Bannai, H., Nakamura, T., Michikawa, T., Inoue, T., and Mikoshiba, K. (2005). Cluster formation of inositol 1,4,5-trisphosphate receptor requires its transition to open state. J. Biol. Chem. 280, 6816–6822. Demuro, A., and Parker, I. (2004). Imaging single-channel calcium microdomains by total internal reflection microscopy. Biol. Res. 37, 675–679. Parker, N.J., Begley, C.G., Smith, P.J., and Fox, R.M. (1996). Molecular cloning of a novel human gene (D11S4896E) at chromosomal region 11p15.5. Genomics 37, 253–256. Sabbioni, S., Barbanti-Brodano, G., Croce, C.M., and Negrini, M. (1997). GOK: a gene at 11p15 involved in rhabdomyosarcoma and rhabdoid tumor development. Cancer Res. 57, 4493–4497.
Department of Pharmacology, University of Minnesota Medical School, Minneapolis, Minnesota 55455, USA. E-mail: [email protected] DOI: 10.1016/j.cub.2005.06.035
“ontogeny is the short and rapid recapitulation of phylogeny”. A recent study on a freshwater nematode [2] has now revealed that the link to the postulated gastraea has been maintained in extant creatures even among animals, such as the star of all nematodes, Caenorhabditis elegans, that undergo a bizarre mode of gastrulation in which there is no recognizable hollow diploblast stage. This finding unifies the nematodes ontogenically with all other triploblast phyla, as anticipated by earlier molecular phylogeny studies [3]. While it is a key attribute of all multi-blastic metazoans, a striking feature of gastrulation is that the manner by which cells internalize, leading to formation of the endoderm and mesoderm, differs remarkably across phylogeny.
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Although gastrulation generates the endoderm and mesoderm in most metazoa, the existing ‘diploblasts’, such as cnidarians, gastrulate without forming a mesoderm layer [4]. There is a great diversity in the form of gastrulating embryos, ranging from a circular dorsal blastopore in amphibians to a ventral groove in arthropods and a primitive streak in birds and mammals. Gastrulation is notably odd in C. elegans: it is initiated very early in a solid ball of 28 cells by ingression of two endodermal precursor cells through a posteroventral cleft, followed by ingression of individual mesodermal and germline progenitor cells. These different forms of gastrulation reflect distinct constraints imposed by the underlying architecture of the egg in terms of amount of yolk [5], changes in the adhesive properties of cells, and the number of cell divisions preceding the onset of gastrulation [6]. Four main modes of cellular movements that mediate the formation of the germ layers during gastrulation in metazoa have been described: invagination, epiboly, delamination, and ingression [6]. Invagination, the bending of epithelial sheets, and ingression, the internalization of individual cells, occur as a consequence of the autonomous apical constriction of epithelial cells. Invagination is believed to be the ancestral mode of gastrulation, and has been observed during the primary internalization of epithelial cells in sea urchins, ventral furrow formation in Drosophila, and involution in amphibians and zebrafish. Gastrulation in C. elegans, involving ingression of individual cells exclusively, is highly atypical for the metazoa outside of the phylum nematoda. Might a new mechanism for gastrulation have been invented at the inception of the nematode phylum? C. elegans and a number of other nematodes show highly mosaic mechanisms of cell-fate specification. Thus, perhaps an innovative mode of gastrulation appeared in the nematodes that accommodates
such a mosaic developmental strategy: the rapid mosaic development of a small number of cells may have constrained the available modes of cell rearrangements. Several lines of evidence, however, suggest that this is not the case. Acrobeloides nanus, a soil nematode that is related to C. elegans, but the development of which is five times slower, is highly regulative. The early blastomeres of this animal are multipotent, in that the loss of somatic blastomeres can be compensated for by the regulative change in the fate of a more posterior blastomere [7]. And the distantly related marine nematode Enoplus brevis, unlike C. elegans, shows a highly indeterminate lineage [8]. Despite these dramatic differences in developmental strategies, however, gastrulation in A. nanus and E. brevis is similar to that in C. elegans, suggesting that this unusual mode of gastrulation is not a consequence of a determinate, mosaic developmental program per se. Other studies in C. elegans provide hints that the peculiar form of gastrulation seen in the nematodes may have derived from a more normal, conserved mechanism. The mechanism that specifies the endoderm seems to be evolutionarily conserved across the triplobasts, including nematodes [9]. Further, in C. elegans, as in other animals in which gastrulation occurs via invagination — Drosophila and Xenopus, for example — the ingressing cells undergo cellautonomous apical constriction [10]. Thus, the cell shape changes associated with internalization may be dictated by an evolutionarily conserved program. Schierenberg [2] has now reported the remarkable discovery that the fresh water nematode Tobrilus diversipapillatus shows a mode of gastrulation that is utterly different from that in all other known nematodes, and that looks to all intents and purpose like gastrulation in virtually all other triplobasts. In T. diversipapillatus, early divisions are symmetric, and a distinct blastocoel is detectable
