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How germ cells are specified in the embryos of animals has been a mystery for decades. Unlike most developmental processes, which are highly conserved, embryos specify germ cells in very different ways. Curiously, in mouse embryos germ cells are specified by extracellular signals; they are not autonomously specified by maternal germ cell determinants (germ plasm), as are the germ cells in most animal model systems. We have developed the axolotl (Ambystoma mexicanum), a salamander, as an experimental system, because classic experiments have shown that the germ cells in this species are induced by extracellular signals in the absence of germ plasm. Here, we provide evidence that the germ cells in axolotls arise from naive mesoderm in response to simple inducing agents. In addition, by analysing the sequences of axolotl germ-cell-specific genes, we provide evidence that mice and urodele amphibians share a common mechanism of germ cell development that is ancestral to tetrapods. Our results imply that germ plasm, as found in species such as frogs and teleosts, is the result of convergent evolution. We discuss the evolutionary implications of our findings.

Original publication

DOI

10.1098/rstb.2003.1331

Type

Journal article

Journal

Philos Trans R Soc Lond B Biol Sci

Publication Date

29/08/2003

Volume

358

Pages

1371 - 1379

Keywords

Ambystoma, Animals, Cell Differentiation, Cell Lineage, Embryonic Induction, Fetal Proteins, Gene Expression Regulation, Developmental, Germ Cells, Mesoderm, Phylogeny, Selection, Genetic, T-Box Domain Proteins