A 2014 demography and RNA-seq study of adult Drosophila melanogaster given a single acute Cs-137 gamma exposure found a threshold effect — no lifespan reduction and no persistent gene-expression change below 100 J/kg incident level, and persistent predominantly down-regulated programs at 100–200 J/kg — and reported absorbed dose as the better cross-experiment dose metric. The authors explicitly asked how to convert radiation damage levels between Drosophila and mammals, how multiple or chronic exposures compare with a single large dose, and whether the gene-expression results and repair mechanisms carry over to mammals, while flagging the large early transcriptional response at 0.1 J/kg as either an anomaly or a protective (hormetic) effect. These questions remain open: the same group's 2017 reanalysis of the same dataset developed gene-expression biodosimetry (reading the received dose off expression changes in selected genes) but restated the translation question as future work; adjacent studies provide conditional hormesis evidence in other paradigms (imaginal, embryonic, or chronic pre-imaginal exposure); and no matched-total-absorbed-dose acute-versus-fractionated comparison in adult flies and no validated Drosophila-to-mammal conversion are known. The problem is to determine the exposure-regimen dependence, the status and mechanism of the low-dose response, and the conserved translational mapping of the Drosophila radiation threshold effect.

Background

Ionizing radiation dose is measured in grays (Gy), where 1 Gy corresponds to 1 J of energy deposited per kg of tissue. Experiments on small organisms usually report the incident radiation level (energy crossing the organism) rather than the absorbed dose (energy actually deposited); the two differ because absorption depends on organism size, shape, orientation, and photon energy. The prevailing regulatory assumption in radiation protection is the linear no-threshold (LNT) hypothesis: detriment is proportional to dose all the way down to zero, so any exposure carries risk. Competing pictures are a threshold model (no net detriment below some dose, because cellular repair copes with the damage) and radiation hormesis (a biphasic dose-response in which low doses of an agent are beneficial while high doses are harmful), often attributed to adaptive responses, meaning the upregulation of DNA repair and antioxidant defenses by a mild insult. The fruit fly Drosophila melanogaster is a genetically tractable in vivo system for these questions: adult fly tissues are largely post-mitotic (cells no longer divide), so flies rarely develop radiation-induced cancer, but radiation shortens fly lifespan and alters gene expression, and most fly genes have mammalian homologues. Antosh et al. (2014) irradiated adult flies 24–48 hours after eclosion (emergence of the adult fly from its pupal case) with single acute exposures to a caesium-137 (Cs-137) gamma source at 0.1–1000 J/kg incident level and measured lifespan (demography) and whole-body RNA-seq gene expression (genome-wide measurement of transcript abundance) at 2, 10, and 20 days after irradiation. They found no statistically significant lifespan change at incident levels up to 50 J/kg and significant shortening at 100 J/kg and above; persistent, predominantly down-regulated gene expression at 100–200 J/kg at days 10–20 (roughly 1550–1720 genes), whereas changes at doses up to 50 J/kg reversed by day 20; a large but transient response at the lowest dose (0.1 J/kg: 138 genes up, 545 down at day 2, returning to control by day 10); and that their Cs-137 data reconcile with earlier cobalt-60 (Co-60) demography data only when both are converted to absorbed dose. They interpreted this as a threshold effect contradicting LNT and explicitly listed what remains open. Historical context matters for two of the open questions: mid-twentieth-century demography studies of daily low-dose irradiation of flies reported lifespan extension at low daily doses, and pre-2014 work reported strain- and sex-dependent lifespan hormesis and radioadaptive responses (protection against a challenging high dose by a preceding low dose). A same-group reanalysis of the same RNA-seq dataset (Shrestha et al. 2017) developed gene-expression biodosimetry — candidate genes whose expression changes approximately linearly with dose, several with human homologues — but found the changes largely transient and restated the translation question as open. A dedicated 2025 review of Drosophila radiobiology (Trinca and de Navascués) confirms that the dose-response shape (threshold versus hormesis), its underlying mechanism, and the fly-to-mammal correspondence remain unresolved, while noting that the genetic underpinning of the fly radiation response appears conserved with that of humans.

Problem Statement

The problem is to determine the low-dose ionizing radiation dose-response of adult Drosophila melanogaster beyond the single-acute-exposure configuration in which the threshold effect was established, and its translational correspondence to mammals. Three linked questions, posed by the source study and left open by subsequent literature, define the objective: (1) Exposure-regimen dependence — do multiple smaller or chronic exposures delivering the same total absorbed dose as one acute exposure produce the same, weaker, or qualitatively different (including protective) effects on lifespan and on the persistent gene-expression program? The source study could only note that historical daily-irradiation demography, alone among comparable studies, reported lifespan extension at low daily doses; no matched-total-dose acute-versus-fractionated comparison in adult flies with lifespan and transcriptome readouts has been reported. (2) Status and mechanism of the low-dose response — is the large, transient transcriptional response at the lowest tested incident level (0.1 J/kg) a reproducible adaptive (hormetic) program or an experimental anomaly, and can a protective phenotype be demonstrated, and by what assays? The underlying mechanism, including the conjectured transcriptional memory (a self-maintaining altered expression state that would keep protective genes expressed), remains unproven. (3) Translational scaling — which components of the radiation response (DNA repair genes, transcriptional programs) are conserved between Drosophila and mammals, and can a validated conversion between Drosophila and mammalian damage levels be established, or is the response fundamentally species-specific? Each sub-question is settled only by direct evidence on the ask it states — (1) a matched-total-absorbed-dose comparison of acute versus fractionated or chronic exposure in adult flies; (2) evidence on whether the low-dose response is reproducible, its mechanism, and any protective phenotype; and (3) evidence on which response components are conserved and whether a validated Drosophila-to-mammal conversion can be established or the response is fundamentally species-specific. The three sub-questions are jointly posed, and each can be resolved independently. These questions jointly decide whether the threshold effect is a general, mechanistically grounded, and translatable phenomenon rather than an artifact of one species, life stage, and exposure schedule. Two features of the source study form relevant background for any investigation of them: its absorbed-dose calculation rests on idealized fly geometry, random orientations, and electron detour factors, and two of its gene-expression dose points were left unprocessed (1 J/kg, never processed; 400 J/kg, not processed).

References

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