as early as the 16 cell stage (Figure 1A). By the 64 cell stage, cells from the future anterior pole start to invaginate, and by 128 cells, a third layer begins to appear between the surrounding ectoderm and the inner cell mass. Embryonic development in T. diversipapillatus thus resembles that seen across metazoa: invagination of a sheet of cells within a hollow ball surrounding a large blastocoel. This surprising finding means that gastrulation is not so radically different among phyla as was previously thought: there is likely a single conserved mechanism in all phyla — now including even nematodes — that is subject to phenomenal variation over relatively short phylogenetic differences. Molecular phylogeny based on 18S ribosomal DNA sequences has placed the nematodes in the same clade as other protostomes, within the ecdysozoa [3]. But a definitive embryological basis for this classification had been lacking. The localization of the blastopore at the anterior end of the gastrulating embryo in T. diversipapillatus, resembling that in a classic protostome gastrula, cements the classification of nematodes as protostomes, based on one of the key defining characteristics: formation of a blastopore at the site of the future mouth, thereby resolving a long-standing conundrum on this issue. In contrast to Haeckel’s biogenetic law, the view espoused by Garstang in 1922 that ontogeny creates phylogeny is the prevalent view regarding the relationship between cladistics and development [11]. Given that T. diversipapillatus is the only nematode known to exhibit a classical gastrula stage reminiscent of more primitive organisms, Schierenberg [2] proposes that the family Triplonchida might occupy a position at the base of the nematode phylogenetic tree; T. diversipapillatus gastrulation might be considered as archetypical and what had been regarded as the phylotypic mode of nematode gastrulation as a derived state. Gastrulation in this
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creature appears to be a relic of the original mode used by the ancient nematodes. A recent molecular phylogeny divides the nematodes into five major clades [12]. Members of clades I and III–V undergo asymmetric divisions and produce blastomeres with distinct fates [13]. Clade II, near the base of the phylum, includes two sister taxa, Enoplida and Triplonchida, of which T. diversipapillatus is a member. Members of this group are unique in that the early divisions are symmetric, resulting in blastomeres with indeterminate fate. But in Enoplus brevis, a member of the Enoplida group, the endoderm precursor is specified at the eight cell stage and gastrulation resembles that seen in C. elegans [8]. So a basal position for Triplonchida would be consistent with the apparent gradual progression from indeterminate to determinate development and specification of germ layer fates at an earlier stage of development as one goes from the base to the top of the nematode phylogenetic tree. How might the prevalent mode of nematode gastrulation — involution of single cells starting very early in development — have arisen during nematode evolution? And why was it fixed? Proper development requires the tight spatial and temporal coordination of sequential events. Changes in the relative timing of developmental processes (heterochrony) and position relative to the ancestral state (heterotopy) are attractive mechanisms by which to link the evolution of diversity in form with modifications in developmental strategies [11]. Traditionally, heterochrony has been associated with changes in size and shape of body plans [14]. But with the renewed interest in evolutionary developmental biology, the concept of heterochrony has been extended to include cellular, molecular and genetic events [15]. The spatiotemporal shift in specification of the germ layer progenitors may explain the unusual mode of gastrulation in nematodes, which, though superficially very different, may
A
L. variegatus B
T. diversipapillatus Invagination begins at 28-cell stage
Invagination begins at 64-cell stage Heterochrony of endoderm specification?
Heterotopy?
Current Biology
Figure 1. Proposed model for a spatiotemporal shift in developmental events leading to the unusual mode of gastrulation in derived nematodes. (A) Similarity between the blastula stages (16 cell stage) of the sea urchin L. variegatus and the freshwater nematode T. diversipapillatus, showing a clear blastocoel (asterisk). The image of the sea urchin was adapted from Jeff Hardin’s Sea Urchin embryology tutorial (http://worms.zoology.wisc.edu/urchins/SUcleavage_stages.html). (T. diversipapillatus image reproduced with permission from [2]; L. variegatus image courtesy of Charles Ettensohn and Jeff Hardin.) (B) In T. diversipapillatus embryos, internalization (assumed here to be invagination) initiates with the apical constriction of a group of cells located anteriorly (left in the cartoon) at the 64 cell stage, resulting in gastrulation reminiscent of that in other protostomes. In more derived nematodes, exemplified by C. elegans, it is postulated that a change in the position of prospective endoderm cells shifts the gastrulation cleft posteroventrally. A heterochronic shift in the developmental program causes endoderm specification to begin at the seven-cell stage, endowing endoderm precursor cells (red nuclei) with the ability to invaginate earlier (28 cell stage) and in the absence of a prominent blastocoel.
