Genetic architecture and molecular mechanisms underlying light entrainment of the Drosophila circadian clock

Despite significant progress in the understanding of how the circadian clock is entrained by light, the genetic architecture and molecular basis of this process are still largely unknown. This study was undertaken to identify biological pathways underlying light entrainment in Drosophila melanogaster. Complementary approaches that combined quantitative trait loci (QTL) mapping, complementation tests, and genome-wide gene expression profiling were used. One hundred and twenty-three recombinant inbred lines (RIL) were assayed for circadian photosensitivity. Composite interval mapping identified a single significant QTL. Quantitative deficiency complementation test refined this QTL interval into two smaller QTLs consisting of 140 candidate genes. Complementation tests with null mutant strains suggested segregating alleles of timeless and cycle may contribute to the variation in light response. In addition, two genes CG9879 and Lilliputian located within the QTL showed a significant differential expression in two RIL that were analysed by microarrays. Interestingly, Lilliputian interacts with several genes such as Shaggy and nejire which have been previously implicated in the circadian clock. Global profiling of gene expression following a light pulse at ZT15 revealed 209 differentially expressed genes in a laboratory strain (Canton-S). These genes are involved in several biological processes, however genes related to signal transduction, gene regulation, glutamate receptor activity, cellular communication and chromatin remodelling were statistically over-represented. RNA interference mediated knockdown further supported the role of these genes in the light response. Notable among these genes were nrv1, Neurofibromin 1, still life and Thor. In addition, the microarray experiments indicated that histone modifications may also play an important role in light entrainment of the clock. Consistently, an aberrant light response was found in various mutants and transgenic strains in which histone acetylation, de-acetylation, and methylation (of DNA and histones) are defective.