This thesis, supervised by Victoria G. Castiglioni, Santiago F. Elena, studies the effect of below-background radiation and microgravity on host and virus mutual responses. Part of the results were published in Frontiers in Microbiology and Microbiology Spectrum. The thesis was defended on 22 April 2026.

This PhD thesis, entitled “Characterization of the pathosystem C. elegans - Orsay virus in the presence of environmental stresses”, studied how biotic stress (viral infection) and abiotic stress (simulated microgravity and below‑background radiation) interact to shape host physiology, gene expression, and evolution. Using the Caenorhabditis elegans–Orsay virus pathosystem, the study combined phenotypic assays, transcriptomics, and experimental evolution, with relevance to space biology.

Microgravity and reduced radiation had contrasting effects on viral dynamics: below‑background radiation increased viral load and altered replication kinetics, while microgravity reduced viral accumulation. Both stresses lowered host reproductive success, yet their combination often produced antagonistic, non‑additive effects. Transcriptomic analyses revealed shared stress responses involving oxidative stress, innate immunity (IPR), proteostasis, and lipid metabolism, alongside a marked attenuation of gene expression under combined stresses.

Experimental evolution showed that Orsay virus evolved under microgravity displayed reduced replication and transmission, indicating environment‑dependent viral fitness trade‑offs. Host evolution experiments in the genetically diverse nematode C. remanei revealed adaptation in lifespan but not fertility. Overall, the work demonstrates that combined stresses generate complex, non‑linear host–pathogen responses.

Ana Villena carried out his doctoral research in the Evolutionary Systems Virology group at the Institute for Integrative Systems Biology I2SysBio (UV-CSIC) under the supervision of Victoria G. Castiglioni and Santiago F. Elena, both I2SysBio researchers. During her research, Ana Villena held a contract under the SpaceWorms project funded by the European Space Agency (ESA). The examination board consisted of Ester Lázaro Lázaro (Centro de Astrobiología, CSIC-INTA), Mark P. Zwart (Institut of Ecology-KNAW, The Netherlands), and Christina Toft (I2SysBio, CSIC-UV), who awarded the thesis a grade of ‘outstanding’.