Abstract :
[en] As opposed to the fixed mechanism of sex determination of endotherms, the sex determination system of poikilotherms is variable and can depend (partially or totally) on environmental parameters, mainly temperature. In fish, high temperature generally drives masculinization of the progenies independently from their genotype, creating individuals presenting a mismatch between their genotype and phenotype. Nile tilapia (Oreochromis niloticus) is a cichlid species presenting a male heterogamety (XY/XX). If temperature is superior to 32°C during the period of sex differentiation, genetic XX progenies are masculinized into XX “neomales”. In adulthood, these neomales sire 100% female progenies that can in turn be masculinized if temperature allows so. This process is naturally relevant, as neomales have been described in the wild. Indeed, climate change could increase the frequency of sex reversals, eventually leading to the decline of a population. As Nile tilapia is a species of high value in aquaculture, sex reversal has been investigated mainly through hormonal treatments. Consequently, few data are available on temperature-induced sex reversed individuals and the consequences in adulthood are unknown. This thesis aimed at determining the consequences of temperature-induced sex reversal via two main axes of research whose goals were to:
1. Investigate the influence of sex reversal on testis maturation and physiology, and on two neuronal populations: the gonadotropin-releasing hormone (GnRH) neuron population and the arginine-vasotocin (AVT) neurons populations (each divided into three subpopulations). These were specifically chosen because they are sexually different and are implicated in the regulation of socio- sexual behaviors. We found that neomales present a reduced number of GnRH1 neurons compared to both XY males and females. Surprisingly though, neomales present the same characteristics regarding testis maturation and hormone secretion (testosterone, 11-ketotestosterone, 17β-estradiol) as XY males. As GnRH1 neurons stand at the apex of the hypothalamo-pituitary-gonadal (HPG) axis, we formulated several hypotheses regarding a potential compensation mechanism that could take place at the three functional levels of this HPG axis. Neomales also presented differences in the number and area of AVT neurons compared to XY males and females, which we hypothesized to be linked with an increased risk to adopt a subordinate behavior, probably reflected in high cortisol circulating concentrations.
2. Investigate the consequences of sex reversal on behavior. The hypothesis regarding neomales subordination could be directly tested. We compared the behavior of neomales and XY males in agonistic interactions, as well as other parameters (size of the nest, hormone concentration, sound
production). During dyadic confrontations, neomales present a higher aggression level compared to iii
XY males, but almost always end up being subordinate. In this context, sound production seemed unnecessary. We also investigated if neomales were as attractive as XY males for gravid females. These females do not prefer any type of male, even though neomales are once again extremely aggressive. When next to a female, both male types emitted sounds that differed in duration and number of pulses. Surprisingly, no difference could be detected regarding hormone circulating concentrations (testosterone, 11-ketotestosterone, cortisol) in both contexts between the two male types. Interestingly, neomales were found to build smaller nests than XY males only when housed with a XY male.
Overall, our results suggest that the sex chromosomes and/or the sex reversal procedure impacted the brain of neomales in a long-lasting manner. Neomales, presenting different brain characteristics than XY males in a region implicated in the integration and treatment of various internal/external informations, would react differently than XY males to the same stimuli.
