Lophotrochozoan Zic Genes.

Lophotrochozoa is a sister taxon of Ecdysozoa in the Protostomia that includes mollusks, annelids, brachiopods, and platyhelminths. Recent studies have clarified the structure, expression, and roles of lophotrochozoan Zic family genes. Zic genes in oligochaete annelid Tubifex tubifex (freshwater sludge worm) and polychaete annelid Capitella teleta (bristle worm) are commonly expressed in a subset of developing brain and mesoderm derivatives. The latter includes the naïve mesoderm and the associated chaetal sacs in each body segment, although the segmentation processes differ between the two species. Furthermore, in brachiopod Terebratalia transversa (lamp shell), Zic is expressed in the anterior ectodermal domains and mesodermal derivatives, including those associated with the chaetal sacs. This result suggests the common involvement of Zic genes in the development of chaetae, a lophotrochozoan novelty acquired in the course of evolution. In addition, the highly simplified lophotrochozoan Dicyema acuticephalum (dicyemid mesozoan, a cephalopod endoparasite), which lost its gut, nervous system, and muscles during evolution, expresses its Zic genes in hermaphroditic gonads, highlighting the role of Zic genes in germ cell development. The role of Zic in head regeneration was revealed in studies on platyhelminth Schmidtea mediterranea (freshwater planarian). Planarian Zic expression was induced in a subpopulation of neoblasts that includes adult pluripotent stem cells. It is needed for head regeneration and production of an anterior signaling center. Suppression of Wnt-β-catenin signaling underlies Zic-mediated head regeneration, reminiscent of Wnt-β-catenin suppression by vertebrate Zic genes. Taken together, studies on the lophotrochozoan Zic genes are essential to understanding not only the roles of these genes in body plan evolution but also the molecular mechanism underlying adult stem cell regulation.