RT-PCR demonstrated that Dse expression was initiated in late gastrula embryos (Fig. 1B). Dsel was maternally expressed, and abundant zygotic transcripts were identified from the early tailbud stage onwards. DS epimerase activity represents the sum of DS-epi1 and DS-epi2 contributions (Maccarana et al., 2006). In Xenopus, there was a minute maternal level of epimerase activity and an onset of zygotic activity at the late gastrula stage, with a subsequent constant increase until at least stage 45 (Fig. 1C). Whole-mount in situ hybridization of stage 17 embryos indicated Dse expression in the epidermis, notochord and pre-migratory NC that overlapped with Snail2 expression in the cranial neural crest (CNC) (Fig. 1D-E′). Quantitative real-time PCR (qPCR) analysis confirmed Dse but not Dsel mRNA in isolated c-Myc+ CNC explants from stage 18 embryos (Fig. 1F). At stage 30, Dse and Dsel expression coincided with migrating Twist+ CNC cells in the mandibular, hyoid and branchial arches (Fig. 1G-I′).
