Developmental system drift in motor ganglion patterning between distantly related tunicates.

BACKGROUND

The larval nervous system of the solitary tunicate is a simple model for the study of chordate neurodevelopment. The development and connectivity of the motor ganglion have been studied in fine detail, but how this important structure develops in other tunicates is not well known.

METHODS AND RESULTS

By comparing gene expression patterns in the developing MG of the distantly related tunicate we found that its patterning is highly conserved compared to the MG. MG neuronal subtypes in were specified in the exact same positions as in though the timing of subtype-specific gene expression onset was slightly shifted to begin earlier, relative to mitotic exit and differentiation. In transgenic embryos electroporated with reporter plasmids, we were also able to characterize the morphology of the lone pair of descending decussating neurons (ddNs) in revealing the same unique contralateral projection seen in ddNs and their putative vertebrate homologs the Mauthner cells. Although expression labels the ddNs in both species, cross-species transgenic assays revealed significant changes to the regulatory logic underlying transcription. We found that -regulatory DNAs from can drive highly specific reporter gene expression in ddNs, but sequences are not active in ddNs.

CONCLUSIONS

This acute divergence in the molecular mechanisms that underlie otherwise functionally conserved -regulatory DNAs supports the recently proposed idea that the extreme genetic plasticity observed in tunicates may be attributed to the extreme of the spatial organization of their embryonic cell lineages.