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

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 primit...

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Bibliographic Details
Published in:Developmental biology 2009-08, Vol.332 (1), p.90-102
Main Authors: Takeuchi, Masaki, Takahashi, Maiko, Okabe, Masataka, Aizawa, Shinichi
Format: Article
Language:English
Subjects:
Animals
Bichir
Biological Evolution
Body Patterning
Embryo, Nonmammalian - cytology
Endoderm
Endoderm - embryology
Extraembryonic
Fishes - embryology
Fishes - genetics
Gene Expression Regulation, Developmental
Germ Layers - embryology
Holoblastic cleavage
Lamprey
Lampreys - embryology
Lampreys - genetics
Larva - cytology
Mesoderm - embryology
Models, Biological
Petromyzontidae
Phylogeny
Primary germ layer
Reverse Transcriptase Polymerase Chain Reaction
vegetal cell mass
VegT
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Summary: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.
ISSN:0012-1606
1095-564X
DOI:10.1016/j.ydbio.2009.05.543