Cell Tissue Res (1995) 279:411-419 Cell &Tissue Research © Springer-Verlag 1995 Expression of Siah-2, a vertebrate homologue of Drosophila sina, in germ cells of the mouse ovary and testis Neil G. Della l, 2, David D. L. Bowtell 1,*, Felix Beck 1 Howard Florey Instituteof ExperimentalPhysiology and Medicine, Universityof Melbourne, Parkville, Victoria, 3052, Australia 2 Departmentof Ophthalmology, Universityof Melbourne, Parkville, Victoria, 3052, Australia Received: 6 April 1994 / Accepted: 17 July 1994 Abstract. Siah-2 is one of three murine homologues of the Drosophila gene seven in absentia (sina). The sina protein is nuclear localizing and required downstream of Rasl, Raf and the tyrosine kinase sevenless in Drosophi- la. Recent results have demonstrated a high degree of functional conservation between vertebrate and insect tyrosine kinase signalling pathways, involving such pro- teins as Son of sevenless, Grb2 and GAR These find- ings, together with the high degree of sequence conser- vation between the Siah proteins and sina, suggest that the mammalian Siah proteins may also participate in sig- nal transduction by some tyrosine kinases. Here, we re- port a high level of expression of Siah-2 in a specific population of germ cells within both the mouse ovary and testis. Siah-2 expression was absent in primordial oocytes but was detected in all growing oocytes, coinci- dent with their recruitment from the pool of quiescent cells. The level of Siah-2 mRNA increased as the oo- cytes matured and was readily detectable in Graafian follicles and in fertilized zygotes up until the two cell stage, a time of extensive maternal transcript degradation and zygotic gene activation. The expression of Siah-2 in the testis was first detected in postmeiotic spermatids. These represented a comparable stage of germ cell de- velopment to those cells first expressing Siah-2 in the ovary. The expression pattern of Siah-2 in germ cells was similar to that described for the proto-oncogene c- mos, and the possibility that Siah-2 lies downstream of p39 mos in signal transduction within the mouse oocyte requires further investigation. Finally, expression of Siah-2 appears to be an excellent marker for the incep- tion of gonadotrophin-independent oocyte growth. Key words: Genes, homologous - Gene seven in absen- tia - Germ cells - Ovary - Testis - Hybridization, in situ - Mouse * Present address and address for correspondence: Peter MacCal- lure Cancer Institute, Locked Bag 1, A. Beckett St., Melbourne, 3000, Australia Introduction The strategy of cross-species hybridization has enabled the identification of a number of vertebrate homologues of important developmental genes in Drosophila mela- nogaster. Many of these genes participate in signalling pathways within the cell which are remarkably con- served across widely divergent species. The Siah (Seven in absentia homologue) family of genes in the mouse were isolated in this way by virtue of their extensive se- quence homology to the Drosophila gene seven in ab- sentia or sina (Della et al. 1993; Carthew and Rubin 1990). Both the sina and Siah predicted proteins include a RING finger motif (Freemont 1993; Carthew and Rub- in 1990; Della et al. 1993) and a second unique cystei- ne/histidine-rich domain, suggesting that these proteins may bind nucleic acids and/or other proteins. Experi- ments in Drosophila have shown that the sina protein lo- calizes to the nucleus (Carthew and Rubin 1990) and is required downstream of Drosophila Rasl and Raf in a signal transduction pathway that commences with the sevenless tyrosine kinase receptor (Dickson et al. 1992; Fortini et al. 1992; Gaul et al. 1992). Recently, a number of the more proximal components of this pathway have been found to be conserved in both a structural and a biochemical sense in vertebrate cells (Williams 1992; Roberts 1992; Pawson and Schlessinger 1993). These components include Son of sevenless (Simon et al. 1991; Bowtell et al. 1992), Grb2 (termed drk in Drosophila; Simon et al. 1993; Olivier et al. 1993; Rozakis-Adcock et al. 1993; Li et al. 1993; Gale et al. 1993), and the mi- togen-activated protein kinases (MAPK, also referred to as ERKs, extracellular signal-regulated kinases; Biggs and Zipursky 1992; Pelech and Sanghera 1992). The ex- act relationship of sina and the Siah proteins to these more proximal elements in tyrosine kinase signalling is not clear at present. The molecular characterization and expression pat- tern of the Siah family of genes have been described in a recent report by our group (Della et al. 1993). The Siah family consists of five members, two of which are pseu-