The Society for Integrative and Comparative Biology
Introduction to the Symposium1
1 Department of Biological Sciences, Northern Illinois University, DeKalb, Illinois 60115
2 Department of Biological Chemistry and the Developmental Biology Center, 240 D Med Sci 1, University of California, Irvine, California 92697-1700
Forty-five years ago a symposium on the "Lower Metazoa" was held in Pacific Grove, California. The resulting volume (Dougherty et al., 1963
) sampled much of the then current study and thought about simple animals—sponges, cnidarians, ctenophores, flatworms, pseudocoelomates, and more. Extensive discussions of homology and phylogeny formed the basis for testing a variety of hypotheses concerning the origin of and relationships among various metazoan taxa. A number of well-developed theories (e.g., the ciliate-acoel and the planula-acoel hypotheses) were widely discussed. As is clear from the reference lists, these theories have a long history. Some of the speculative ideas advanced seem perhaps less far-fetched today, e.g., Dougherty (1963, p. 3): "...is novel genetic information, once acquired, too precious to be thrown away; may it perhaps be shifted about and put to seemingly different use, but, like the genie in the bottle, remain to emerge again in its earlier form?" For example, consider in this context the possibility of medusa loss and reappearance in cnidarian lineages (Boero et al., 1996; Marques and Migotto, 2001). Throughout the Dougherty et al. volume, the premise that is clear above all else is that data from the basal metazoans will be decisive in testing various hypotheses and theories of the origin of animals in general and bilaterians in particular.
A number of symposia since 1960 (often sponsored by the American Society of Zoologists and its successor, the Society for Integrative and Comparative Biology) reaffirm and update these themes (see Garey, 2002). The basic assumption remains intact as well: without detailed knowledge of the basal metazoans, it is impossible to provide an effective comparative framework for the study of animal evolution. Recent views of animal phylogeny (e.g., Halanych, 2004) favor somewhat altered hypotheses for the early-diverging taxa compared to those current when the Dougherty et al. (1963) volume was compiled. Most investigations of some well-studied simple animals including the nematodes and most or all of the flatworms no longer find these to be basal bilaterians. The position of the ctenophores is now much debated as well—are they sister to all other tissue-grade metazoans or are they sister to the bilaterians? Interest in the placozoans has also been rekindled, particularly because of their small genome (Grell and Ruthman, 1991). There is currently considerable debate as to whether sponges or placozoans are the earliest-diverging animals (e.g., Ender and Schierwater, 2003). Despite these debates, there is a broad consensus that the non-bilaterian metazoans constitute at least four groups: sponges (which may be paraphyletic), placozoans, cnidarians, and ctenophores.
These early-diverging metazoans are crucial to comparative and evolutionary studies. The reconstructed character states of basal nodes for any character will clearly depend on the character states of the basal taxa, and for animals these include the above-mentioned sponges, placozoans, cnidarians, and ctenophores. Nevertheless, the complexity of biological systems makes it unrealistic to analyze many representatives of these taxa in depth, particularly at the genomic level. Thus the "model system" approach can and should complement the comparative approach (Hanken, 1993). In biology, a model is a symbolic representation of a hypothesis, thus facilitating its testing (Kohn et al., 1977). We use the term "model system" as typically employed in developmental biology, e.g., Robert (2004, p. 1008), "...a model system comprises both a model organism and the network of relationships in which it figures, both within the laboratory and between research centers." In this context, character evolution in metazoans—whether genomic, cellular, or organismal—should be illuminated by well-developed model systems in these basal groups. At this time, sponges seem deserving of an entirely separate treatment, in part because they are a large and diverse group and in part because they may be united by shared primitive rather than derived characters (thus their systematics are in a state of flux). Appropriately, the symposium "Sponges: new views of old animals," sponsored by the Division of Invertebrate Zoology, complemented our efforts (see Integrative and Comparative Biology, Vol. 45, #2). The focus here then is on the other non-bilaterian metazoans—placozoans, cnidarians, and ctenophores.
