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Abstract

How new morphologies are generated has been a subject of speculation for centuries by such likes as Charles Darwin and Jean-Baptiste Lamarck. Changes in the shape of organs are an essential component to the development of new body plans, new species, and the capacity to take advantage of new ecological niches. However molecular investigations into the advent of novelties are in their infancy. Here, we show that within Diplogastridae, the hermaphrodite gonad has a novel path of gonad arm extension and makes a unique ventral migration in comparison to many other representative nematodes including C. elegans. This novel ventral migration can be prevented in Pristionchus pacificus by laser ablation or genetic ablation of the vulva. Ablation of all three vulval precursor cells (VPCs) results in all gonad arms failing to migrate ventrally. Wnt signaling plays a putative direct role in instructing the distal tip cells (DTCs) to migrate from the dorsal to the ventral side of the animal as the extending arms come back to the anterior-posterior center of the animal. In P. pacificus hermaphrodite animals where the Wnt pathway members bar-1, mom-2 and cwn-2 are mutated, gonad arms fail to extend ventrally. Ppa-BAR-1 is expressed in the nuclei of DTCs consistent with a signaling role for BAR-1 in these cells. Ppa-MOM-2 is expressed in the developing vulva at the appropriate stage suggesting it is part of a complex Wnt signal from the vulva to the DTCs. Taken together it is likely that cooption of Wnt signaling members has allowed for the advent of new organ morphology. We hypothesize that the Wnt signal is permissive and acts through the Netrin cell guidance system in P. pacificus; this hypothesis is based upon the observation that members of this pathway are necessary for all post-embryonic dorsal-ventral migrations in C. elegans. We are currently investigating the expression of Netrin receptors in the DTCs of P. pacificus hermaphrodites. Intriguingly analysis of the P. pacificus genome and the available expressed sequence tags (ESTs), suggest the composition of the Netrin pathway and its molecular functional organization operate differently in P. pacificus than C. elegans. This system provides in roads towards investigating the acquisition of new signaling functions and altered molecular networks and their roles in generating new organ shapes and body plans.