Development 114, 769-786 (1992) Printed in Great Britain © The Company of Biologists Limited 1992 769 Early mRNAs, spatially restricted along the animal-vegetal axis of sea urchin embryos, include one encoding a protein related to to Moid and BMP-1 SUSAN D. REYNOLDS, LYNNE M. ANGERER, JAMES PALIS*, ADNAN NASIR and ROBERT C. ANGERERf Department of Biology, University of Rochester, Rochester NY 14627, USA •Current address: Department of Pediatrics, University of Rochester School of Medicine and Dentistry, Rochester NY 14642, USA •(•Corresponding author Summary The cloning and characterization of cDNAs representing four genes or small gene families that are coordinate^ expressed in a spatially restricted pattern during the very early blastula (VEB) stage of sea urchin develop- ment are presented. The VEB genes encode multiple transcripts that are expressed transiently in embryos of Strongylocentrotus purpuratus between 16-cell stage and hatching, with peak abundance 12 to 15 hours post- fertilization (-150-250 cells). The VEB transcripts share the same spatial pattern in the early blastula embryo: they are asymmetrically distributed along the animal- vegetal axis but their distribution around this axis is uniform. Thus, the VEB transcripts are the earliest messages to reveal asymmetry along the primary axis in the sea urchin embryo. The temporal and spatial patterns of VEB transcript accumulation are not consistent with involvement of these gene products in cell division or in tissue-specific functions. Furthermore, VEB messages cannot be detected in either ovary or adult tissues, suggesting that these genes function exclusively during embryogenesis. We suggest that the VEB genes function in constructing the early blastula. Two VEB genes encode metalloendoproteases: one (SpHE) is hatching enzyme and the other (SpAN) is similar to bone morphogenetic protein-1 (BMP-1; Wozney et al., Science 242: 1528-1534, 1988) and the Tolloid gene product (tld) (Shimell et al., Cell 67: 459- 482, 1991). Several lines of evidence suggest that the VEB genes are regulated directly by factors or regulat- ory activities localized along the maternally specificed animal-vegetal axis. Key words: sea urchin embryo, sea urchin blastula, coordinate early gene expression, animal-vegetal axis, metalloendoprotease. Introduction The very early blastula (VEB, morula-hatching) period of sea urchin development is similar but not identical to the mid-blastula transition (MBT) of Xenopus embryos and the cellular blastoderm (CB) stage of Drosophila embryogenesis. In all three organisms, cell cycles, which are synchronous and rapid during cleavage, become asynchronous and the overall rate of cell division slows greatly (Hinegardner, 1974; Satoh et al., 1976; Edgar et al., 1986). In addition, similar changes in cell motility are observed in each of these embryos. Considerable pulsatile activity is apparent at the basal surface of VEB-stage sea urchin blastomeres (Wol- pert and Mercer, 1963) and in cells of post-MBT Xenopus embryos (e.g., Newport and Kirschner, 1982a,b) and cell movement begins during the CB- stage of Drosophila embryogenesis (Foe and Alberts, 1983). In the sea urchin embryo, several morphological changes are also evident at VEB stage. The blastocoel enlarges rapidly resulting in large increases in the apical and basal surfaces of blastomeres, and this is ac- companied by vectorial deposition and modification of extracellular matrices both inside and outside the embryo (McCarthy and Burger, 1987; Alliegro et al., 1988; Bisgrove et al., 1991; for a recent review, see McClay et al., 1990). A single cilium forms on the apical surface of cells, nuclei are positioned in the basal cytoplasm, and desmosomes form at the apical ends of intercellular junctions (reviewed in Wolpert and Mercer, 1963).