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Fig. 1. Time course of maternal transcript degradation in activated, unfertilized eggs. (A) The same Northern blot probed for rpA1 (stable) and string, Hsp83, or nanos (unstable) transcripts. (B) Quantitative analysis of the time course of Hsp83 transcript degradation. The points represent the ratio of Hsp83 transcripts to stable rpA1 transcripts relative to the initial (0-0.5 h) concentration. It can be seen that more than 95% of the Hsp83 transcripts have disappeared by 3.0 to 3.5 h after egg activation. Data from two independent experiments are presented. Half-hour time windows are shown. See ref. 9 for details. lated region (UTR) of Hsp83 and nanos that, when deleted, result in substantial stabilization of corresponding transgenic transcripts in both unfertilized and fertilized eggs (Fig. 2) (9, 14-16). It was comparison of the stability of the cis element-deleted transcripts in unfertilized versus fertilized eggs that led to the discovery of the zygotic degradation pathway (9): transcripts deleted for an element required for maternal degradation are fully stabilized in unfertilized eggs (Fig. 2 A) but are destabilized starting 2 h after fertilization in developing embryos (Fig. 2 B). Thus, there must be additional cis-acting elements that are still present in these transcripts and that mediate zygotic degradation. Spatial Control of Maternal RNA Stability in Drosophila Although unstable maternal transcripts such as string and Hsp70 are eliminated throughout the egg or early embryo (Fig. 3 A and B), unstable transcripts such as Pgc, Hsp83, and nanos are eliminated from the bulk cytoplasm of the egg or embryo but remain stable at the posterior (Fig. 3 C-F) (9). Uniform instability is the default state for the unstable classes of transcripts: if the Hsp83 3′ UTR is replaced with a 3′ UTR from a uniformly degraded transcript (e.g., Hsp70) or if a cis-acting "protection" element is deleted (see below), then the resulting transgenic Hsp83 transcripts are degraded throughout the embryo (see Fig. 5 
Fig. 2. Removal of a maternal Hsp83 degradation element stabilizes transgenic transcripts in unfertilized (A) but not fertilized (B) eggs. Northern blots are shown that were simultaneously probed for (i) endogenous Hsp83 transcripts; (ii) transgenic reporter transcripts carrying the Hsp83 5′ UTR, the first 111 codons of the Hsp83 ORF, an Escherichia coli ß-galactosidase RNA tag, and the Hsp83 3' UTR deleted for a 97-nt element referred to as the Hsp83 degradation element (HDE) (for a detailed description of this transgene see ref. 9); (iii) endogenous rpA1 transcripts. On both blots it can be seen that endogenous Hsp83 transcripts are unstable and endogenous rpA1 transcripts are stable. However, although transgenic ∆HDE transcripts are stable in unfertilized eggs (1A), they are degraded commencing 2 h after fertilization in developing embryos (B). Half-hour time windows after egg activation or fertilization are shown. See ref. 9 for details. A and B) (9). This experiment proves that the endogenous Hsp83, nanos, and Pgc transcripts that remain at the posterior of the egg and early embryo are protected from degradation in that region of the cytoplasm (i.e., the degradation machinery is 
Fig. 3. Certain classes of maternal transcripts are degraded throughout the cytoplasm of activated, unfertilized eggs whereas others are protected from degradation in the posterior polar plasm, string transcripts are initially present throughout the egg (A) and are subsequently degraded (B). In contrast, whereas Hsp83 (C) and nanos (E) transcripts are initially present in both the posterior polar plasm and the presumptive somatic region (C and E), degradation is limited to the somatic region whereas transcripts are protected from degradation in the posterior polar plasm (D and F). (A, C, and E) One to 2 h after egg activation; (B, D, and F) 3-4 h after egg activation. Whole-mount RNA in situ hybridizations are shown, with anterior to the left and dorsal toward the top of the page. See ref. 9 for details.
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