Article

Expression of Dazl and Vasa in turtle embryos and ovaries: evidence for inductive specification of germ cells

Department of Cell and Developmental Biology, Weill Medical College of Cornell University, New York, NY 10065, USA.
Evolution & Development (Impact Factor: 2.68). 09/2009; 11(5):525-34. DOI: 10.1111/j.1525-142X.2009.00360.x
Source: PubMed

ABSTRACT In bilaterian animals, germ cells are specified by the inductive/regulative mode or the predetermined (germ plasm) mode. Among tetrapods, mammals and urodeles use the inductive mode, whereas birds and anurans use the predetermined mode. From histological data it has been predicted that some reptiles including turtles use the inductive mode. Examining turtle oocytes, we find that Dazl RNA, Vasa RNA, and Vasa protein are not localized, suggesting that germ plasm is not present. In turtle embryos at somite stages, primordial germ cells (PGCs) expressing Dazl lie on a path from the lateral posterior extraembryonic endoderm through the gut to the gonad as previously described. In gastrulating embryos, cells expressing Dazl are found in the blastoporal plate and subsequently below the blastoporal plate, indicating that PGCs are generated at the equivalent of the early posterior primitive streak of mammals. Vasa RNA is expressed in somatic cells of gastrula to early somite stages, and Vasa RNA and protein are expressed in PGCs of later embryos. Taken together the evidence strongly suggests that turtles, and other reptiles (lacertoid lizards) with the same location of PGCs in embryos, use the inductive mode of germ cell specification. Phylogenetic analysis of the available evidence supports the following hypotheses: (1) the inductive mode is basal among reptiles, indicating that this mode was maintained as basal tetrapods evolved to amniotes, (2) the predetermined mode arose twice within reptiles, and (3) the induced mode may be used in several lepidosaurs whose PGCs are located in an unusual pattern distributed around the embryo.

Download full-text

Full-text

Available from: Brian I. Crother, Jun 16, 2015
0 Followers
 · 
82 Views
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: A common feature of development in most vertebrate models is the early segregation of the germ line from the soma. For example, in Xenopus and zebrafish embryos primordial germ cells (PGCs) are specified by germ plasm that is inherited from the egg; in mice, Blimp1 expression in the epiblast mediates the commitment of cells to the germ line. How these disparate mechanisms of PGC specification evolved is unknown. Here, in order to identify the ancestral mechanism of PGC specification in vertebrates, we studied PGC specification in embryos from the axolotl (Mexican salamander), a model for the tetrapod ancestor. In the axolotl, PGCs develop within mesoderm, and classic studies have reported their induction from primitive ectoderm (animal cap). We used an axolotl animal cap system to demonstrate that signalling through FGF and BMP4 induces PGCs. The role of FGF was then confirmed in vivo. We also showed PGC induction by Brachyury, in the presence of BMP4. These conditions induced pluripotent mesodermal precursors that give rise to a variety of somatic cell types, in addition to PGCs. Irreversible restriction of the germ line did not occur until the mid-tailbud stage, days after the somatic germ layers are established. Before this, germline potential was maintained by MAP kinase signalling. We propose that this stochastic mechanism of PGC specification, from mesodermal precursors, is conserved in vertebrates.
    Development 06/2014; 141(12):2429-2440. DOI:10.1242/dev.105346 · 6.27 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Vasa has been extensively used as a germline marker to trace the origin and migration pathway of primordial germ cells (PGCs) in many organisms, but little work has been reported on vasa genes and the origin of PGCs in holothurians. Using in situ hybridization and immunohistochemistry, vasa mRNA and protein of the sea cucumber Apostichopus japonicus (Aj-vasa) was detected in the cytoplasm of the unfertilized egg and was equally distributed in the cytoplasm of early embryos, from the 2-cell embryo to the blastula, indicating that Aj-vasa mRNA is maternally supplied. Later, expression of both Aj-vasa mRNA and protein centralizes gradually in newly organized structures from blastula to five-tentacle larva, and then is restricted to PGC-like cells of the original gonad in juveniles with 0.1-cm body length. The structure of the gonad develops further from a simple tubular gonad in 0.5-cm-length juveniles to a branched gonad in 3-cm-length juveniles. Our findings showed that the maternal supply of the vasa gene products in A. japonicus is different from that in sea urchin Strongylocentrotus purpuratus, of echinoderm, and suggested that the specialization of PGCs is an epigenesis mechanism in A. japonicus. Mol. Reprod. Dev. © 2013 Wiley Periodicals, Inc.
    Molecular Reproduction and Development 07/2013; DOI:10.1002/mrd.22207 · 2.68 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The embryonic development of germ cells in tetrapods is described, focusing on groups with the inductive mode of germ cell specification. In mammals PGCs are induced early in the gastrulation process, they are internalized with future extraembryonic mesoderm in the early posterior primitive streak, and specified soon thereafter. Strong evidence indicates that a similar process occurs in turtles and some other reptiles. In amniotes, the PGCs appear well before formation of the gonad in the posterior trunk, resulting in a period in which they are located outside the embryo before their migration to the gonad. In contrast, in urodeles the PGCs appear relatively late, and throughout development maintain a position close to precursors of the somatic cells of the gonad so that migration is not required. In lampreys early development of germ cells is strikingly similar to that in urodeles, suggesting this is the primitive process. As amniotes evolved large yolky eggs and better access to nutrition, development of the posterior half of the trunk became more dependent on cell proliferation; this was followed or accompanied by a shift of early germ cell development to the equivalent of the early primitive streak. A similar process may have occurred as some basal vertebrates developed large yolky eggs.
    Evolution & Development 09/2009; 11(5):603-9. DOI:10.1111/j.1525-142X.2009.00366.x · 2.68 Impact Factor