The epidermis was the first epithelium to express
dlg-1 mRNA. It was initially detected at the late 4E stage but with no detectableDLG-1 protein. The level of
dlg-1 mRNA increased during the 8E stage and was maintained throughout the 16E and elongation stages (comma, 1.5-fold). DLG-1 protein was first observed during the late 8E stage, with puncta of protein visible on the membrane of nascent epidermal cells. These puncta began to coalesce at the early 16E stage and formed a continuous, circumferential junction by the
mid-16E stage. The level of DLG-1 increased during the elongation stages (comma, 1.5-fold), as the cells changed shape to convert the embryo from a ball into a vermiform.The digestive tract began to express
dlg-1 mRNA at the 8E stage, and, similar to the epidermis, the levels of RNA increased throughout the 8E and 16E stages. DLG-1 protein was first observed in midgut precursors at the early 16E stage, where puncta of protein appeared at the lateral surface and rapidly coalesced at the apical surface, in agreement with previous studies (Leung et al., 1999; Totong et al., 2007; Achilleos et al., 2010). By the
mid-16E stage (20-40 min later), the puncta of DLG-1 had banded together to form cell junctions, which continued to expand and mature as the embryo elongated (comma, 1.5-fold stages). The RNA remained expressed in the intestine throughout all of these stages. In the foregut, DLG-1 protein was first detectable by the
mid-16E stage, suggesting that translation of
dlg-1 mRNA was delayed in this tissue by 20-40 min. We observed membrane-associated DLG-1 puncta on cell surfaces throughout the foregut at the 16E stage. These spots accumulated at the nascent apical surface by the bean stage, where they joined together to form connected junctions by the comma stage. The RNA remained expressed throughout these stages. The arcade cells are born during the
mid-16E stage, starting 290 min after the first division (Sulston et al., 1983). The majority of these cells are anterior to the foregut primordium and express
dlg-1 mRNA from birth. DLG-1 protein accumulated 100 min later in the arcades, after the epidermis and foregut had both formed epithelia and soon before the arcade cells became an epithelium (i.e., between the comma and 1.25-fold stage, 390-400 min after the first division; Portereiko and Mango, 2001; Portereiko et al., 2004). The presence of RNA but lack of protein was detectable by the 16E stage but was clearest at the comma stage, when the arcade cells clustered together as a group anterior to the foregut epithelium. Thus there was a delay in protein accumulation, suggesting that either the RNA was translationally repressed or protein was made but degraded immediately.