Characterization of Notch-class gene expression in segmentation stem cells and segment founder cells in Helobdella robusta (Lophotrochozoa; Annelida; Clitellata; Hirudinida; Glossiphoniidae)

Summary To understand the evolution of segmentation, we must compare segmentation in all three major groups of eusegmented animals: vertebrates, arthropods, and annelids. The leech Helobdella robusta is an experimentally tractable annelid representative, which makes segments in anteroposterior progression from a posterior growth zone consisting of 10 identified stem cells. In vertebrates and some arthropods, Notch signaling is required for normal segmentation and functions via regulation of hes‐class genes. We have previously characterized the expression of an hes‐class gene (Hro‐hes) during segmentation in Helobdella, and here, we characterize the expression of an H. robusta notch homolog (Hro‐notch) during this process. We find that Hro‐notch is transcribed in the segmental founder cells (blast cells) and their stem‐cell precursors (teloblasts), as well as in other nonsegmental tissues. The mesodermal and ectodermal lineages show clear differences in the levels of Hro‐notch expression. Finally, Hro‐notch is shown to be inherited by newly born segmental founder cells as well as transcribed by them before their first cell division.