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F. Raible and P.R.H. Steinmetz
complexity and variation. In the marine context, however, the diversity of insects
is only moderate. Other ecdysozoan groups, including crustaceans and free-living
nematodes, are more abundant in the sea, but suitable marine models and molecular
data for these groups are still scarce.
One emerging marine model system among the arthropods is the amphipod
Parhyale hawaiiensis, a developmental model species that is also subject to directed
genome sequencing. In Parhyale, unlike the conventional arthropod model systems,
early cleavages are total, unequal and invariant (Gerberding et al. 2002). Therefore,
early patterning processes act in a fundamentally different context than in the insect
model Drosophila, where early nuclear divisions generate a syncytium in which patterning molecules can diffuse. Future genetic analyses are thus necessary in order to
reveal the extent to which the genetic machineries in arthropods are actually related.
Such studies can rely on a growing repertoire of tools, including cell lineage tracing (Gerberding et al. 2002) and transgenesis (Pavlopoulos and Averof 2005). By
allowing comparisons between distant arthropod taxa, these studies will also permit a clearer view of how complex the ancestral patterning machinery at the base
of arthropods was. Likewise, studies in Parhyale reveal ancestral aspects of leg patterning in pancrustaceans, and thus provide data for the comparison – and ultimately
the evolution – of the respective gene regulatory networks (Prpic and Telford 2008).
Most of these studies can follow a candidate gene approach, yet a more unbiased
view of the genetic constitution of amphipods will rely on genomic sequencing.
Whereas the Parhyale genome is very large (3.6 Gbp), the smaller-sized genome
(690 Mbp) of a related amphipod, Jassa slatteryi, has recently been proposed for
genome sequencing and shall provide such unbiased insights into the amphipod
genome.
5.7 Lophotrochozoans: An Evolutionary Branch Leading
to New Perspectives
As outlined in Chapter 4, lophotrochozoans are an animal superphylum that combines, among others, molluscs and annelids (Halanych 2004, Halanych et al.
1995), and – along with the ecdysozoans – represents a large share of the protostomes. Although lophotrochozoans represent a major branch of eubilaterian
evolution, they have for a long time remained relatively poorly covered by molecular descriptions, mostly owing to the success of ecdysozoan species (Caenorhabditis
elegans, Drosophila melanogaster) as molecular genetic model systems. In recent
years, expressed sequence tag- (EST), bacterial artificial chromosome- (BAC) and
full genome-sequencing projects have started to close this gap by exploring the
genomic repertoire of lophotrochozoans. Although none of the genome projects
has so far been published, the emerging data already provide interesting facets of
lophotrochozoan biology that are also of relevance for the understanding of animal
complexity.
F. Raible and P.R.H. Steinmetz
complexity and variation. In the marine context, however, the diversity of insects
is only moderate. Other ecdysozoan groups, including crustaceans and free-living
nematodes, are more abundant in the sea, but suitable marine models and molecular
data for these groups are still scarce.
One emerging marine model system among the arthropods is the amphipod
Parhyale hawaiiensis, a developmental model species that is also subject to directed
genome sequencing. In Parhyale, unlike the conventional arthropod model systems,
early cleavages are total, unequal and invariant (Gerberding et al. 2002). Therefore,
early patterning processes act in a fundamentally different context than in the insect
model Drosophila, where early nuclear divisions generate a syncytium in which patterning molecules can diffuse. Future genetic analyses are thus necessary in order to
reveal the extent to which the genetic machineries in arthropods are actually related.
Such studies can rely on a growing repertoire of tools, including cell lineage tracing (Gerberding et al. 2002) and transgenesis (Pavlopoulos and Averof 2005). By
allowing comparisons between distant arthropod taxa, these studies will also permit a clearer view of how complex the ancestral patterning machinery at the base
of arthropods was. Likewise, studies in Parhyale reveal ancestral aspects of leg patterning in pancrustaceans, and thus provide data for the comparison – and ultimately
the evolution – of the respective gene regulatory networks (Prpic and Telford 2008).
Most of these studies can follow a candidate gene approach, yet a more unbiased
view of the genetic constitution of amphipods will rely on genomic sequencing.
Whereas the Parhyale genome is very large (3.6 Gbp), the smaller-sized genome
(690 Mbp) of a related amphipod, Jassa slatteryi, has recently been proposed for
genome sequencing and shall provide such unbiased insights into the amphipod
genome.
5.7 Lophotrochozoans: An Evolutionary Branch Leading
to New Perspectives
As outlined in Chapter 4, lophotrochozoans are an animal superphylum that combines, among others, molluscs and annelids (Halanych 2004, Halanych et al.
1995), and – along with the ecdysozoans – represents a large share of the protostomes. Although lophotrochozoans represent a major branch of eubilaterian
evolution, they have for a long time remained relatively poorly covered by molecular descriptions, mostly owing to the success of ecdysozoan species (Caenorhabditis
elegans, Drosophila melanogaster) as molecular genetic model systems. In recent
years, expressed sequence tag- (EST), bacterial artificial chromosome- (BAC) and
full genome-sequencing projects have started to close this gap by exploring the
genomic repertoire of lophotrochozoans. Although none of the genome projects
has so far been published, the emerging data already provide interesting facets of
lophotrochozoan biology that are also of relevance for the understanding of animal
complexity.