simply reflect the same process occurring with a very limited number of cells (Figure 1). Precocious mesendoderm specification relative to number of cell divisions would result in the acquisition of competency for invagination at a stage at which there are too few cells to form a conspicuous blastocoel. Nance and Priess [16] have suggested that, at the time gastrulation is initiated, C. elegans embryos possess a minor blastocoel with a variable volume that is less than that of individual cells. In addition to such a heterochronic shift, altered cell adhesion properties in the early embryos might also prevent the formation of a substantial blastocoel. An anterior to posteroventral shift in the initial position of the endoderm progenitor cells would explain the location of the
gastrulation cleft in most nematodes, which is atypical for protostomes. This hypothesized temporal and spatial shift in the assignment of cells that initiate gastrulation, and hence the change in the appearance of gastrulation, could be tested by comparing development at the cellular and molecular levels between nematode species. Such comparisons should provide exciting insights into how small changes in developmental programs result in such dramatic changes in developmental strategies. Recent studies in Drosophila and Xenopus have explored the intimate relationship between cell division and gastrulation and the requirement for lengthening of the cell cycle prior to gastrulation, a phenomenon that has also been observed in C. elegans [17]. These
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studies suggest that the cytoskeletal architecture of actively dividing cells is incompatible with that required for morphogenesis. Mutations in two genes in C. elegans, gad-1 and emb-5, lead to premature division of the endoderm cells and their failure to invaginate [18,19]. Concomitant with a heterochrony in cell fate specification, a delay in the lengthening of the cell cycle, required for the reorganization of the cytoskeleton of gastrulating cells, might also contribute to the distinct morphology of the T. diversipapillatus ‘gastrula’. Gastrulation was among the key innovations of metazoan evolution. It seems likely that, once it was invented, the basic mechanics were preserved while many unique properties were allocated to the various phyla of animals. The radically different appearance of gastrulation in C. elegans and other metazoans may have arisen from relatively modest changes in the location and timing of specification. Secondarily simplified animals like C. elegans have become exceedingly parsimonious with their cells: for example, the function of the kidney in C. elegans has been relegated to a single cell [20]. Similarly, it may be appropriate to regard the two endoderm cells that initiate gastrulation in this
animal as an invaginating sheet, albeit a very small one. It is conceivable that the cellular mechanisms involved in mobilizing germ layer progenitors into the embryo interior may be virtually identical, whether this movement involves a large sheet of cells or only two cells. Though embryos appear very different on the surface, inwardly they may be closely similar. References 1. Beetschen, J.C. (2001). Amphibian gastrulation: history and evolution of a 125 year-old concept. Int. J. Dev. Biol. 45, 771–795. 2. Schierenberg, E. (2005). Unusual cleavage and gastrulation in a freshwater nematode: developmental and phylogenetic implications. Dev. Genes Evol. 215, 103–108. 3. Aguinaldo, A.M., Turbeville, J.M., Linford, L.S., Rivera, M.C., Garey, J.R., Raff, R.A., and Lake, J.A. (1997). Evidence for a clade of nematodes, arthropods and other moulting animals. Nature 387, 489–493. 4. Martin, V.J., Littlefield, C.L., Archer, W.E., and Bode, H.R. (1997). Embryogenesis in hydra. Biol. Bull. 192, 345–363. 5. Arendt, D., and Nubler-Jung, K. (1999). Rearranging gastrulation in the name of yolk: evolution of gastrulation in yolk-rich amniote eggs. Mech. Dev. 81, 3–22. 6. Technau, U., and Scholz, C.B. (2003). Origin and evolution of endoderm and mesoderm. Int. J. Dev. Biol. 47, 531–539. 7. Wiegner, O., and Schierenberg, E. (1999). Regulative development in a nematode embryo: a hierarchy of cell fate transformations. Dev. Biol. 215, 1–12. 8. Voronov, D.A., and Panchin, Y.V. (1998). Cell lineage in marine nematode Enoplus brevis. Development 125, 143–150. 9. Leptin, M. (2005). Gastrulation movements: the logic and the nuts and bolts. Dev. Cell 8, 305–320.
