1 2 A major challenge in modern biology is to understand how naturally occurring variation in DNA 3 sequences affects complex organismal traits through networks of intermediate molecular 4 phenotypes. Here, we performed deep RNA sequencing of 200 Drosophila Genetic Reference 5 Panel inbred lines with complete genome sequences, and mapped expression quantitative trait 6 loci for annotated genes, novel transcribed regions (most of which are long noncoding RNAs), 7transposable elements and microbial species. We identified host variants that affect expression 8 of transposable elements, independent of their copy number, as well as microbiome 9 composition. We constructed sex-specific expression quantitative trait locus regulatory 10 networks. These networks are enriched for novel transcribed regions and target genes in 11 heterochromatin and euchromatic regions of reduced recombination, and genes regulating 12 transposable element expression. This study provides new insights regarding the role of natural 13 genetic variation in regulating gene expression and generates testable hypotheses for future 14 functional analyses.15 Introduction 1 2 Understanding how naturally occurring genetic variation affects variation in organismal 3 quantitative traits by modifying underlying molecular networks is a key challenge in modern 4 biology. Most traits are highly polygenic 1-3 and associated molecular variants have small 5 additive effects on trait variation 4 . Most of these variants are in intergenic regions, up-or down-6 stream of coding regions, or in introns, and presumably play a regulatory role in modulating 7 gene expression.8 Systems genetics analysis seeks to determine how naturally occurring molecular 9 variation gives rise to genetic variation in organismal phenotypes by examining genetic variation 10 in gene expression (expression quantitative trait loci, or eQTLs) and other intermediate 11 molecular phenotypes 2, 5-13 . Polymorphic variants associated with variation in gene expression 12 are classified as cis-or trans-eQTLs depending on whether they are proximal or distal to the 13 gene encoding the transcript, respectively. Genetic variation in gene expression is pervasive; 14 cis-eQTLs can have large effects on gene expression that are detectable in small samples; and 15variants associated with human diseases and quantitative traits tend to be enriched for cis-16 eQTLs 2, 5-15 . eQTLs with both cis-and trans-effects can be assembled into directed 17 transcriptional networks of regulator and target genes [16][17][18] . Elucidating such regulatory 18 transcriptional networks will facilitate understanding how the effects of individual variants 19 propagate through the network, and how multiple variants together regulate gene expression 20 and affect complex traits 15-18 . 21 Here, we performed deep RNA sequencing of the Drosophila melanogaster Genetic 22 Reference Panel (DGRP) of inbred lines with complete DNA sequences 19,20 . We mapped eQTLs 23 for annotated genes, novel transcribed region (NTRs, which are largel...