Predictors of TE-derived piRNA abundance. The coefficient of determination (R²) is reported for individual predictors of variation in sense (dark bars) and antisense (light bars) piRNA abundance between TE families. R² values are from a bivariate regression model including the indicated predictor variable and a factor for genotype (w^K or w¹¹¹⁸). Variables are grouped into four categories: (A) genome-wide abundance, (B) piRNA cluster abundance, (C) TE transcript level, and (D) piRNA production. piRNA clusters are the 15 major sources of uniquely mapping piRNAs defined in Brennecke et al. (2007) including the following: cluster1/42AB–2R:2144349 … 2386719; cluster2/20A–X:21392175 … 21431907; cluster3/102E–4:1258473 … 1348320; cluster4/XTAS–GenBank L03284.1; cluster5/38C–2L:20148259 … 20227581; cluster6/80E–F 3L:23273964 … 23314199; cluster7–U:4015849 … 4029971; cluster8/flamenco–X:21505666 … 21684449; cluster9/20B–X:21759393 … 21844063; cluster10–U:5766708 … 5772171; cluster11/100F–3R:27895169 … 27905030; cluster12–3LHet:1402377 … 1557939; cluster13–3LHet:2011004 … 2180268; cluster14–U:7542733 … 7545114; and cluster15–3LHet:238123 … 332969. *P < 0.05; **P < 0.01; ***P < 0.001; N.S., P > 0.1.