Muncaster, Simonvan der Burg, ChloéLongney, Jana2026-06-162026-06-162026https://hdl.handle.net/10289/18373Teleost fish display exceptional diversity in their reproductive strategies, including sequential hermaphroditism, in which individuals undergo complete functional sex change during adulthood (monandry) and in some cases, a second male morph can arise prior to puberty (diandry). While the endocrine and genetic mechanisms underlying sex change have been extensively studied, the contribution of the immune system to gonadal remodelling remains poorly understood. This thesis investigates the role of immune processes during female-tomale prepubertal sex change in initial-phase members of the New Zealand spotty wrasse (Notolabrus celidotus). This study integrates histological analysis with gene expression profiling to examine immune involvement across transitional stages of gonadal sex change. Histological examination revealed stage-dependent changes in gonadal leukocyte (eosinophilic granular cell) abundance and localisation, coinciding with ovarian degeneration and testicular development. Gene expression analyses demonstrated coordinated increases in immuneassociated markers (cd68, il-1β, and tnf-α), particularly during late transitional and male stages. Notably, immune activation was not confined to early degenerative phases but persisted into the male phase, suggesting roles beyond debris clearance, likely including tissue organisation and maintenance. Although inter-individual variability limited statistical significance for some markers, consistent directional trends across genes and concordance with histological observations support a biologically meaningful pattern of immune modulation during sex change. By building a body of evidence that links immune gene expression with histological evidence of leukocyte involvement, this study highlights a likely localised immune function response as a key mechanism associated with vertebrate sex change. Importantly, it identifies immune cells as active contributors to sex change rather than passive responders.enAll items in Research Commons are provided for private study and research purposes and are protected by copyright with all rights reserved unless otherwise indicated.sex changehermaphroditeimmune cellleukocytehistologyThesis