Expression of Axolotl DAZL RNA, a Marker of Germ Plasm: Widespread Maternal RNA and Onset of Expression in Germ Cells Approaching the Gonad

Abstract

How germ cell specification occurs remains a fundamental question in embryogenesis. The embryos of several model organisms contain germ cell determinants (germ plasm) that segregate to germ cell precursors. In other animals, including mice, germ cells form in response to regulative mechanisms during development. To investigate germ cell determination in urodeles, where germ plasm has never been conclusively identified, we cloned a DAZ-like sequence from axolotls, Axdazl. Axdazl is homologous to Xdazl, a component of Xenopus germ plasm found in the vegetal pole of oocytes and eggs. Axdazl RNA is not localized in axolotl oocytes, and, furthermore, these oocytes do not contain the mitochondrial cloud that localizes Xdazl and other germ plasm components in Xenopus. Maternal Axdazl RNA is inherited in the animal cap and equatorial region of early embryos. At gastrula, neurula, and tailbud stages, Axdazl RNA is widely distributed. Axdazl first shows cell-specific expression in primordial germ cells (PGCs) approaching the gonad at stage 40, when nuage (germ plasm) appears in PGCs. These results suggest that, in axolotls, germ plasm components are insufficient to specify germ cells.

Evolution of the germ line–soma relationship in vertebrate embryos

“…Germ plasm RNA localizes in Xenopus oocytes through an 'early' pathway, while a second 'late' pathway translocates RNAs involved in somatic patterning to the vegetal pole, for example the RNAs encoding Vg1 and VegT (King et al 2005). Interestingly, neither the early nor late pathway is present in oocytes from axolotls (Johnson et al 2001, Nath & Elinson 2007. Considering that urodeles retain primitive amphibian traits (Johnson et al 2003b, Ahlberg et al 2005, it can be concluded that evolution of the MC was an innovation of frogs.…”

“…Considering that urodeles retain primitive amphibian traits (Johnson et al 2003b, Ahlberg et al 2005, it can be concluded that evolution of the MC was an innovation of frogs. Importantly, its evolution resulted in a repositioning of PGCs to the endodermal compartment, away from the ancestral mesodermal origin of amphibian PGCs (Johnson et al 2001, Bachvarova et al 2004. Repositioning of the PGCs in early embryos has been associated with major innovations that characterize frog development, most notably the dramatic anteriorization of the trunk (Johnson et al 2003b).…”

“…Vertebrate embryology is, in general, highly conserved, and vertebrate evolutionary history is well established, so it is possible to draw unambiguous conclusions about the ancestry of embryological traits. Thus, mammalian embryos do not contain germ plasm (Eddy 1975); germ plasm is found in birds (Tsunekawa et al 2000), but not in turtles (Bachvarova et al 2009b); among amphibians, anurans (frogs) employ germ plasm, but urodeles (salamanders) do not (Smith 1966, Johnson et al 2001; teleost embryos contain germ plasm (Olsen et al 1997, Yoon et al 1997, but those of actinopterygian fish probably do not (Bachvarova et al 2009a). Viewing this pattern at a glance, two opposing hypotheses are possible.…”

“…In contrast, PGCs form in very different locations in bird, frog, and teleost embryos, and germ plasm is inherited in each of these embryos in different ways (Johnson et al 2003b). From comparisons such as these, it has been concluded that epigenesis is conserved and germ plasm evolved repeatedly (Johnson et al 2001, 2003a, Extavour & Akam 2003, Crother et al 2007). However, if this is true, it raises the intriguing question: why has germ plasm evolved so many times?…”

Differential expression of fertility genes boule and dazl in Chinese sturgeon (Acipenser sinensis), a basal fish

“…This gene family consists in a Y-linked DAZ gene cluster and the autosomal dazl and boule genes, all of which encode germ cellspecific RNA-binding proteins with a common RNArecognition motif and at least one copy of DAZ repeat (Xu et al 2001). The Y-linked DAZ gene cluster is restricted to catarrhine primates (Agulnik et al 1998;Gromoll et al 1999), while dazl homologs are found only in vertebrates, such as zebrafish (Maegawa et al 1999), medaka (Xu et al 2007), rainbow trout (Li et al 2011), gibel carp (Peng et al 2009), Xenopus (Houston et al 1998), axolotl (Johnson et al 2001), chicken (Rengaraj et al 2010), mouse (Cooke et al 1996) and human (Seboun et al 1997), where they are more closely Huan Ye and Chuang-Ju Li contributed equally to this work.…”