In contrast to the general notion of induction of a common set of heat shock polypeptides (HSP) in all cell types, heat shocked malpighian tubules (MT) ofDrosophila melanogaster larvae did not synthesize the common set of HSP induced in salivary glands or brain ganglia of larvae and in gonads (testis or ovary) of adult flies. Instead, heat shocked MT of late 3rd instar larvae and freshly eclosed adults synthesized a novel set of polypeptides (MT-specific HSP) with a major induced band at 58 kd. Surprisingly, the MT of older flies synthesized both the MT-specific as well as the common set of HSP in response to heat shock. Fusion genes with hsp70 or hsp26 promoter linked to the lac-Z (beta galactosidase) or to ADH (alcohol dehydrogenase) reporter gene were also not induced in larval MT but showed good induction in the MT of older flies. This unexpected finding raises intriguing issues regarding the nature of MT-specific HSP, their genes and the cell type dependent induction of the two sets of HSP.

The1(2) gl⁴ mutation (a deletion mutation of a recessive oncogene) ofDwsophila melanogaster causes neuroblastoma in the optic centres of brain of late third instar larvae. We have studied thein situ patterns of DNA synthesis in these brains by immunocytochemical detection of cells incorporating 5-bromodeoxyuridine. It was seen that1 (2)gl⁴ brains from younger 3rd instar larvae had fewer replicating cells than in wild type larvae of comparable age but in brain ganglia of olderl (2) gl⁴ larvae the number of replicating cells was much higher. The spatial distribution of replicating cells in optic lobes of brain ganglia of1 (2) gl⁴ larvae was disturbed from early 3rd instar stage, much before the tumourous growth was morphologically detectable. The stereotyped pattern of asymmetrical cell divisions of the neuroblasts and their progeny cells was also not seen in1 (2) gl⁴ brain ganglia. Therefore, it appears that thel (2)gl⁴ product has an important role early in development to determine the temporal and spatial patterns of neuroblast cell division in developing brains.

Of the several noncoding transcripts produced by thehsrΩ gene ofDrosophila melanogaster, the nucleus-limited >10-kb hsrΩ-n transcript colocalizes with heterogeneous nuclear RNA binding proteins (hnRNPs) to form fine nucleoplasmic omega speckles. Our earlier studies suggested that the noncoding hsrΩ-n transcripts dynamically regulate the distribution of hnRNPs in active (chromatin bound) and inactive (in omega speckles) compartments. Here we show that a P transposon insertion in this gene's promoter (at -130 bp) in the hsrΩ⁰⁵²⁴¹ enhancer-trap line had no effect on viability or phenotype of males or females, but the insertion-homozygous males were sterile. Testes of hsrΩ⁰⁵²⁴¹ homozygous flies contained nonmotile sperms while their seminal vesicles were empty. RNA: RNAin situ hybridization showed that the somatic cyst cells in testes of the mutant male flies contained significantly higher amounts of hsrΩ-n transcripts, and unlike the characteristic fine omega speckles in other cell types they displayed large clusters of omega speckles as typically seen after heat shock. Two of the hnRNPs, viz. HRB87F and Hrp57A, which are expressed in cyst cells, also formed large clusters in these cells in parallel with the hsrΩ-n transcripts. A complete excision of the P transposon insertion restored male fertility as well as the fine-speckled pattern of omega speckles in the cyst cells. Thein situ distribution patterns of these two hnRNPs and several other RNA-binding proteins (Hrp40, Hrb57A, S5, Sxl, SRp55 and Rb97D) were not affected by hsrΩ mutation in any of the meiotic stages in adult testes. The present studies, however, revealed an unexpected presence (in wild-type as well as mutant) of the functional form of Sxl in primary spermatocytes and an unusual distribution of HRB87F along the retracting spindle during anaphasetelophase of the first meiotic division. It appears that the P transposon insertion in the promoter region causes a misregulated overexpression of hsrΩ in cyst cells, which in turn results in excessive sequestration of hnRNPs and formation of large clusters of omega speckles in these cell nuclei. The consequent limiting availability of hnRNPs is likely totrans-dominantly affect processing of other pre-mRNAs in cyst cells. We suggest that a compromise in the activity of cyst cells due to the aberrant hnRNP distribution is responsible for the failure of individualization of sperms in hsrΩ⁰⁵²⁴¹ mutant testes. These results further support a significant role of the noncoding hsrΩ-n transcripts in basic cellular activities, namely regulation of the availability of hnRNPs in active (chromatin bound) and inactive (in omega speckles) compartments.