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| Germ cell differentiation and development in chaetognaths |
| The main reason I find this type of germ cell differentiation interesting, is that I am always looking for possibilities to manipulate the investment into the male and female function of a hermaphrodite. The organisation of germ cell differentiation in chaetognaths may allow exactly that. The Figures here are from a wonderful paper on Spadella cephaloptera and Sagitta inflata by Carré et al. (2002) (I added the full citation and abstract below). They describe the development of the primary germ cells, which was observed for the first time almost a century ago, using modern developmental biology tools. The scheme shows how he germ granules form (green), and how they are involved in germ cell differentiation (see their paper for a detailed description). |
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| Below is another Figure from the same paper, depicting the stage 9 in the above scheme (i.e. a late gastrula). Now the four primary germ cells have formed. Each of them will provide the germ cells for the left and right ovary and testis respectively. These cells are visible in the developing embryo, and it could hence be possible to destroy any of them using laser ablation, or a similar technique. That would probably permit to manipulate the allocation to male and female reproduction (e.g. to make a worm that invests less in the male function). Equally, it could be possible to produce males and females by destroying both female and male germ cells respectively. If such worms are viable this would be a fantastic tool to study sex allocation and mating conflicts. |
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| Formation of a large Vasa-positive germ granule and its inheritance by germ cells in the enigmatic Chaetognaths |
| Development 129, 661-670 (2002) |
| Danièle Carré, Chakib Djediat and Christian Sardet |
| Laboratoire de Biologie du Développement, Station Zoologique, Observatoire Océanologique, Villefranche-sur-mer, France |
| Chaetognaths (arrow worms) are abundant hermaphrodite marine organisms whose phylogenetic position amongst protostomes and deuterostomes is still debated. Ancient histological observations dating from a century ago described the presence in eggs of a large granule, presumed to be a germ plasm, and its probable inheritance in four primary germ cells (PGCs). Using videomicroscopy, electron microscopy and immunocytochemistry (labelling with anti-Vasa antibodies) we have followed the cycle of aggregation and dispersion of germ plasm and nuage material in eggs, embryos, PGCs and oocytes in several species of benthic (Spadella) and planktonic (Sagitta) chaetognaths. In these animals, germ cells and gametes can be observed in vivo throughout the 1-2 month life cycle. After describing internal fertilization in live animals we show that the single large (15 mm diameter) germ granule forms by a spiralling aggregation movement of small germ islands situated in the vegetal cortex at the time of first mitosis. We also demonstrate that the granule forms autonomously in unfertilized activated eggs or fertilized egg fragments. Once formed, the germ granule first associates with the cleavage furrow and is segregated into one of the first two blastomeres. The germ granule is then translocated from the cortex to the mitotic spindle during 3rd cleavage and remains in the single most-vegetal blastomere until the 32-cell stage. At the 64-cell stage the germ granule is partitioned as nuage material into two founder PGCs and further partitioned into four PGCs situated at the tip of the archenteron during gastrulation. These four PGCs migrate without dividing to reach the transverse septum, then proliferate and differentiate into oocytes and spermatocytes of two ovaries and two testes. We noted that germ plasm and nuage material were associated with mitochondria, the nucleus, the spindle and the centrosome during some stages of development and differentiation of the germ line. Finally, we demonstrate that a Vasa-like protein is present in the germ granule, in PGCs and in the electron-dense material associated with the germinal vesicle of oocytes. These features stress the conservation of cellular and molecular mechanisms involved in germ cell determination. |
this page was last updated on Sunday, February 20, 2011 |