Comparative approach to developmental analysis: the case of the dalyellid flatworm, Gieysztoria superba

Dalyellida represents a taxon of small rhabdocoel flatworms that occur in f reshwater habitats all over the world. Combining histology and electron mic roscopy we have analyzed the embryonic development of a new dalyellid speci es, Gieysztoria superba, in order to obtain more comparative data pertainin g to morphogenesis and organogenesis in flatworms. We have used a morpholog ical staging system that we recently introduced for another rhabdocoel, Mes ostoma lingua (Younossi-Hartenstein et al., 2000). Our data show that in ma ny fundamental respects, such as the irregular cleavage, mesenchymal embryo nic primordium, and lack of gastrulation movements, Gieysztoria is highly s imilar to Mesostoma. During cleavage (stages 1 and 2) the embryo is located in the center of the egg where it is surrounded by a layer of yolk cells. Cleavage leads up to a solid, disc shaped cell cluster. During stage 3, the embryo migrates to the ventral side of the egg and acquires bilateral symm etry. Stages 4/5 sees the emergence of the first organ primordia, the brain , epidermis and pharynx. A peculiar invagination of the epidermal layer pus hes the embryo back into the center of the yolk ("embryonic invagination"). Organogenesis takes place during stages 5 and 6 while the embryo is invagi nated. A junctional complex, consisting initially of small septate junction s, followed later by a more apically located zonula adherens, is formed in all epithelial tissues, including epidermis, protonephridia, and pharynx. D uring late stages (6-8), Gieysztoria embryos evert back to the surface wher e the epidermal primordium expands and grows around the yolk to close dorsa lly. During this phase of development cytodifferentiation of the different organ systems takes place. Stage 7 is characterized by the appearance of ey e pigmentation, brain condensation and spindle shaped myocytes. Stage 8 des cribes the fully dorsally closed and differentiated embryo. In comparison t o other rhabdocoels, including Mesostoma, Gieysztoria exhibits a precocious differentiation of an intestinal epithelium and male genital apparatus. In Mesostoma, these structures are formed post hatching.