Social Cognition: Imitation, Imitation, Imitation Monkeys recognize when they are being imitated, but they seem unable to learn by imitation. These facts make sense if imitation is seen as two different capacities: social mirroring, when actions are matched and have social benefits; and learning by copying, when new behavioural routines are acquired by observation. Richard W. Byrne Imitation gets a bad press: we know it is the sincerest form of flattery, and of course for effective education the learner must be able to copy the teacher, but on the whole, ‘imitation’ is linked to shallow, cheap and even fraudulent behaviour. It comes as a shock to discover that, as far as
we know, most non-human animals are unable to imitate [1]: is imitation, after all, rather clever? In everyday human life, imitation is remarkably prevalent: babies imitate the facial movements of adults within minutes of birth [2]; lovers find themselves unconsciously mirroring the other’s posture, and sycophants do the same with the stance and
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Putzke, A.P., and Rothman, J.H. (2003). Gastrulation: PARtaking of the bottle. Curr. Biol. 13, R223–R225. Hall, B.K. (1999). In Evolutionary Developmental Biology, Second Edition Edition. (Kluwer Academic Publishers). Blaxter, M.L., De Ley, P., Garey, J.R., Liu, L.X., Scheldeman, P., Vierstraete, A., Vanfleteren, J.R., Mackey, L.Y., Dorris, M., Frisse, L.M., et al. (1998). A molecular evolutionary framework for the phylum Nematoda. Nature 392, 71–75. Voronov, D.A., Panchin, Y.V., and Spiridonov, S.E. (1998). Nematode phylogeny and embryology. Nature 395, 28. Gould, S.J. (1992). Ontogeny and phylogeny–revisited and reunited. Bioessays 14, 275–279. Smith, K.K. (2003). Time’s arrow: heterochrony and the evolution of development. Int. J. Dev. Biol. 47, 613–621. Nance, J., and Priess, J.R. (2002). Cell polarity and gastrulation in C. elegans. Development 129, 387–397. Duncan, T., and Su, T.T. (2004). Embryogenesis: coordinating cell division with gastrulation. Curr. Biol. 14, R305–R307. Nishiwaki, K., and Miwa, J. (1998). Mutations in genes encoding extracellular matrix proteins suppress the emb-5 gastrulation defect in Caenorhabditis elegans. Mol. Gen. Genet. 259, 2–12. Knight, J.K., and Wood, W.B. (1998). Gastrulation initiation in Caenorhabditis elegans requires the function of gad-1, which encodes a protein with WD repeats. Dev. Biol. 198, 253–265. Nelson, F.K., and Riddle, D.L. (1984). Functional study of the Caenorhabditis elegans secretory-excretory system using laser microsurgery. J. Exp. Zool. 231, 45–56.
Department of MCD Biology, University of California, Santa Barbara, California 93106, USA. E-mail: [email protected] DOI: 10.1016/j.cub.2005.06.030
mannerisms of the powerful [3]; when you copy a friend’s wave in a dense crowd it shows them immediately you’ve seen them; and even the most inarticulate mechanic can show us what to do to fix our car’s engine. Imitation certainly comes naturally to humans. The idea that imitation is a special faculty, critical in child development and perhaps a central aspect of human uniqueness, has gained ground in psychology over recent years [4,5]. The discovery of ‘mirror neurons’ [6,7] — cells in the premotor area of the brain that are activated by a hand performing a simple goal-directed action and respond equally whether the hand is one’s own or another person’s
know where to look. With a molecular suspect now identified, surveillance of these nearmembrane foci for Ca2+ influx may provide a breakthrough in the study of these ephemeral coupling domains. Evanescent wave imaging has a solid pedigree for this task [14], as the unparalled optical resolution of this method can detect Ca2+ signals resulting from the activity of single Ca2+ channels at the cell surface [18]. Finally, we should remember that STIM1 was originally cloned from a chromosomal region linked to several cancers [19]. STIM1 overexpression induces growth arrest in certain cells, leading to its characterization as a potential tumor growth suppressor [20]. Unraveling the pathophysiological linkage between STIM1, SOCE and cell proliferation will be a further area of research catalyzed by these provocative new data. References 1. Putney, J.W., Jr. (1986). A model for receptor-regulated calcium entry. Cell Calcium 7, 1–12. 2. Venkatachalam, K., van Rossum, D.B., Patterson, R.L., Ma, H.-T., and Gill, D.J. (2002). The cellular and molecular basis of store-operated calcium entry. Nat. Cell Biol. 4, E263–E272. 3. Berridge, M.J. (2004). Conformational coupling: A physiological calcium entry mechanism. Sci. STKE 243, pe33.
