Xdazl is an RNA component of Xenopus germ plasm and encodes an RNA-binding protein that can act as a functional homologue of Drosophila boule. boule is required

In most sexually reproducing organisms, the gametes are derived from a precursor stem cell population, the primordial germ cells (PGCs). These cells originate extragonadally early in development and migrate through somatic tissues to reach the developing gonads. Once ensheathed by somatic gonad cells, PGCs divide and differentiate into definitive gametes. Discovering how the PGCs are initially specified and develop separate from the somatic lineage is a fundamental problem in developmental biology. In many organisms, germ plasm material, present in the egg, can be found in PGCs and germ cells throughout the life of the organism and is thought to act as a determinant of germ cell fate. The asymmetric inheritance of maternal factors is a common strategy used by diverse organisms such as nematodes, insects, crustaceans and Anuran amphibians to specify PGCs (Nieuwkoop and Sutasurya, 1979, 1981). Germ plasm is morphologically similar in all organisms where it is found and is typically composed of a fibrillar ‘germinal cytoplasm,’ electron-dense germinal granules, mitochondria and ribosomes (Czolowska, 1972). In Xenopus, the germ plasm is present in eggs as numerous discrete islands at the vegetal pole. These islands aggregate after fertilization, a process that requires a kinesin-like protein, Xklp-1 (Robb et al., 1996). Germ plasm is segregated unequally during cleavage stages until gastrulation, at which time the germ plasm becomes perinuclear and is divided equally among daughter cells (Whitington and Dixon, 1975). During subsequent embryogenesis, PGCs remain in the endoderm and are thought to undergo 2-3 cell divisions. Around stage 32/33 (late tailbud stage), the PGCs begin to migrate dorsally through the lateral endoderm. By early tadpole (stage 40), the PGCs accumulate in the dorsal crest of the posterior endoderm and are subsequently incorporated into the lateral plate mesoderm that forms the dorsal mesentery. In later stages, PGC migration continues to the dorsal body wall and then laterally to the forming genital ridges (Wylie and Heasman, 1976). Exactly how the germ plasm functions in PGC specification remains largely unknown. Removal of vegetal pole cytoplasm (Buehr and Blackler, 1970) or low dose ultraviolet (u.v.) irradiation of the vegetal pole (Züst and Dixon, 1975; Holwill et al., 1987) cause sterility or delayed migration of PGCs. Importantly, introduction of vegetal pole cytoplasm into irradiated eggs can restore fertility (Smith, 1966; Wakahara, 1977). This observation led Smith (1966) to conclude that indeed germ plasm contained germ cell determinants. Other evidence, however, suggests that Xenopus germ plasm does not act as a ‘true determinant’ as is thought for Drosophila pole plasm. Mis-localization of pole plasm, either through cytoplasmic transfer (Illmensee and Maholwald, 1974) or misexpression of oskar RNA, a key pole plasm component, at the anterior pole (Ephrussi and Lehmann, 1992) can cause the formation of ectopic, functional pole cells. Similar experiments 447 Development 127, 447-456 (2000) Printed in Great Britain © The Company of Biologists Limited 2000 DEV3100