PhD thesis - Biologisk Institut - Københavns Universitet

37 Conclusions and future perspectivesConclusions and future perspectivesDespite the fact that circular body wall muscles do not develop in a segmentalmanner, the data presented herein document for the first time traits of segmentation inSipuncula. For a short period of time during sipunculan ontogeny, a repeated patternis generated from the posterior pole of the larvae in their nervous system, whichresembles the annelid-like mode of segmentation. In agreement with recent molecularphylogenetic analyses, this strongly argues for a segmented sipunculan and annelidlast common ancestor, which had four longitudinal body wall muscles that developedfrom anterior to posterior. However, the absence of circular body wall muscles in anumber of adult polychaetes and their larvae casts doubt on the assumption that ringmuscles were part of the annelid ground pattern, although multiple, independent lossof these muscles in the respective lineages are also possible. Accordingly, thedifferent pathways of body segmentation in annelids (including sipunculans,echiurans, and myzostomids) might be due to adaptations to different modes of life,thus indicating that segmentation is more complex as well as labile to evolutionarychanges than previously assumed. The fact that a segmented body is established andlater secondarily lost during ontogeny renders sipunculans an ideal group to study theontogeny and evolution of segmentation, in particular in comparison to the wellstudied annelid, arthropod and chordate model organisms. This might help to shedlight on the contradicting conclusions on morphological urbilaterian characteristics(i.e., a complex, segmented versus a simple, non-segmented species at the base of thebilaterian tree of life).In this context, one primary aim of future studies would be to deepen theknowledge on the “segmentation” process in Sipuncula from a molecular perspective.The way segments are specified in insects, especially Drosophila, is different from theway segments are controlled in vertebrates or annelids (Chipman 2010). Therefore,knowing which genes are involved in sipunculan segmentation, which unlike theabove mentioned model organisms looses its segmentation during ontogeny, can be agood objective for understanding the evolution of segmental specification and loss.Modern high-throughput sequencing technologies such as 454 reads orillumina are able to generate a large EST dataset of almost all genes expressed duringthe development of an animal. Accordingly, the analysis of the EST dataset and thegene expression of so-called “segmentation” genes (i.e., hairy, notch, delta, wingless,