PhD thesis - Biologisk Institut - Københavns Universitet

15 General introductionthey have been variously seen as either a sister taxon to Annelida or to Mollusca (e.g.,Åkesson 1958, Rice 1975b, c, Scheltema 1993, 1996, Cutler 1994, Zrzavy et al. 1998,Giribet et al. 2000). Nowadays, a growing number of morphological and molecularanalyses strongly suggest a close relationship to annelids, leaving the only questionwhether sipunculans cluster within Annelida or constitute their sister taxon (e.g.,Wanninger et al. 2005a, Tzetlin and Purschke 2006, Struck et al. 2007, Dunn et al.2008, Mwinyi et al. 2009, Shen et al. 2009, Sperling et al. 2009, Zrzavy et al. 2009,Dordel et al. 2010, Hausdorf et al. 2010, Struck et al. 2011). Accordingly, theinclusion of Sipuncula within Annelida suggests a secondary loss of a previouslysegmented body plan in Sipuncula rather than an initial evolutionary step towardssegmentation in these animals (Dordel et al. 2010, Struck et al. 2011).Thesis objectivesIn the light of the proposed sipunculan-annelid assemblage, it was the aim of thepresent PhD thesis to elucidate sipunculan body plan patterning and to deal with thefollowing scientific issues:1. Neuro- and myogenesis were described in three different sipunculan species,Phascolosoma agassizii, Themiste pyroides and Thysanocardia nigra (Fig. 2).Since nervous and muscle systems follow a segmental development inannelids, these experiments should clarify whether or not sipunculans showcryptic segmentation during their ontogeny.2. The distribution of proliferating cells in T. pyoides and T. nigra weredescribed throughout development to assess whether sipunculans possess aposterior growth zone similar to the segmented annelids.3. Comparative analysis of the acquired developmental data with those availablefor annelids and other lophotrochozoans (e.g., Mollusca) were carried out inorder to infer shared and possible ancestral neuromuscular features.4. The establishment and application of an in situ hybridisation protocol foranalyses of tempo-spatial expression patterns of selected candidate genesknown to be involved in body patterning (e.g., neural, muscular and so-called“segmentation” genes (hox1-9, even-skipped, hairy)).