Induction of indora expression in pole cells

Germ-line development involves the interaction between germline cells and adjacent somatic cells. Studies in a number of organisms have shown that mesodermal cells of the adult ovary and testis support the differentiation of germ-line cells into mature gametes (Kimble and White, 1981; Tres and Kierszenbaum, 1983; Abe, 1988; Lin and Spradling, 1993; Henderson et al., 1994; Tax et al., 1994; Miura et al, 1996). Despite the importance of the somatic cells to gametogenesis, the role of these cells in germ-line development during embryogenesis is not well understood. In many animal groups, the germ line is proposed to be specified autonomously by maternal factors localized in the germ plasm, a histologically distinct region of the egg cytoplasm (Beams and Kessel, 1974; Eddy, 1975). Experimental studies in Xenopus and Drosophila have demonstrated that factors capable of directing germ-line development are localized in the germ plasm (Illmensee and Mahowald, 1974; Ikenishi et al., 1986). However, germ plasmbearing cells do not necessarily differentiate into functional germ cells. Wylie et al. (1985) reported that in Xenopus embryos, germ plasm-bearing cells isolated during their migration toward the genital ridge are unable to become germ line when implanted into ectopic sites. Thus, germ-line development is not simply the consequence of autonomous function of germ plasm, but is also influenced by cellular environment (Wylie et al., 1985). In Drosophila, the germ plasm is localized in the posterior pole region of early embryos and partitioned into the germline progenitors, or pole cells (Mahowald, 1962, 1968; Underwood et al., 1980; Technau and Campos-Ortega, 1986; Hay et al., 1988). During gastrulation, the pole cells move along the dorsal surface of the embryos, along with the posterior midgut rudiment. They then migrate through the midgut epithelium toward the overlying mesodermal layer, where they associate with the dorsolateral mesoderm in parasegments (PS) 10-12 (Boyle and DiNardo, 1995; Boyle et al., 1997; Broihier et al., 1998; Moore et al., 1998a). The dorsolateral mesoderm in PS 10-12 is specified as somatic gonadal precursors (SGPs), and they coalesce with the associating pole cells to form the embryonic gonads (Boyle and DiNardo, 1995; Boyle et al., 1997; Moore et al., 1998a). The physical proximity of pole cells to the mesodermal cells raises the question as to whether germ-line development could be partly controlled by the mesodermal cells. Indeed, several mutations which abolish the gonadal mesoderm affect pole cell migration (Broihier et al., 1998; Moore et al., 1998a, b). For example, in tinman (tin), zinc finger homeodomain protein-1 (zfh-1) double-mutant embryos where the dorsolateral mesoderm is ablated, pole cells pass through the midgut epithelium, but subsequently they are dispersed around the midgut (Broihier et al., 1998; Moore et al., 1998b). Furthermore, in abdominal-A (abd-A) mutants which fail to specify the dorsolateral mesoderm into SGPs, pole cells 1023 Development 126, 1023-1029 (1999) Printed in Great Britain © The Company of Biologists Limited 1999 DEV3908