Fig. 3

Spatial restriction of vasa-1, pl10, piwi, nanos-1, and nanos-2 expression during gastrozooid ontogenesis indicates cell differentiation and restriction of i-cells to particular domains. a–e, h Ontogenetic series of gastrozooids, stained blue for vasa-1 transcript. Distal is up. a Young gastrozooid buds close to the siphosomal horn. b Young gastrozooid with strong vasa-1 expression in the developing tentacle bud. Within the basigaster region transcript was found predominantly in deeper tissue layers. Anterior view. c Slightly older gastrozooid with vasa-1 expression in the gastrozooid tip (open arrowhead) and faint signal in deeper layers of the basigaster (filled arrowhead). Posterior view. d Early stage of tentacle formation with developing tentilla branching off the tentacle. Anterior view. e vasa-1 transcript disappears from maturing gastrozooid within the developing tip (empty arrowhead) and from the basigaster region (filled arrowhead) but remains present in tentacle bases and developing tentilla. Lateral view, anterior to the left. f, g Ontogenetic series of gastrozooids, stained blue for piwi (f) and nanos-1 (g) transcript. Transcript gets restricted to deeper tissue layers within the basigaster. h–l Mature gastrozooids stained blue for vasa-1 (h), pl10 (i), piwi (j), and nanos-1 transcript (k). Expression is not detectable in the body of the mature gastrozooid. l nanos-2 expression in the basigaster region and the tentacle base of a gastrozooid. m Semi-thin longitudinal section of a mature gastrozooid basigaster, stained with toluidine blue. Undifferentiated cells (pc) are present along the mesoglea in ectodermal tissue and nematoblasts (nb) in outer ectodermal layers. ba basigaster, dte developing tentilla, ect ectoderm, end endoderm, g gastrozooid, h horn, m mesoglea, mgo male gonophore, nb nematoblast, pc putative nematocyte progenitor cells, tb tentacle bud, tba tentacle base, te tentillum