Figure 2

Expression of Sycon Pax and Six genes during embryogenesis. A, Summary of embryogenesis in Sycon, with consecutive stages depicted from left to right: oocyte, cleavage, pre-inversion, post-inversion, larva. Abbreviations: pin, pinacocytes; ch, choanocytes; ac, accessory cells; cc, cruciform cells; mi, micromeres ma, macromeres; and mc, maternal cells. B-G, SciPaxB expression is detectable in the oocytes (B), and in all blastomeres during late cleavage (C). During early pre-inversion it is detectable in the micromeres and cruciform cells and in late pre-inversion (E), post-inversion (F) and in larvae (G) SciPaxB expression is restricted to the equatorial micromeres. H-M, SciPaxF expression is detectable in the oocytes (H) and is gradually restricted to become predominant in the cross cells (J to K), and then the equatorial micromeres in the larva (M). N-S , SciSixC expression is present in the oocytes (N) and then all blastomeres (O), but strongest in the cruciform cells by early pre-inversion (P). During late pre-inversion (Q) the expression is localized to the cruciform cells and the macromeres, and becomes limited to the macromeres in the post-inversion embryos and larva (R,S). T-Z, SciSixA is expressed in the oocytes (T), but not in cleavage stages or early pre-inversion stage embryos (U, V), when strong expression in a ring of accessory cells is apparent (U). Transient expression in the anterior micromeres is detectable in post-inversion stage embryos (W-Y) but not larvae (Z). All images are of whole mounted, glycerol-cleared specimens, except Y, which is a resin section. Embryos in pre-inversion, post-inversion and larvae were isolated from the tissue, except from K. The asterisk indicates macromeres at the posterior pole of the embryo and larva. Black arrows, white arrows and white arrowheads indicate the cruciform cells, micromeres and the accessory cells, respectively.