5 Metazoan Complexity
163
5.8 Aplysia: From Neural Circuits to Neurotranscriptomics
Molluscs comprise one of the largest phyla in the animal kingdom, second only
to arthropods (Brusca and Brusca 2003). Many of the mollusc species are marine.
Moreover, molluscs comprise the cephalopods, which are counted among the most
highly evolved invertebrates. The sea hare, Aplysia californica, has become a classic model system for neurobiology, primarily due to the pioneering analyses on the
setup and modulation of its simple neural circuitry, and the molecular mediators
of learned behaviour (reviewed in Kandel 2001). The analysis of the Aplysia nervous system is greatly facilitated by the large and easy-to-recognize neurons of this
species, some of which reach a diameter of 1 mm. A recent study made use of the
size of these cells for RNA extractions to determine specific transcriptomes of the
Aplysia nervous system. Specific libraries were generated from single types of neuron, for example the metacerebral cells (MCC) or even its neurites (Moroz et al.
2006). As a complementary approach to the ongoing Aplysia genome sequencing project, this EST sampling already revealed many interesting components.
Indicative of the general mollusc gene repertoire, the search revealed additional
genes missing from the representative ecdysozoan models, such as the zinc finger
transcription factor churchill (also described in Kortschak et al. 2003), P2X receptor
genes involved in pain sensing and forms of long-term synaptic plasticity, selenoprotein N homologs or major vault proteins with RNA binding capacity. Moreover, the
sequences uncovered correlates of DNA methyl transferase 1, DNA methyl transferase associating protein, and the transcriptional repressor Methyl-CpG binding
Domain Protein 2. The combination of these factors suggests that Aplysia, in contrast to Caenorhabditis elegans or insects, possesses a CpG methylation pathway,
representing an independent route to gene regulatory complexity in this species.
This EST dataset can now be used to determine the cell-type-specific transcriptomes that will for instance allow the differences between two neuron types to be
studied, or between neurons before and after stimulation. These data will not only
be interesting for the particular neurobiological question, but will also have wider
implications for the comparison of complexity: currently, it is unclear how easily
new cell types can arise in evolution, and how many distinct molecular characteristics it takes to generate cell types of separate function. Moreover, a transcriptomic
approach to define cell types could add a more quantitative and objective component
to the analysis of cell type complexity, as this typically depends on more subjective
measures. Empirical values for the molecular distinctions of individual cell types
could in turn also provide new means to assess the evolution of cell types.
5.9 Platynereis: Ancestral Complexity of Cells
and Genomic Features
Over the last decade, the marine annelid worm Platynereis dumerilii has begun to
emerge as suitable model species for molecular evolutionary comparisons. Current
molecular resources include high-quality sequences for more than 70,000 ESTs and
163
5.8 Aplysia: From Neural Circuits to Neurotranscriptomics
Molluscs comprise one of the largest phyla in the animal kingdom, second only
to arthropods (Brusca and Brusca 2003). Many of the mollusc species are marine.
Moreover, molluscs comprise the cephalopods, which are counted among the most
highly evolved invertebrates. The sea hare, Aplysia californica, has become a classic model system for neurobiology, primarily due to the pioneering analyses on the
setup and modulation of its simple neural circuitry, and the molecular mediators
of learned behaviour (reviewed in Kandel 2001). The analysis of the Aplysia nervous system is greatly facilitated by the large and easy-to-recognize neurons of this
species, some of which reach a diameter of 1 mm. A recent study made use of the
size of these cells for RNA extractions to determine specific transcriptomes of the
Aplysia nervous system. Specific libraries were generated from single types of neuron, for example the metacerebral cells (MCC) or even its neurites (Moroz et al.
2006). As a complementary approach to the ongoing Aplysia genome sequencing project, this EST sampling already revealed many interesting components.
Indicative of the general mollusc gene repertoire, the search revealed additional
genes missing from the representative ecdysozoan models, such as the zinc finger
transcription factor churchill (also described in Kortschak et al. 2003), P2X receptor
genes involved in pain sensing and forms of long-term synaptic plasticity, selenoprotein N homologs or major vault proteins with RNA binding capacity. Moreover, the
sequences uncovered correlates of DNA methyl transferase 1, DNA methyl transferase associating protein, and the transcriptional repressor Methyl-CpG binding
Domain Protein 2. The combination of these factors suggests that Aplysia, in contrast to Caenorhabditis elegans or insects, possesses a CpG methylation pathway,
representing an independent route to gene regulatory complexity in this species.
This EST dataset can now be used to determine the cell-type-specific transcriptomes that will for instance allow the differences between two neuron types to be
studied, or between neurons before and after stimulation. These data will not only
be interesting for the particular neurobiological question, but will also have wider
implications for the comparison of complexity: currently, it is unclear how easily
new cell types can arise in evolution, and how many distinct molecular characteristics it takes to generate cell types of separate function. Moreover, a transcriptomic
approach to define cell types could add a more quantitative and objective component
to the analysis of cell type complexity, as this typically depends on more subjective
measures. Empirical values for the molecular distinctions of individual cell types
could in turn also provide new means to assess the evolution of cell types.
5.9 Platynereis: Ancestral Complexity of Cells
and Genomic Features
Over the last decade, the marine annelid worm Platynereis dumerilii has begun to
emerge as suitable model species for molecular evolutionary comparisons. Current
molecular resources include high-quality sequences for more than 70,000 ESTs and