Summary and Conclusions

Tw'o main conclusions can be drawn from the work presented here, summarizing our current understanding of segmentation in leeches. The first and most obvious is that, as developmental biologists, we are just scratching the surface of understanding how this embryo develops and how' it generates segments. The degree of understanding of Helobodella development that has been achieved by a handful of research groups over roughly four decades pales in comparison with that achieved for standard model organisms studied by hundreds or thousands or workers, and often for much longer.

Nonetheless, a second and equally significant conclusion is that there is much to be learned from the careful study of non-model organisms such as Helobdella, representing comparatively unexplored branches of the phylogenetic tree. The tremendous diversity of animal genomes and animal body plans are linked by a corresponding diversity of developmental mechanisms, and we can only come to understand the full breadth of extant developmental mechanisms by investigating how development has been modified along different evolutionary branches.

Helobdella provides particularly fruitful grounds for such investigations, not only because of its phylogenetic position and its experimental tractability, but also because its highly rearranged genome has effectively accelerated the evolutionary exploration of “developmental hyperspaceThus, in addition to the retrospective approach of evo-devo, in which developmental comparisons are used primarily to infer ancestral mechanisms of development, studying Helobdella and its clitellate annelid allies should provide insights into the possibilities of divergent or novel developmental mechanisms, starting w'ith the mechanisms of genome diversification itself (Simakov et al. 2013; Cho et al. 2014).

In this chapter, we have summarized how studies of leech segmentation have revealed a number of evolutionary developmental novelties: axial growth from a PGZ comprising just five bilateral pairs of lineage-restricted stem cells; the production of segmental founder cells by parental and grandparental stem cell lineages operating in parallel; segmentation by lineage-dependent rather than boundary-dependent mechanisms; non-canonical expression of homologs of the Drosophila segmentation genes; dorsoventral patterning by evolutionarily divergent, cell contact-mediated BMP signaling; HOX cluster fragmentation and HOX gene duplication within a paradigmatically segmented animal; production of segmental gut structures by a syncytial yolk cell; and the evolution of a body plan with fixed segment number from an ancestor that presumably underwent indeterminate segmentation. This list of intriguing developmental phenomena is probably far from complete, and the application of emerging techniques in omics, optics, and experimental manipulation to the tractable Helobdella embryo should continue to provide insights into the evolution of developmental mechanisms.

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