4. Bolotina, V.M. (2004). Store-operated channels: diversity and activation mechanisms. Sci. STKE 243, pe34. 5. Parekh, A.B., and Putney, J.W., Jr. (2005). Store-operated calcium channels. Physiol. Rev. 85, 757–810. 6. Rosado, J.A., Redondo, P.C., Sage, S.O., Pariente, J.A., and Salido, G.M. (2005). Store-operated Ca2+ entry: vesicle fusion or reversible trafficking and de novo conformational coupling? J. Cell. Physiol., in press. 7. Roos, J., DiGregorio, P.J., Yeromin, A.V., Ohlsen, K., Lioudyno, M., Zhang, S., Safrina, O., Kozak, J.A., Wagner, S.L., Cahalan, M.D., et al. (2005). STIM1, an essential and conserved component of store-operated Ca2+ channel function. J. Cell Biol. 169, 435–445. 8. Liou, J., Kim, L.M., Heo, W.D., Jones, J.T., Myers, J.W., Ferrell, J.E., Jr., and Meyer, T. (2005). STIM is a Ca2+ sensor essential for Ca2+ store depletion triggered Ca2+ influx. Curr. Biol. 15, this issue. 9. Williams, R.T., Manji, S.S.M., Parker, N.J., Hancock, M.S., Van Stekelenburg, L., Eid, J.-P., Senior, P.V., Kazenwadel, J.S., Shandala, T., Saint, R., et al. (2001). Identification and characterization of the STIM (stromal interaction molecule) gene family: coding for a novel class of transmembrane proteins. Biochem. J. 357, 673–685. 10. Putney, J.W., Jr. (2005). Capacitative calcium entry: sensing the calcium stores. J. Cell Biol. 169, 381–382. 11. Manji, S.S.M., Parker, N.J., Williams, R.T., van Stekelenburg, L., Person, R.B., Dziadek, M., and Smith, P.J. (2000). STIM1: a novel phosphoprotein located at the cell surface. Biochim. Biophys. Acta 1481, 147–155. 12. Williams, R.T., Senior, P.V., van Stekelenburg, L., Layton, J.E., Simth, P.J., and Dziadek, M.A. (2002). Stromal interaction molecule 1 (STIM1), a transmembrane proteins with growth supressor activity, contains an extracellular SAM domain modified by N-
Nematode Gastrulation: Having a BLASTocoel! During gastrulation of the nematode worm Caenorhabditis elegans, individual cells ingress into a solid ball of cells. Gastrulation in a basal nematode, in contrast, has now been found to occur by invagination into a blastocoel, revealing an unanticipated embryological affinity between nematodes and all other triploblastic metazoans. Pradeep M. Joshi and Joel H. Rothman The reorganization of an amorphous ball of cells during embryogenesis into distinct germ layers with unique developmental potentials results in the formation of the gut and other internal organs. This process is called gastrulation, from the Greek word ‘gaster’, or belly. In 1872, Haeckel defined a gastrula as a hollow diploblastic embryonic stage
common to all metazoans, wherein the inner layer, the endoderm, delimits a gastric cavity connected to the exterior by an opening interpreted to be the mouth [1]. The gastrula was the basis for Haeckel’s gastraea theory in which he proposed that all metazoa evolved from a primitive organism, a ‘gastraea’, retaining the gastrula stage of development throughout their evolutionary history [1]. This hypothesis was crystallized in Haeckel’s famous claim that
13.
14.
15. 16.
17.
18.
19.
20.
linked glycosylation. Biochim. Biophys. Acta 1596, 131–137. Oritani, K., and Kincade, P.W. (1996). Identification of stromal cell products that interact with pre-B cells. J. Cell Biol. 134, 771–782. Axelrod, D. (2001). Selective imaging of surface fluorescence with very high aperture microscope objectives. J. Biomed. Opt. 6, 6–13. Berridge, M.J. (1995). Capacitative calcium entry. Biochem. J. 312, 1–11. Park, M.K., Petersen, O.H., and Tepikin, A.V. (2000). The endoplasmic reticulum as one continuous Ca2+ pool: visualization of rapid Ca2+ movements and equilibration. EMBO J. 19, 5729–5739. Tateishi, Y., Hattori, M., Nakayama, T., Iwai, M., Bannai, H., Nakamura, T., Michikawa, T., Inoue, T., and Mikoshiba, K. (2005). Cluster formation of inositol 1,4,5-trisphosphate receptor requires its transition to open state. J. Biol. Chem. 280, 6816–6822. Demuro, A., and Parker, I. (2004). Imaging single-channel calcium microdomains by total internal reflection microscopy. Biol. Res. 37, 675–679. Parker, N.J., Begley, C.G., Smith, P.J., and Fox, R.M. (1996). Molecular cloning of a novel human gene (D11S4896E) at chromosomal region 11p15.5. Genomics 37, 253–256. Sabbioni, S., Barbanti-Brodano, G., Croce, C.M., and Negrini, M. (1997). GOK: a gene at 11p15 involved in rhabdomyosarcoma and rhabdoid tumor development. Cancer Res. 57, 4493–4497.
