Dr Helen White-Cooper
Regulation of Gene Expression in Drosophila Spermatogenesis
Cell differentiation is driven by co-ordinated changes in the gene expression profile of the cell: some genes are switched on, others are switched off. One of the most astonishing developmentally regulated changes in cell morphology occurs in spermatogenesis. The mature sperm is a highly specialised cell, whose formation from a simple primary spermatocyte involves an unusual cell division (meiosis) to form a round spermatid, followed by complex changes in the cell architecture to form the final elongated motile sperm. These differentiation events require many gene products used at no other time in development. Underlying this there is a dramatic switch in the gene expression profile of male germ-line cells: as they enter the primary spermatocyte stage they activate transcription of a large set of genes required for sperm production. We have recently discovered that another small set of genes is transcribed after meiosis, in elongating spermatids, and are investigating their role in sperm function.
The meiotic arrest loci:
The meiotic arrest class of Drosophila genes regulate transcription in spermatogenesis; specifically they are required for activation of expression of numerous genes required for spermatid differentiation. A series of micro-array experiments revealed that approximately half of all Drosophila protein coding genes are expressed in testes, and that 15-20% of these are regulated by our genes, ie, the meiotic arrest genes control transcription of up to 10% of all Drosophila protein coding genes. We cloned and characterised five of these meiotic arrest genes (aly, comr, achi/vis, topi and tomb). aly and comr have unknown functions, topi, tomb and achi/vis encode DNA binding proteins. In normal primary spermatocytes all the meiotic arrest proteins are chromatin associated, consistent with their role in gene-expression regulation. Their localisation in different mutant backgrounds varies: e.g. aly function is required for the nuclear localisation of Comr, and vice versa, indicating that formation and localisation of an active complex is highly regulated. We are currently investigating the formation of the complex, and its activity at target promoters.
Gene function in spermatogenesis
In 1998 I drew a model of the function of the meiotic arrest genes, in which I postulated that aly would regulate many genes required for spermatid differentiation. This model was based on expression data for about 20 genes. The array experiment was designed to test the model with many more genes. However, the main observation revealed by the array analysis is that we know very little about gene function in Drosophila spermatogenesis. I could initially not answer my question “do the meiotic arrest loci predominantly regulate genes required for spermiogenesis, and not genes required before meiosis?” because the function of the majority of genes that changed significantly in the mutants was not known. We therefore began a large-scale functional-genetics project in which we are using RNA in situ hybridisation to describe the expression pattern, and genetic regulation, of more than 1000 genes in testes. We have examined the expression patterns of about 1200 genes, and it seems my initial prediction was correct – genes that act after meiosis tend to be regulated by the meiotic arrest genes, whereas genes that act earlier tend not to require the meiotic arrest genes for their expression.
Post-meiotic gene expression in Drosophila
It has been generally accepted that there is no post-meiotic transcription in Drosophila spermatogenesis. However, in our in situ hybridisation project, we have identified about 25 genes that are transcribed during the elongation stage of spermatid development. Moreover, their transcripts are localised to the distal ends of the spermatids. The localised transcripts fall into two classes - "cups" and "comets". A mutation in one of the comet genes, scotti, is male sterile, with defects late in spermatogenesis. We are using in vivo reporter constructs and genetic analysis to examine the mechanisms controlling the mRNA and protein localisations and functions.
Research in my lab is funded by the Royal Society, The Wellcome Trust and BBSRC.