2 TYPES OF POLE CELLS ARE PRESENT IN THE DROSOPHILA EMBRYO, ONE WITH AND ONE WITHOUT SPLICING ACTIVITY FOR THE 3RD-P-ELEMENT INTRON

In Drosophila, it has been postulated that the third intron of the P-e lement is spliced only in germ-line cells. To test whether this postul ate is applicable to pole cells, the progenitor cells of germ line, we carried out a histochemical assay to detect the splicing activity in embryos. The splicing activity was detected in pole cells and primordi al germ cells. The activity increased to reach a maximum at 5-6 hours AEL (after egg laying), then decreased to an undetectable level by 8-9 hours AEL. The splicing activity showed a small second peak at 12-15 hours AEL. It was rather unexpected that not all pole cells were capab le of splicing the third intron. Almost all pole cells that had the sp licing activity at 5-6 hours AEL penetrated the embryonic gonads and d ifferentiated into primordial germ cells. Our findings suggest that po le cells are selected to penetrate the gonads while they are migrating from the proctodeal cavity to the gonads. Furthermore, these results suggest that the machinery to splice the P-element is active in some p ole cells, and that this activity is used for processing transcripts o f genes that play important roles in the differentiation of pole cells into primordial germ cells.