Department of Pharmacology, University of Minnesota Medical School, Minneapolis, Minnesota 55455, USA. E-mail: [email protected] DOI: 10.1016/j.cub.2005.06.035
“ontogeny is the short and rapid recapitulation of phylogeny”. A recent study on a freshwater nematode [2] has now revealed that the link to the postulated gastraea has been maintained in extant creatures even among animals, such as the star of all nematodes, Caenorhabditis elegans, that undergo a bizarre mode of gastrulation in which there is no recognizable hollow diploblast stage. This finding unifies the nematodes ontogenically with all other triploblast phyla, as anticipated by earlier molecular phylogeny studies [3]. While it is a key attribute of all multi-blastic metazoans, a striking feature of gastrulation is that the manner by which cells internalize, leading to formation of the endoderm and mesoderm, differs remarkably across phylogeny.
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Although gastrulation generates the endoderm and mesoderm in most metazoa, the existing ‘diploblasts’, such as cnidarians, gastrulate without forming a mesoderm layer [4]. There is a great diversity in the form of gastrulating embryos, ranging from a circular dorsal blastopore in amphibians to a ventral groove in arthropods and a primitive streak in birds and mammals. Gastrulation is notably odd in C. elegans: it is initiated very early in a solid ball of 28 cells by ingression of two endodermal precursor cells through a posteroventral cleft, followed by ingression of individual mesodermal and germline progenitor cells. These different forms of gastrulation reflect distinct constraints imposed by the underlying architecture of the egg in terms of amount of yolk [5], changes in the adhesive properties of cells, and the number of cell divisions preceding the onset of gastrulation [6]. Four main modes of cellular movements that mediate the formation of the germ layers during gastrulation in metazoa have been described: invagination, epiboly, delamination, and ingression [6]. Invagination, the bending of epithelial sheets, and ingression, the internalization of individual cells, occur as a consequence of the autonomous apical constriction of epithelial cells. Invagination is believed to be the ancestral mode of gastrulation, and has been observed during the primary internalization of epithelial cells in sea urchins, ventral furrow formation in Drosophila, and involution in amphibians and zebrafish. Gastrulation in C. elegans, involving ingression of individual cells exclusively, is highly atypical for the metazoa outside of the phylum nematoda. Might a new mechanism for gastrulation have been invented at the inception of the nematode phylum? C. elegans and a number of other nematodes show highly mosaic mechanisms of cell-fate specification. Thus, perhaps an innovative mode of gastrulation appeared in the nematodes that accommodates
such a mosaic developmental strategy: the rapid mosaic development of a small number of cells may have constrained the available modes of cell rearrangements. Several lines of evidence, however, suggest that this is not the case. Acrobeloides nanus, a soil nematode that is related to C. elegans, but the development of which is five times slower, is highly regulative. The early blastomeres of this animal are multipotent, in that the loss of somatic blastomeres can be compensated for by the regulative change in the fate of a more posterior blastomere [7]. And the distantly related marine nematode Enoplus brevis, unlike C. elegans, shows a highly indeterminate lineage [8]. Despite these dramatic differences in developmental strategies, however, gastrulation in A. nanus and E. brevis is similar to that in C. elegans, suggesting that this unusual mode of gastrulation is not a consequence of a determinate, mosaic developmental program per se. Other studies in C. elegans provide hints that the peculiar form of gastrulation seen in the nematodes may have derived from a more normal, conserved mechanism. The mechanism that specifies the endoderm seems to be evolutionarily conserved across the triplobasts, including nematodes [9]. Further, in C. elegans, as in other animals in which gastrulation occurs via invagination — Drosophila and Xenopus, for example — the ingressing cells undergo cellautonomous apical constriction [10]. Thus, the cell shape changes associated with internalization may be dictated by an evolutionarily conserved program. Schierenberg [2] has now reported the remarkable discovery that the fresh water nematode Tobrilus diversipapillatus shows a mode of gastrulation that is utterly different from that in all other known nematodes, and that looks to all intents and purpose like gastrulation in virtually all other triplobasts. In T. diversipapillatus, early divisions are symmetric, and a distinct blastocoel is detectable
