Identification of germ plasm-enriched mRNAs in Drosophila melanogaster by the cDNA microarray technique

The development of embryonic germ cells in Drosophila depends on the germ plasm, the most posterior part of the ooplasm. The germ plasm is devoted to the formation of future germ cells and is known to contain all the factors that are necessary to induce germ cell fate. Besides having a characteristic organelle and protein distribution, the germ plasm also contains a large number of localized RNA species that have been shown to play crucial roles in germ cell determination. To identify germ plasm-enriched, localized transcripts, we used a two-step method composed of cDNA microarray (containing 3200 annotated Drosophila cDNAs) and in situ RNA hybridization techniques. We compared germ plasm deficient, normal and ectopic germ plasm conditions in the cDNA microarray experiments. RNA species whose concentration increased when ectopic germ plasm was present and decreased when the germ plasm was missing were selected. These candidates were then subjected to a second screen which compared the distribution of the given RNA in wild type embryos and in eggs with ectopic germ plasm. Finally, 17 RNA species were found to be enriched in the germ plasm. Based on these data, we estimate that around 1% of the Drosophila genes encode for germ plasm-enriched, localized transcripts. We conclude that this combination of microarray and in situ hybridization techniques is a simple but powerful experimental design for the genome-wide identification of genes coding for germ plasm localized transcripts.