Germ layer patterning in bichir and lamprey; an insight into its evolution in vertebrates

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.ydbio.2009.05.543 Byline: Masaki Takeuchi (a), Maiko Takahashi (a), Masataka Okabe (b), Shinichi Aizawa (a) Keywords: Bichir; Lamprey; Holoblastic cleavage; Primary germ layer; Endoderm; Extraembryonic; vegetal cell mass; VegT Abstract: 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 present marker analyses in basal ray-finned fish bichir and agnathan lamprey embryos indicated 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. Eomesodermin (eomes), but not VegT, orthologs are expressed maternally in these animals, 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; amphibian-type holoblastic development would have been acquired secondarily, accompanied by the exploitation of new molecular machinery such as maternal VegT. Author Affiliation: (a) Laboratory for Vertebrate Body Plan, Center for Developmental Biology (CDB), RIKEN Kobe, 2-2-3 Minatojima Minamimachi, Chuo-ku, Hyogo 650-0047, Japan (b) Department of Anatomy, The Jikei University School of Medicine, 3-25-8 NishiShinbashi, Minatoku, Tokyo 105-8461, Japan Article History: Received 28 January 2009; Revised 21 April 2009; Accepted 5 May 2009