Characterization and developmental expression of the amphioxus homolog of notch (AmphiNotch): Evolutionary conservation of multiple expression domains in amphioxus and vertebrates

Notch encodes a transmembrane protein that functions in intercellular signa ling. Although there is one Notch gene in Drosophila, vertebrates have thre e or more with overlapping patterns of embryonic expression. We cloned the entire 7575-bp coding region of an amphioxus Notch gene (AmphiNotch), encod ing 2524 amino acids, and obtained the exon/intron organization from a geno mic cosmid clone. Southern blot and PCR data indicate that AmphiNotch is th e only Notch gene in amphioxus. AmphiNotch, like Drosophila Notch and verte brate Notch1 and Notch2, has 36 EGF repeats, 3 Notch/lin-12 repeats, a tran smembrane region, and B ankyrin repeats. Phylogenetic analysis places it at the base of all the vertebrate genes, suggesting it is similar to the ance stral gene from which the vertebrate Notch family genes evolved. AmphiNotch is expressed in all three embryonic perm layers in spatiotemporal patterns strikingly similar to those of all the vertebrate homologs combined. In th e developing nerve cord, AmphiNotch is first expressed in the posteriormost part of the neural plate, then it becomes more broadly expressed and later is localized dorsally in the anteriormost part of the nerve cord correspon ding to the diencephalon. In late embryos and larvae, AmphiNotch is also ex pressed in parts of the pharyngeal endoderm, ill the anterior gut diverticu lum, and, like AmphiPax2/5/8, in the rudiment of Hatschek's kidney. A compa rison with Notch1 and Pax5 and Pax8 expression in the embryonic mouse kidne y helps support homology of the amphioxus and vertebrate kidneys. AmphiNotc h is also an early marker for presumptive mesoderm, transcripts first being detectable at the gastrula stage in a ring of mesendoderm just inside the blastopore and subsequently in the posterior mesoderm, notochord, and somit es. As in sea urchins and vertebrates, these domains of AmphiNotch expressi on overlap with those of several Wnt genes and brachyury. These relationshi ps suggest that amphioxus shares with other deuterostomes a common mechanis m for patterning along the anterior/posterior axis involving a posterior si gnaling center in which the Notch and Wnt pathways and brachyury interact. (C) 2001 Academic Press.