as early as the 16 cell stage (Figure 1A). By the 64 cell stage, cells from the future anterior pole start to invaginate, and by 128 cells, a third layer begins to appear between the surrounding ectoderm and the inner cell mass. Embryonic development in T. diversipapillatus thus resembles that seen across metazoa: invagination of a sheet of cells within a hollow ball surrounding a large blastocoel. This surprising finding means that gastrulation is not so radically different among phyla as was previously thought: there is likely a single conserved mechanism in all phyla — now including even nematodes — that is subject to phenomenal variation over relatively short phylogenetic differences. Molecular phylogeny based on 18S ribosomal DNA sequences has placed the nematodes in the same clade as other protostomes, within the ecdysozoa [3]. But a definitive embryological basis for this classification had been lacking. The localization of the blastopore at the anterior end of the gastrulating embryo in T. diversipapillatus, resembling that in a classic protostome gastrula, cements the classification of nematodes as protostomes, based on one of the key defining characteristics: formation of a blastopore at the site of the future mouth, thereby resolving a long-standing conundrum on this issue. In contrast to Haeckel’s biogenetic law, the view espoused by Garstang in 1922 that ontogeny creates phylogeny is the prevalent view regarding the relationship between cladistics and development [11]. Given that T. diversipapillatus is the only nematode known to exhibit a classical gastrula stage reminiscent of more primitive organisms, Schierenberg [2] proposes that the family Triplonchida might occupy a position at the base of the nematode phylogenetic tree; T. diversipapillatus gastrulation might be considered as archetypical and what had been regarded as the phylotypic mode of nematode gastrulation as a derived state. Gastrulation in this
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creature appears to be a relic of the original mode used by the ancient nematodes. A recent molecular phylogeny divides the nematodes into five major clades [12]. Members of clades I and III–V undergo asymmetric divisions and produce blastomeres with distinct fates [13]. Clade II, near the base of the phylum, includes two sister taxa, Enoplida and Triplonchida, of which T. diversipapillatus is a member. Members of this group are unique in that the early divisions are symmetric, resulting in blastomeres with indeterminate fate. But in Enoplus brevis, a member of the Enoplida group, the endoderm precursor is specified at the eight cell stage and gastrulation resembles that seen in C. elegans [8]. So a basal position for Triplonchida would be consistent with the apparent gradual progression from indeterminate to determinate development and specification of germ layer fates at an earlier stage of development as one goes from the base to the top of the nematode phylogenetic tree. How might the prevalent mode of nematode gastrulation — involution of single cells starting very early in development — have arisen during nematode evolution? And why was it fixed? Proper development requires the tight spatial and temporal coordination of sequential events. Changes in the relative timing of developmental processes (heterochrony) and position relative to the ancestral state (heterotopy) are attractive mechanisms by which to link the evolution of diversity in form with modifications in developmental strategies [11]. Traditionally, heterochrony has been associated with changes in size and shape of body plans [14]. But with the renewed interest in evolutionary developmental biology, the concept of heterochrony has been extended to include cellular, molecular and genetic events [15]. The spatiotemporal shift in specification of the germ layer progenitors may explain the unusual mode of gastrulation in nematodes, which, though superficially very different, may
A
L. variegatus B
T. diversipapillatus Invagination begins at 28-cell stage
Invagination begins at 64-cell stage Heterochrony of endoderm specification?
Heterotopy?
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Figure 1. Proposed model for a spatiotemporal shift in developmental events leading to the unusual mode of gastrulation in derived nematodes. (A) Similarity between the blastula stages (16 cell stage) of the sea urchin L. variegatus and the freshwater nematode T. diversipapillatus, showing a clear blastocoel (asterisk). The image of the sea urchin was adapted from Jeff Hardin’s Sea Urchin embryology tutorial (http://worms.zoology.wisc.edu/urchins/SUcleavage_stages.html). (T. diversipapillatus image reproduced with permission from [2]; L. variegatus image courtesy of Charles Ettensohn and Jeff Hardin.) (B) In T. diversipapillatus embryos, internalization (assumed here to be invagination) initiates with the apical constriction of a group of cells located anteriorly (left in the cartoon) at the 64 cell stage, resulting in gastrulation reminiscent of that in other protostomes. In more derived nematodes, exemplified by C. elegans, it is postulated that a change in the position of prospective endoderm cells shifts the gastrulation cleft posteroventrally. A heterochronic shift in the developmental program causes endoderm specification to begin at the seven-cell stage, endowing endoderm precursor cells (red nuclei) with the ability to invaginate earlier (28 cell stage) and in the absence of a prominent blastocoel.
