5 Metazoan Complexity
161
ligands. Also, cnidarians have independently expanded some gene families, such as
light-perceiving opsins (Suga et al. 2008) or muscle contraction-regulating myosin
light chains (unpublished) that might have led to an independent increase of cell
types in some cnidarian lineages.
5.6 Ecdysozoans: Going Beyond the Established Systems
Ecdysozoans comprise a superphylum of molting animals that include, among
others, nematodes, arthropods and priapulids (Aguinaldo et al. 1997) (also see
Chapter 4). Two of the best-studied model systems of molecular biology – the nematode Caenorhabditis elegans and the fruitfly Drosophila melanogaster – belong to
the ecdysozoans. Driven by the availability of powerful genetic tools, these two
models have had an enormous impact on biological research, covering a broad range
of topics that reaches from basic cellular principles to systems analyses. The significance of these studies for our understanding of biology remains undisputed. As
outlined already above, however, more and more evidence challenges the unspoken
assumptions that these findings are representative for the majority of invertebrates,
or that they provide direct approximations of the processes prevailing in a simple
animal predecessor. In contrast, these data suggest that some of the more simple features displayed by extant ecdysozoan models are the result of secondary
simplification of ancestrally complex characters.
Notwithstanding the restricted retention of ancestral complexity, several ecdysozoan groups have evolved fascinatingly complex characters. For instance, the
Drosophila Down syndrome cell adhesion molecule (Dscam) gene is an intron-rich
gene that displays the highest number of splice variants identified in any species to
date (Schmucker et al. 2000). In Drosophila melanogaster, Dscam isoforms appear
to convey specific identity to migrating neurons, and mediate homophilic repulsions,
an important prerequisite for the establishment of neural circuits. Dscam transcripts
also display high diversity in related groups, including the crustacean Daphnia, suggesting that the cellular diversity mediated by Dscam alternative splicing could be
a more basal arthropod feature (Brites et al. 2008). In contrast, vertebrate Dscam
orthologs do not seem to undergo extensive alternative splicing, even though they
also appear to be involved in homophilic interactions (reviewed in Hattori et al.
2008).
Examples like this illustrate that the evaluation of animal complexity, even when
analysed on a quantitative molecular basis, needs to distinguish between the conservation of ancestral complexity and secondarily gained features of complexity. As
it is still unclear how these two types of changes relate to one another, complex
features of a given species or group will always have to be assessed for their evolutionary time of origin. It is also noteworthy in this context that both insects and
nematodes are among the most diverse animal groups, with insects being the most
species-rich group of all animals (Brusca and Brusca 2003). Despite several lost features of complexity, insects have thus evolved broad ranges of new morphological
161
ligands. Also, cnidarians have independently expanded some gene families, such as
light-perceiving opsins (Suga et al. 2008) or muscle contraction-regulating myosin
light chains (unpublished) that might have led to an independent increase of cell
types in some cnidarian lineages.
5.6 Ecdysozoans: Going Beyond the Established Systems
Ecdysozoans comprise a superphylum of molting animals that include, among
others, nematodes, arthropods and priapulids (Aguinaldo et al. 1997) (also see
Chapter 4). Two of the best-studied model systems of molecular biology – the nematode Caenorhabditis elegans and the fruitfly Drosophila melanogaster – belong to
the ecdysozoans. Driven by the availability of powerful genetic tools, these two
models have had an enormous impact on biological research, covering a broad range
of topics that reaches from basic cellular principles to systems analyses. The significance of these studies for our understanding of biology remains undisputed. As
outlined already above, however, more and more evidence challenges the unspoken
assumptions that these findings are representative for the majority of invertebrates,
or that they provide direct approximations of the processes prevailing in a simple
animal predecessor. In contrast, these data suggest that some of the more simple features displayed by extant ecdysozoan models are the result of secondary
simplification of ancestrally complex characters.
Notwithstanding the restricted retention of ancestral complexity, several ecdysozoan groups have evolved fascinatingly complex characters. For instance, the
Drosophila Down syndrome cell adhesion molecule (Dscam) gene is an intron-rich
gene that displays the highest number of splice variants identified in any species to
date (Schmucker et al. 2000). In Drosophila melanogaster, Dscam isoforms appear
to convey specific identity to migrating neurons, and mediate homophilic repulsions,
an important prerequisite for the establishment of neural circuits. Dscam transcripts
also display high diversity in related groups, including the crustacean Daphnia, suggesting that the cellular diversity mediated by Dscam alternative splicing could be
a more basal arthropod feature (Brites et al. 2008). In contrast, vertebrate Dscam
orthologs do not seem to undergo extensive alternative splicing, even though they
also appear to be involved in homophilic interactions (reviewed in Hattori et al.
2008).
Examples like this illustrate that the evaluation of animal complexity, even when
analysed on a quantitative molecular basis, needs to distinguish between the conservation of ancestral complexity and secondarily gained features of complexity. As
it is still unclear how these two types of changes relate to one another, complex
features of a given species or group will always have to be assessed for their evolutionary time of origin. It is also noteworthy in this context that both insects and
nematodes are among the most diverse animal groups, with insects being the most
species-rich group of all animals (Brusca and Brusca 2003). Despite several lost features of complexity, insects have thus evolved broad ranges of new morphological