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"Adult neurogenesis, i.e., the production of new neurons in the adult brain, has been studied intensively in the past years, both in humans and in animal models, as the understanding of this process can have major clinical implications. The study of neurogenesis in fish has been receiving more attention as, unlike mammals, they possess remarkably high levels of adult neurogenesis and a high capability for neuronal regeneration and replacement where neuronal death has occurred. Less is known, however, on the importance of adult neurogenesis for behavioural plasticity, i.e., for the capacity to change behaviour according to context. As a product of the brain, behaviour relies on functional neuronal networks and it may be expected that more permanent changes in behavioural states imply structural reorganization of neuronal circuits, with the integration of new neurons. Interestingly, the high level of brain plasticity of fish is paralleled by a high degree of behavioural plasticity, with many examples of species that change, either reversibly or irreversibly, their behavioural phenotype during their lifetime, as illustrated by species with functional sex-change and alternative reproductive phenotypes. Flexibility in behaviour may thus require a reorganization of neuronal networks underlying these behaviours with recruitment of new neurons. In this thesis, the link between brain and behavioural plasticity was studied in a small marine fish that inhabits the Mediterranean and adjacent Atlantic coasts, the peacock blenny Salaria pavo. In this species, males adopt nests in rock crevices and attract females into the nest for egg laying, with the male taking care of the eggs until hatching. In some populations, a scarcity of nest sites drives smaller and young males to adopt an alternative reproductive tactic to reproduce. These “sneaker” males mimic the females’ morphology and reproductive behaviour in order to illude the larger nesting males and parasitically fertilize eggs during mating events. Sneaker males later transition into the nesting male phenotype, and this major behavioural transformation in the same animal, first courting males and afterwards courting females, may imply significant reorganization of brain areas associated with reproductive behaviour. During the study, a brain atlas for the species was developed and the main cell proliferation regions, i.e. niches of stem cells birth that may differentiate into cells of the nervous system, characterized. Proliferative areas were observed throughout the whole brain and paralleled the pattern described for other teleosts. Proliferative cells were abundant namely in areas like the olfactory bulbs (granular and glomerular), the anterior subdivision of the dorsomedial telencephalon (DMa), the dorsal and ventral part of the ventral subdivision of the dorsomedial telencephalon (DMvd and DMvv), the dorsal part of the dorsal subdivision of the dorsomedial telencephalon (DMdd), the posterior subdivision of the dorsolateral telencephalon (DLp), the posterior zone of the dorsal telencephalic area (DP), the preoptic area (POA), the dorsal, supracommissural and ventral nucleus of the ventral telencephalic area (Vd, Vs and Vv), the optic tectum and its periventricular grey zone (TeO and PGZ), the ventral zone of the periventricular hypothalamus (Hv), the cerebellum, mainly the molecular layer (CCeM) and the caudal lobe (LCa). A study of the brain nuclei activated during female courtship events using immediate early-genes suggested that some of the areas of the social behaviour network (SBN), a set of brain nuclei underlying the expression of social behaviour across vertebrates, are implicated in female courtship, in particular nuclei in the ventral telencephalic regions. This was followed by an experiment to investigate the possible link between cell proliferation and male tactic switch. Nest availability was manipulated to allow a fraction of sneaker males to adopt a nest and start the transition to nesting males. Ten days after the experiment, some of the smaller males had indeed started switching into nesting males, adopting a nest and starting to develop male secondary sexual characters. The pattern of brain proliferation was studied in these fish to try to confirm that the irreversible behavioural transition would be associated with the reorganization of brain nuclei, assuming that cell proliferation relates to neurogenesis and structural reorganization. Transitional males had elevated cell proliferation levels, as compared to males that remained sneakers, in the dorsolateral anterior and posterior telencephalic regions, thought to be homologous to the hippocampus in mammals. Cell proliferation levels were generally elevated in ventral and ventromedial telencephalic nuclei in both sneakers and transitional males, as compared with nesting males and females, areas considered to be homologous to nuclei of the amygdaloid complex of mammals. There was large variation in proliferation levels within transitional males, and in particular one male more advanced in the transition had higher numbers of BrdU-positive cells than the others. This suggests that a longer time-window for detecting the peak in brain cell proliferation associated with tactic transition in some fish may have been needed. Overall, the study supports the hypothesis that behavioural transition in males of this species is paralleled by an increase in cell proliferation in nuclei potentially relevant for the expression of reproductive behaviours, and establishes the peacock blenny as a new relevant model for the study of neuronal plasticity in vertebrates."