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Introduction: The hedgehog signaling pathway plays a crucial role in inducing segment polarity through cell-cell interactions in various metazoans, including arthropods and annelids. However, its involvement in organogenesis and segmentation among lophotrochozoans remains inconsistent. This study aimed to explore the role of the hedgehog gene during gut development in the freshwater leech, Helobdella austinensis.

Methods: Developmental RT-PCR and in situ hybridization were performed to examine the expressions of hedgehog genes. In addition, embryos were treated with cyclopamine (a hedgehog signaling antagonist) and purmorphamine (a Smo agonist) to examine the potential interactions between Helobdella orthologs to hedgehog and two NKL genes: Hau-NK2 and Hau-NK4.

Results: We examined the expressions of four core pathway members – Hedgehog (Hh), Patched (Ptc), Smoothened (Smo), and the downstream transcription factor Gli – spatiotemporally during the embryonic stages of H. austinensis. All four genes were expressed in the developing gut and proboscis during organogenesis but not during the segmentation stage. Additionally, the treatment of embryos with cyclopamine and purmorphamine revealed that NK genes are regulated by hedgehog signaling. Furthermore, NK2 and NK4 were expressed in the developing gut rather than in a segmental stripe pattern.

Conclusion: This study confirms that the hedgehog signaling pathway is associated with gut development in the freshwater leech, H. austinensis. The expression patterns of hedgehog pathway genes and their interaction with NK genes suggest a role of hedgehog signaling in regulating gut development rather than segmentation in the freshwater leeches.