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. 2009 May 1;137(3):522-35.
doi: 10.1016/j.cell.2009.03.040. Epub 2009 Apr 23.

Specialized piRNA pathways act in germline and somatic tissues of the Drosophila ovary

Colin D Malone  1 , Julius Brennecke, Monica Dus, Alexander Stark, W Richard McCombie, Ravi Sachidanandam, Gregory J Hannon
Affiliations

Specialized piRNA pathways act in germline and somatic tissues of the Drosophila ovary

Colin D Malone et al. Cell. .

Abstract

In Drosophila gonads, Piwi proteins and associated piRNAs collaborate with additional factors to form a small RNA-based immune system that silences mobile elements. Here, we analyzed nine Drosophila piRNA pathway mutants for their impacts on both small RNA populations and the subcellular localization patterns of Piwi proteins. We find that distinct piRNA pathways with differing components function in ovarian germ and somatic cells. In the soma, Piwi acts singularly with the conserved flamenco piRNA cluster to enforce silencing of retroviral elements that may propagate by infecting neighboring germ cells. In the germline, silencing programs encoded within piRNA clusters are optimized via a slicer-dependent amplification loop to suppress a broad spectrum of elements. The classes of transposons targeted by germline and somatic piRNA clusters, though not the precise elements, are conserved among Drosophilids, demonstrating that the architecture of piRNA clusters has coevolved with the transposons that they are tasked to control.

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Figures

Figure 1
Figure 1. Maternal Deposition of piRNAs Defines Somatic and Germline piRNA Pathways
Ovarian and early embryonic piRNAs were mapped to transposons (independent of mapping number) and piRNA clusters (only piRNAs with unique genome-wide mapping). (A) The log2 fold ratio between ovarian and embryonic piRNAs over the 86 most targeted transposons is shown (right). The extent of maternal piRNA deposition is defined as strong (red), intermediate (yellow), or weak (green). Gypsy-family LTR retrotransposons are shown in red. For each element, the Piwi bias (log2 fold ratio of Piwi-bound piRNAs to Aub/AGO3-bound piRNAs) is shown in heat map form (center; green indicates strong, red weak Piwi bias). To the left, the sequence contribution of each element to the 42AB and flamenco piRNA clusters is shown in orange and black, respectively. (B) Ovarian (black) and embryonic (red) piRNAs were plotted over elements with strong (roo and F element) or weak (ZAM and gyspy 5) maternal piRNA deposition (identical y axes). For ZAM, a strong correlation between piRNA density and sequence fragments present in the flamenco cluster (blue) was found. (C) Total number (left) and ratio (right) of ovarian and embryonic piRNAs mapping to the 15 major piRNA clusters is shown. The corresponding Piwi-bias heat map is shown for each cluster as in (A). (D) piRNA densities over the 42AB and flamenco piRNA clusters (identical y axes) are shown.
Figure 2
Figure 2. Transposons Segregate into Distinct Regulatory Classes
Transposons segregated by maternal deposition and Piwi bias display differential levels of ping-pong amplification. (A)–(C) display piRNAs from the wK strain. (A) Ping-pong signal and Piwi bias of transposons with strong (red), intermediate (yellow), or weak (green) maternal piRNA deposition are displayed as a scatter plot (0 MM, zero mismatches). (B) Depiction of the ping-pong signature for indicated transposons. Graphs indicate the likelihood (in percent) that a complementary piRNA exists with a 5′ end at the indicated distance (x axis) for the average piRNA mapping to a particular transposon. The ping-pong signal was defined as the value at position 10 nt. (C) piRNA densities over the indicated transposons are shown in black. Those are split into piRNAs with (red) and piRNAs without (green) a sequenced ping-pong partner. (D) Ping-pong signals (value at 10 nt in [B]) are displayed as heat maps for aub and piwi heterozygote (+/−) and mutant (−/−) libraries. (E) Aub, AGO3, and Piwi protein localization in wild-type, aubaubQC42/HN2, and piwi1/2 mutant ovaries. We note some variability in piwi mutants due to their aberrant morphology.
Figure 3
Figure 3. Evolutionary Conservation of the flamenco piRNA Cluster
Transposon composition and chromosomal organization of the somatic flamenco piRNA cluster. (A) Schematic of the Drosophila melanogaster X chromosome with the flamenco cluster enlarged. Below, the transposon annotation at the flamenco loci in two other Drosophilid species is shown. Uniquely mapping D. erecta piRNAs are plotted over the putative flamenco cluster (bottom). (B) The transposon makeup of the 42AB, flamenco, and putative flamenco clusters are displayed. Pie charts display transposon orientation percentages. All graphs display the percentage of total annotated Repbase transposons in the cluster. Known and putative errantiviruses are indicated by black and gray dots.
Figure 4
Figure 4. Genetic Dissection of the Germline and Somatic piRNA Pathways
Analysis of piRNA populations in nine piRNA pathway mutants. (A) Size profiles of Piwi- (green), Aub- (yellow), and AGO3- (red) bound ovarian piRNAs are plotted as a percentage. Below, siRNA-normalized small RNA size profiles are shown for ovaries mutant (red) or heterozygous (black) for the indicated genes. (B) Uniquely mapping piRNAs are plotted over the 42AB and flamenco clusters. A typical heterozygote situation (here aub) is shown in black, all mutants in red. Libraries were normalized to allow for a direct comparison of piRNA densities between all libraries (Supplemental Experimental Procedures and Supplemental Glossary). For the krimp mutant, the flamenco density is scaled to 50%, with all other axes identical.
Figure 5
Figure 5. Mutation of spindle-E Defines Features of the Germline piRNA Pathway
Mutations in spn-E and flamenco display reciprocal effects on piRNA profiles and ping-pong signatures. (A) Shown are the fold changes of cluster-derived piRNAs in spn-E and flamenco mutants compared to their respective heterozygotes (left). To the right, piRNA densities from heterozygote (black) and mutant (yellow) libraries are plotted over the 42AB and flamenco clusters. (B) A bar diagram shows the log2 fold changes of piRNA levels mapping to all analyzed transposons in spn-E and flamenco mutants compared to their respective heterozygotes (center). The identity of several transposons is given (color coded according to the degree of maternal inheritance [rightmost bar diagram]). Also shown are piRNA densities for selected elements (germline elements to the left, somatic elements to the right). (C) Heat maps indicating ping-pong signals for typical germline (red), intermediate (yellow), and somatic transposons (green) in flamenco and spn-E mutants. (D) Immunocytochemical analysis of Aub, AGO3, and Piwi protein localization in wild-type and spn-E1/100.37 mutant ovaries.
Figure 6
Figure 6. Piwi Localization and Loading in Germline Cells Requires Armitage
(A) Piwi protein localization in wild-type and armi1/72.1 mutant ovaries. (B) Densities of uniquely mapping, Piwi-bound piRNAs over the 42AB and flamenco clusters from the wild-type (Oregon R, black) and armi mutants (yellow). (C) Annotation of repeat derived, Piwi-bound piRNAs from wild-type and armi mutant ovaries. (D) Log2 fold changes of cluster-derived piRNAs in an armi mutant total RNA library compared to heterozygote. (E) Log2 fold changes of piRNAs mapping antisense to indicated transposons in armi and spn-E mutants compared to respective heterozygotes are shown in heat map form. Corresponding ping-pong signal heat maps are shown (right). (F) piRNA densities over indicated transposons in heterozygote (black) and armi mutant (yellow) libraries. (G) F element ping-pong profiles in armi heterozygote (black), armi mutant (yellow), and aub mutant (blue) libraries.
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