SPATIAL EXPRESSION OF A FORKHEAD HOMOLOG IN THE SEA-URCHIN EMBRYO

Echinoderms are the sister group of the chordates and hemichordates wi thin the deuterostomes. They lack a notochord or any structures obviou sly homologous with it. To gain insight into developmental mechanisms important in the origin and early evolution of chordates, we investiga ted sea urchin homologues of chordate genes that are implicated in not ochord formation, viz. Brachyury and HNF-3 beta. Here we report the pa ttern of expression of a sea urchin orthologue of forkhead, Hphnf3 whi ch is present as a single copy per haploid genome. An Hphnf3 transcrip t of 3.0 kb was first detected at the swimming blastula stage, accumul ated maximally at the gastrula and prism-embryo stages, and decreased at the pluteus-larva stage. In situ hybridization signals were found i n cells of the vegetal plate of the swimming blastula. During gastrula tion, intense staining was evident in the cells surrounding the blasto pore, whereas weak staining was detected in the invaginating archenter on. At the prism-embryo stage, the entire archenteron stained intensel y; then, at pluteus stage, the larva staining decreased in intensity. The forkhead and Brachyury genes begin to be expressed almost simultan eously in sea urchin embryos, in the vegetal plate at the late blastul a stage. After the onset of gastrulation, however, Hphnf3 is expressed in the posterior part of the archenteron, whereas the Brachyury ortho logue, HpTa, is expressed in the secondary mesenchyme founder cells, w hich occupy the anterior tip of archenteron. Hphnf3 may contribute to specification of embryonic cells as archenteron, and the role of HpTa may be directed towards specification of mesodermal founder cells. Exc ept for the basal character of expression in endoderm and endomesoderm , these transcription factors are clearly utilized differently in chor dates.