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: 2.72). 09/2009; 11(5):525-34. DOI: 10.1111/j.1525-142X.2009.00360.x
Source: PubMed


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.

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    • "Given that our analysis of C. calyptratus gastrulation has provided greater insight into its mechanism of gastrulation, we decided to revisit gastrulation in the turtle Trachemys scripta to understand the degree of conservation or diversity in these reptiles. Turtle embryos have been analyzed with some modern techniques (e.g., in situ hybridization and IHC) (Coolen et al., 2008; Bachvarova et al., 2009; Bertocchini et al., 2013). We decided to take the morphological study of these embryos a step further. "
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    ABSTRACT: Background Amniote gastrulation is often described with respect to human, mouse and chick development by the presence of the primitive streak, a posterior-to-anterior midline morphological cell ingression feature that has come to define Amniote gastrulation. How this midline, ingression-based strategy of gastrulation evolved from the ancestral blastopore, a circumferential involution event in Anamniotes, is unknown. However, within the Amniote clade there exists a more diverse range of gastrulation strategies than just the primitive streak. Investigating gastrulation in a wider range of Amniotes provides a way to understand evolutionary transition from blastopore to the primitive streak. Results We analysed early to late gastrulation stages of Chameleo calyptratus, showing their unique morphology through confocal imaging of F-actin and laminin-stained embryos to visualise cell morphology and assess basal lamina integrity. We analysed the expression pattern of core mesodermal markers Brachyury and Fgf8 and complimented this analysis with that of the turtle, Trachemys scripta. Conclusions Our analysis suggests that reptile gastrulation is bi-modal; primary internalisation occurs anteriorly via an incomplete blastopore-like opening, while posteriorly the cells undergo ingression in the Brachyury-expressing blastoporal plate. This strategy stands mid-way between Anamniotes and Avians/Mammals, suggesting that blastoporal plate is a precursor of the avian primitive streak. This article is protected by copyright. All rights reserved. © 2015 Wiley Periodicals, Inc.
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    • "A consequence of Blimp1, therefore, is that germline development in mouse is initiated from a quasi-pluripotent state (Leitch and Smith, 2013), rather than mesoderm. It is clear that the mechanism of PGC specification in urodeles was adapted as primitive amniotes and mammals evolved (Bachvarova et al., 2009a,b). We argue, therefore, that elucidation of this mechanism could aid in the development of in vitro methods for producing PGCs from non-rodent mammals, such as humans, because current methods for these species remain inefficient (Hayashi et al., 2012). "
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    • " there are more PGCs in the anterior region of the extra - embryonic me - soderm than the posterior region , with a gradient of den - sity of PGCs [ Bachvarova et al . , 2009b ] . Due to the similar localization of PGCs to the predetermined specification at early somite stages , and the same pathway of PGCs mi - gration as the predetermined mode [ Bachvarova et al . , 2009b ] ( see fig . 2 for summary ) , PGCs in this group of animals may be specified by the predetermined mode . However , further investigations are required to evaluate this hypothesis ."
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