Dying occurs as a defined molecular progression in Drosophila rather than as nonspecific physiological collapse

While we know much about the biology of aging, the biology of dying remains mysterious. Here, we show that old-age death in Drosophila is a discrete and ordered process that follows, but is distinguishable from, aging itself. We apply pseudotime trajectory analysis to transcriptomic data from individual, old flies as they progress toward death. This pseudotime reordering reveals a reproducible molecular organization underlying the process of dying, as demonstrated by systematic changes in the pattern of gene expression as the process advances. Analysis of these transcriptomic changes reveals not only the continuation of some characteristic processes of mid-life aging, but also the activation of a host of novel processes that are not typically associated with aging, and which instead seem to define the progression of an individual towards death. Finally, we reanalyze an existing C. elegans dataset and find that many of the processes we observe changing through the death progression in flies also vary among dying worms, suggesting that features of the death progression have been maintained across a wide span of evolution. These data challenge the idea that dying is simply a nonspecific collapse of physiology that is the inherent conclusion to the dysfunctions of aging. Overall design: Bulk RNA-Seq profiling of 113 flies representing 73 for natural aging characterization and 40 for supine death phenotype characterization.