By virtue of their greater diversity and correspondingly greater scrutiny, particularly at the molecular level, cnidarians received the bulk of the attention. Indeed, with the recent initiation of genomic projects using cnidarians (e.g., the Hydra expressed sequenced tags and Hydra magnipapillata genome project, the production of a Nematostella vectensis bacterial artificial chromosome library, and the N. vectensis genome project, see Steele et al., 2004; Darling et al., 2005), as well as the increasing interest in the ecological condition of coral reefs (e.g., Bellwood et al., 2004), cnidarians are deservedly receiving considerable attention. It is timely to consider what sorts of cnidarian model organisms should be developed and in what ways these organisms can serve as model systems. In some sense, every model organism needs to be developed and selected from its natural progenitors, and no organism will be an entirely ideal model (Robert, 2004). While H. magnipapillata and N. vectensis will no doubt provide very useful benchmarks for the evolution of animal genomes, these two species by no means exhaust cnidarian diversity (e.g., the life cycle of neither species has a medusa or colonial polyp stage). Clearly, there are opportunities for other cnidarians to serve as models of other biological systems. Considerations of how ctenophores and placozoans can complement and contrast to cnidarian models are also essential.
In some respects, cnidarians are currently well established as model systems. Most notably, for decades studies of cnidarians have illuminated developmental mechanisms in general (e.g., Steele, 2002; Ball et al., 2004) and axial patterning in particular (Meinhardt and Gierer, 2000). While several presentations at the symposium discussed these aspects, much of the focus of the following papers is in less well-established directions, in efforts to broaden the relevance of basal metazoan models. In these papers, early-diverging animals are considered as models for investigating symbiosis, eye development, programmed cell death, environmental signaling, histocompatibility, asexual fission, and more. While axial patterning is currently a principal topic in developmental evolution, these papers show that basal metazoans have broader relevance as well. Generally, the symposium was devoted to three interrelated themes: phylogenetics, genomics, and a better understanding of phenotypes. In phylogenetics, new characters and methods are needed to resolve the relationships among the basal metazoans. At the same time, specific basal metazoans are being selected and promoted within the scientific community as models for genome-level projects. Finally and perhaps most importantly, to facilitate functional assays, all levels of the phenotype must be further characterized by detailed laboratory and field studies.
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ACKNOWLEDGMENTS |
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Many thanks to the Society for Integrative and Comparative Biology in general and the Divisions of Invertebrate Zoology and Evolutionary Developmental Biology in particular for hosting and supporting this symposium. The National Institutes of Health, the National Science Foundation, and Northern Illinois University also provided generous support. Thanks again to all the speakers for their stimulating presentations and to the audience for putting up with the sometimes crowded venue! The editor, Alan Kohn, and a number of reviewers provided helpful comments on all the manuscripts.
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FOOTNOTES |
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1 From the Symposium Model Systems for the Basal Metazoans: Cnidarians, Ctenophores. and Placozoans presented at the Annual Meeting of the Society for Integrative and Comparative Biology, 5– 9 January 2004, at New Orleans, Louisiana.
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Bellwood, D. R., T. P. Hughes, C. Folke, and M. Nyström. 2004. Confronting the coral reef crisis. Nature, 429:827-833.[CrossRef][Medline]
Boero, F., J. Bouillon, and S. Piraino. 1996. Classification and phylogeny in the Hydroidomedusae (Hydrozoa, Cnidaria). In S. Piraino, F. Boero, J. Bouillon, P. F. S. Cornelius, and J. M. Gilli (eds.), Advances in hydrozoan biology.Sci. Mar, 60:17-33.
Darling, J. A., A. R. Reitzel, R. M. Burton, M. E. Mazza, J. F. Ryan, J. C. Sullivan, and J. R. Finnerty. 2005. Rising starlet: The starlet sea anemone, Nematostella vectensis. BioEssays, 27:211-221.[CrossRef][Web of Science][Medline]
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Grell, K. G., and A. Ruthman. 1991. Placozoa. In F. W. Harrison, and J. A. Westfall (eds.), Microscopic anatomy of invertebrates, Vol. 2. Placozoa, Porifera, Cnidaria, and Ctenophora, pp. 13– 27. Wiley-Liss, New York.
Halanych, K. M. 2004. The new view of animal phylogeny. Annu. Rev. Ecol. Evol. Syst, 35:229-256.
Hanken, J. 1993. Model systems versus outgroups: Alternative approaches to the study of head development and evolution. Amer. Zool, 33:448-456.
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Marques, A. C., and A. E. Migotto. 2001. Cladistic analysis and new classification of the family Tubulariidae (Hydrozoa, Anthomedusae). Papéis Avulsos Zool., S. Paulo, 41:465-488.
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