15-P031 Evolution of bivalve bodyplan

Vertebrate ancestors would have increased their egg size to store yolk, and the increase is considered to have altered the cleavage pattern and germ layer formation. Amphibian holoblastic cleavage in which all blastomeres contribute to any one of the three primary germ layers has been widely thought to be a developmental pattern in the stem lineage of vertebrates, and meroblastic cleavage to have evolved independently in each vertebrate lineage. In extant primitive vertebrates, agnathan lamprey and basal bony fishes also undergo holoblastic cleavage, and their vegetal blastomeres have been generally thought to contribute to embryonic endoderm. However, the identification of their primary germ layers based on molecular evidences was not reported. We performed the marker analyses in most basal ray-finned fish bichir and agnathan lamprey embryos, resulting that their mesoderm and endoderm develop in the equatorial marginal zone, and their vegetal cell mass is extraembryonic nutritive yolk cells, having non-cell autonomous meso-endoderm inducing activity. Furthermore, Eomesodermin, but not VegT, orthologs are expressed maternally in these animals as well as zebrafish, mouse and protochordates, suggesting that VegT is a maternal factor for endoderm differentiation only in amphibian. The study raises the viewpoint that the lamprey/bichir type holoblastic development would have been ancestral to extant vertebrates and retained in their stem lineage as a preliminary state toward the meroblastic development; amphibian-type holoblastic development would have been acquired secondarily, accompanied by the exploitation of new molecular machinery such as maternal VegT.