Post-senescence reproductive rebound in Daphnia associated with reversal of age-related transcriptional changes.

A long-lived species of zooplankton microcrustaceans, Daphnia magna, sometimes exhibits late-life rebound of reproduction, briefly reversing reproductive senescence. Such events are often interpreted as terminal investments in anticipation of imminent mortality. We demonstrate that such post-senescence reproductive events (PSREs) neither cause nor anticipate increased mortality. We analyze an RNAseq experiment comparing young, old reproductively senescent, and old PSRE Daphnia females. We first show that overall age-related transcriptional changes are dominated by the increased transcription of guanidine monophosphate synthases and guanylate cyclases, as well as two groups of presumed transposon-encoded proteins, and by a drop in transcription of protein synthesis-related genes. We then focus on gene families and functional groups in which full or partial reversal of age-related transcriptional changes occur. This analysis reveals a reversal, in the PSRE individuals, of age-related up-regulation of apolipoproteins D, lysosomal lipases, and peptidases as well as several proteins related to mitochondrial and muscle functions. While it is not certain which of these changes enable reproductive rejuvenation, and which are by-products of processes that lead to it, we present some evidence that post-senescence reproductive events are associated with the reversal of age-related protein and lipid aggregates removal and apoptosis.