This thesis, supervised by Santiago F. Elena and Rubén González, explores the relationships between host developmental stages and virus infection and evolution on different host-pathogen systems. Part of the results of this research has been published in the journals Philosophical Transactions of the Royal Society, Journal of General Virology, and iScience. The thesis was defended on 19 December 2025.

Existing in the borderline between life and death, viruses are the most abundant biological entities on Earth and important agents contributing to the evolution of species. In that process, viruses face host population with different levels of susceptibility to infection, including hosts on different developmental stages. This doctoral thesis, entitled "Unveiling the interplay between host developmental stages and virus infection: A study on viral dynamics and evolution”, explores the impact of a given host developmental stage on the evolution of viruses and the differential responses that hosts display depending on the time of their life they are infected.

The research combines experimental and computational approaches and studies infection processes on different systems such as Arabidpsis thaliana and Turnip mosaic virus; and Caenorhabditis elegans and Orsay virus. They found a high impact of developmental stages on both infection outcomes and virus evolution. Viruses accumulated different mutations after being evolved on different host developmental stages, and each stage imposed unique selective preassures. “Although sometimes overlooked, the host developmental stage at the time of infection is an important aspect impacting host–virus interactions, and we shall consider it when studying viruses infections and evolution, specially in a changing world such as the one we live in”. Izan Melero states. These insights deepen our understanding of how viruses interact with their hosts, and how organisms strategize their resources to overcome infection.

Izan Melero Peris conducted his doctoral research in the Evolutionary Systems Virology group under the supervision of Santiago Elena, CSIC Research Professor at the I2SysBio (UV-CSIC) and Rubén González, post- doctoral researcher on the Pasteur Institute. During his research, he held a contract under the FPI Investigation Staff Training Programme (Ministry of Science, Innovation and Universities). He also enjoyed short stays at University of Sevilla (Sevilla, Spain), and AMOLF (NWO, Amsterdam, The Netherlands). The qualifying committee was formed by Stéphanie Bedhomme (CNRS, Montpellier), Tomás Matus (I2SysBio, CSIC-UV), and Rafael Vázquez (La Fe, Valencia), who graded the thesis as outstanding.