Intercellular bridge and germline cyst formation is a conserved feature of germ cell biology and is crucial for fertility (e.g. In mouse, interaction between the inactive serine-threonine kinase TEX14 and CEP55 regulates intercellular bridge stability in part by blocking abscission factors (Alix, Escrt complex) ( Greenbaum et al., 2011 Greenbaum et al., 2006 Kim et al., 2015 Morita et al., 2007). Arrest of the cytokinetic furrow and maintenance of the midbody involves a complex regulatory network that stabilizes the actomyosin meshwork to arrest abscission and maintain the contractile ring, forming ring canals instead of dividing ( Greenbaum et al., 2011 Haglund et al., 2011 Hime et al., 1996 Robinson and Cooley, 1996). In Drosophila ring canal formation involves regulation of the actin cytoskeleton, which localizes to the cleavage furrows and maintains midbody structures ( Greenbaum et al., 2011). The interconnections generated from incomplete cytokinesis form intercellular bridges (or ring canals) ( Brown and King, 1964 de Cuevas et al., 1997 Fawcett et al., 1959 Greenbaum et al., 2011 Haglund et al., 2011 Koch and King, 1966, 1969 Koch et al., 1967 Lei and Spradling, 2016 Lin and Spradling, 1993 Mahowald, 1971 Marlow and Mullins, 2008 Pepling and Spradling, 1998 Robinson and Cooley, 1996 Spradling, 1993). Type-II divisions generate cystoblast cells that divide mitotically with incomplete cytokinesis to generate interconnected sisters in Drosophila ( Cox and Spradling, 2003), Xenopus ( Kloc et al., 2004) and medaka ( Saito et al., 2007). Cells resulting from type-I divisions do not divide further but directly differentiate as meiotic cells and are observed in juvenile and adult teleost fish (medaka and in zebrafish)( Marlow and Mullins, 2008 Saito et al., 2007). These differentiating divisions have been classified as type-I or type-II divisions ( Saito et al., 2007 Saito and Tanaka, 2009). Although the earliest stages of PGC development in zebrafish have been extensively studied ( Barton et al., 2016 Marlow, 2015 Paksa and Raz, 2015 Raz, 2003), the cellular events and molecular mechanisms involved in the transition from PGC to germline stem cell (GSC) and the earliest phases of gametogenesis are less well understood.Ī common and evolutionarily conserved feature of GCs is the asymmetric division of GSCs to produce a stem cell and a premeiotic daughter. There the PGCs proliferate and eventually enter meiosis and differentiate to produce the gametes, sperm in males and oocytes in females.
Once specified, PGCs ignore somatic differentiation programs and migrate to the site where the gonad anlage forms ( Braat et al., 1999a Gross-Thebing et al., 2017 Nieuwkoop and Sutasurya, 1979 Strome and Updike, 2015). Early germ cells, called primordial germ cells (PGCs), can be specified by maternal factors or by inductive signals ( Farrell et al., 2018 Lawson and Hage, 1994 Marlow, 2015 Nieuwkoop and Sutasurya, 1979).
In many organisms the germline is among the first cell types to be set aside during early development ( Ginsburg, 1994 Illmensee and Mahowald, 1976 Illmensee et al., 1976 Wolf et al., 1983). In addition to promoting cystoblast divisions and meiotic entry, dazl function is required upstream of germline stem cell establishment and fertility. Accordingly, dazl mutant GCs form defective ring canals, and ultimately remain as individual cells that fail to differentiate as meiocytes. Analysis of dazl mutants revealed an essential role for Dazl in regulating incomplete cytokinesis and germline cyst formation before the meiotic transition. Here we investigate cystogenesis in zebrafish and identified Deleted in azoospermia (Dazl), a conserved vertebrate RNA binding protein as a regulator of this process. This evolutionary conserved process involves synchronous and incomplete mitotic divisions of a germ cell daughter (cystoblast) to generate sister cells connected by stable intercellular bridges that facilitate exchange of materials to support a large synchronous population of gamete progenitors. Before entering meiosis, differentiating germ cells (GCs) of sexual animals typically undergo cystogenesis.
Fertility and gamete reserves are maintained by asymmetric divisions of the germline stem cells to produce new stem cells or daughters that differentiate as gametes.
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