simply reflect the same process occurring with a very limited number of cells (Figure 1). Precocious mesendoderm specification relative to number of cell divisions would result in the acquisition of competency for invagination at a stage at which there are too few cells to form a conspicuous blastocoel. Nance and Priess [16] have suggested that, at the time gastrulation is initiated, C. elegans embryos possess a minor blastocoel with a variable volume that is less than that of individual cells. In addition to such a heterochronic shift, altered cell adhesion properties in the early embryos might also prevent the formation of a substantial blastocoel. An anterior to posteroventral shift in the initial position of the endoderm progenitor cells would explain the location of the
gastrulation cleft in most nematodes, which is atypical for protostomes. This hypothesized temporal and spatial shift in the assignment of cells that initiate gastrulation, and hence the change in the appearance of gastrulation, could be tested by comparing development at the cellular and molecular levels between nematode species. Such comparisons should provide exciting insights into how small changes in developmental programs result in such dramatic changes in developmental strategies. Recent studies in Drosophila and Xenopus have explored the intimate relationship between cell division and gastrulation and the requirement for lengthening of the cell cycle prior to gastrulation, a phenomenon that has also been observed in C. elegans [17]. These
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studies suggest that the cytoskeletal architecture of actively dividing cells is incompatible with that required for morphogenesis. Mutations in two genes in C. elegans, gad-1 and emb-5, lead to premature division of the endoderm cells and their failure to invaginate [18,19]. Concomitant with a heterochrony in cell fate specification, a delay in the lengthening of the cell cycle, required for the reorganization of the cytoskeleton of gastrulating cells, might also contribute to the distinct morphology of the T. diversipapillatus ‘gastrula’. Gastrulation was among the key innovations of metazoan evolution. It seems likely that, once it was invented, the basic mechanics were preserved while many unique properties were allocated to the various phyla of animals. The radically different appearance of gastrulation in C. elegans and other metazoans may have arisen from relatively modest changes in the location and timing of specification. Secondarily simplified animals like C. elegans have become exceedingly parsimonious with their cells: for example, the function of the kidney in C. elegans has been relegated to a single cell [20]. Similarly, it may be appropriate to regard the two endoderm cells that initiate gastrulation in this
animal as an invaginating sheet, albeit a very small one. It is conceivable that the cellular mechanisms involved in mobilizing germ layer progenitors into the embryo interior may be virtually identical, whether this movement involves a large sheet of cells or only two cells. Though embryos appear very different on the surface, inwardly they may be closely similar. References 1. Beetschen, J.C. (2001). Amphibian gastrulation: history and evolution of a 125 year-old concept. Int. J. Dev. Biol. 45, 771–795. 2. Schierenberg, E. (2005). Unusual cleavage and gastrulation in a freshwater nematode: developmental and phylogenetic implications. Dev. Genes Evol. 215, 103–108. 3. Aguinaldo, A.M., Turbeville, J.M., Linford, L.S., Rivera, M.C., Garey, J.R., Raff, R.A., and Lake, J.A. (1997). Evidence for a clade of nematodes, arthropods and other moulting animals. Nature 387, 489–493. 4. Martin, V.J., Littlefield, C.L., Archer, W.E., and Bode, H.R. (1997). Embryogenesis in hydra. Biol. Bull. 192, 345–363. 5. Arendt, D., and Nubler-Jung, K. (1999). Rearranging gastrulation in the name of yolk: evolution of gastrulation in yolk-rich amniote eggs. Mech. Dev. 81, 3–22. 6. Technau, U., and Scholz, C.B. (2003). Origin and evolution of endoderm and mesoderm. Int. J. Dev. Biol. 47, 531–539. 7. Wiegner, O., and Schierenberg, E. (1999). Regulative development in a nematode embryo: a hierarchy of cell fate transformations. Dev. Biol. 215, 1–12. 8. Voronov, D.A., and Panchin, Y.V. (1998). Cell lineage in marine nematode Enoplus brevis. Development 125, 143–150. 9. Leptin, M. (2005). Gastrulation movements: the logic and the nuts and bolts. Dev. Cell 8, 305–320.
Social Cognition: Imitation, Imitation, Imitation Monkeys recognize when they are being imitated, but they seem unable to learn by imitation. These facts make sense if imitation is seen as two different capacities: social mirroring, when actions are matched and have social benefits; and learning by copying, when new behavioural routines are acquired by observation. Richard W. Byrne Imitation gets a bad press: we know it is the sincerest form of flattery, and of course for effective education the learner must be able to copy the teacher, but on the whole, ‘imitation’ is linked to shallow, cheap and even fraudulent behaviour. It comes as a shock to discover that, as far as
we know, most non-human animals are unable to imitate [1]: is imitation, after all, rather clever? In everyday human life, imitation is remarkably prevalent: babies imitate the facial movements of adults within minutes of birth [2]; lovers find themselves unconsciously mirroring the other’s posture, and sycophants do the same with the stance and
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Department of MCD Biology, University of California, Santa Barbara, California 93106, USA. E-mail: [email protected] DOI: 10.1016/j.cub.2005.06.030
mannerisms of the powerful [3]; when you copy a friend’s wave in a dense crowd it shows them immediately you’ve seen them; and even the most inarticulate mechanic can show us what to do to fix our car’s engine. Imitation certainly comes naturally to humans. The idea that imitation is a special faculty, critical in child development and perhaps a central aspect of human uniqueness, has gained ground in psychology over recent years [4,5]. The discovery of ‘mirror neurons’ [6,7] — cells in the premotor area of the brain that are activated by a hand performing a simple goal-directed action and respond equally whether the hand is one’s own or another person’s