INTRODUCTION Exposure of cells to various environmental insults including heat shock results in the induction of a small group of conserved proteins known as the heat shock proteins (Hsps) (reviewed by Morimoto et al., 1994). In addition, some of the Hsps have been reported to be expressed in the absence of stress during embryogenesis and development in the fruitfly Drosophila melanogaster (Dm) (Sirotkin and Davidson, 1982). In D. melanogaster, the low molecular mass Hsps (s-Hsps: small heat shock proteins), Hsp27, Hsp26, Hsp23, and Hsp22, are encoded by four of seven heat-inducible genes clustered at chromosomal region 67B (Petersen et al., 1979; Ayme and Tissières, 1985). In addition to being stress-inducible, the synthesis of s-Hsps can be regulated by the molting hormone 20-hydroxyecdysone as shown in developing imaginal discs and in cultured cells (Ireland et al., 1982; Sirotkin and Davidson, 1982; Cheney and Shearn, 1983; Beaulieu et al., 1989). While the synthesis of the four s-Hsps of D. melanogaster is coordinated during heat shock in cultured cells, their expression during fly development is not (reviewed by Arrigo and Tanguay, 1991; Arrigo and Landry, 1994; Michaud et al., 1997). For example, ovarian cells of D. melanogaster contain mRNAs encoding Hsp27 and Hsp26 that are stored in oocytes (Graziosi et al., 1980; Zimmerman et al., 1983; Mason et al., 1984). P-element transformation studies have revealed the presence of multiple regulatory elements controlling the tran- scription of these small Hsp genes (Cohen and Meselson, 1985; Glaser and Lis, 1990). Using a hsp26-lacZ fusion gene, Glaser et al. (1986) showed that the hsp26 gene was expressed in the absence of stress in different tissues including gonads and neu- rocytes. The expression of Hsp27, Hsp26 and Hsp23 in the central nervous system and in germ lines of D. melanogaster has also been reported (Pauli et al., 1990; Haass et al; 1990; Marin et al., 1993, 1996; Marin and Tanguay, 1996). In addition, each s-Hsp appears to be expressed in specific cells of some organs. In gonads of Drosophila, Hsp26 mRNA has been detected in nurse cells of developing ovaries, in primary spermatocytes as well as in some spermatogonia (Glaser et al., 1986). Hsp27 and Hsp26 polypeptides have been detected in specific cells of the germ line and in some somatic parts of the male reproductive system (Pauli et al., 1990; Marin et al., 1993). Hsp23 has also been reported to be expressed in 1989 Journal of Cell Science 110, 1989-1997 (1997) Printed in Great Britain © The Company of Biologists Limited 1997 JCS9612 The developmental and heat-shock-induced expression of two small heat-shock proteins (Hsp23 and Hsp27) was investigated during spermatogenesis in Drosophila melanogaster. Both of these Hsps were expressed in unstressed and stressed male gonads as shown by immunoblotting. Immunostaining of whole-mount organs and thin sections of testes showed that an anti-Hsp23 antibody specifically decorated cells of the somatic lineage, such as the cyst cells and the epithelial cells of the testis and of the seminal vesicle. Hsp27 was expressed in some somatic cells (cyst cells and epithelial cells of the accessory glands) and, in addition, was also visible in the maturing sperma- tocytes of the germline. The same cell-specific pattern of expression was observed after heat shock, and cells which did not express Hsp23 and Hsp27 in the absence of stress were similarly unable to mount a heat shock response for these s-Hsps. However other Hsps such as Hsp70 and Hsp22 were induced under heat-shock conditions in testes. Actinomycin D prevented the heat-induced accumulation of these Hsps indicating that the induction of Hsps was regulated at the transcriptional level. The heat shock tran- scriptional factor of Drosophila (DmHSF), present in sig- nificantly lower amount in testes when compared to other tissues such as the head, was shown to be required for the heat activation of Hsp22 and Hsp70. Immunostaining revealed that HSF expression was restricted to specific cells such as cyst cells, epithelial pigment cells, spermatogonia and spermatids but not the primary spermatocytes. These data show that the expression and induction of the different small Hsps is regulated in a cell-specific manner under both normal and heat shock conditions and suggest that factors other than the DmHSF are involved in this regulation in male gonads. Key words: Spermatogenesis, Hsp27, Hsp23, Heat shock, Drosophila, Heat shock factor SUMMARY Cell-specific expression and heat-shock induction of Hsps during spermatogenesis in Drosophila melanogaster Sébastien Michaud 1 , Raquel Marin 1 , J. Timothy Westwood 2 and Robert M. Tanguay 1 1 CHUL Research Center (CHUQ) and Laboratory of Cellular and Developmental Genetics, RSVS, Université Laval, Ste-Foy, Québec, Canada G1K 7P4 2 Department of Zoology, Erindale College, University of Toronto, Mississauga, Ontario, Canada L5L 1C6 *Author for correspondence (e-mail: Robert.Tanguay@rsvs.ulaval.ca)