Chemical diplomacy in male tilapia: urinary signal increases sex hormone and decreases aggression.

Androgens, namely 11-ketotestosterone (11KT), have a central role in male fish reproductive physiology and are thought to be involved in both aggression and social signalling. Aggressive encounters occur frequently in social species, and fights may cause energy depletion, injury and loss of social status. Signalling for social dominance and fighting ability in an agonistic context can minimize these costs. Here, we test the hypothesis of a 'chemical diplomacy' mechanism through urinary signals that avoids aggression and evokes an androgen response in receiver males of Mozambique tilapia (Oreochromis mossambicus). We show a decoupling between aggression and the androgen response; males fighting their mirror image experience an unresolved interaction and a severe drop in urinary 11KT. However, if concurrently exposed to dominant male urine, aggression drops but urinary 11KT levels remain high. Furthermore, 11KT increases in males exposed to dominant male urine in the absence of a visual stimulus. The use of a urinary signal to lower aggression may be an adaptive mechanism to resolve disputes and avoid the costs of fighting. As dominance is linked to nest building and mating with females, the 11KT response of subordinate males suggests chemical eavesdropping, possibly in preparation for parasitic fertilizations.

A Multi-Component Pheromone in the Urine of Dominant Male Tilapia (Oreochromis mossambicus) Reduces Aggression in Rivals.

Males often use scent to communicate their dominance, and to mediate aggressive and breeding behaviors. In teleost fish, however, the chemical composition of male pheromones is poorly understood. Male Mozambique tilapia, Oreochromis mossambicus, use urine that signals social status and primes females to spawn. The urinary sex pheromone directed at females consists of 5ß-pregnane-3α,17α,20ß-triol 3-glucuronate and its 20α-epimer. The concentration of these is positively correlated with male social rank. This study tested whether dominant male urine reduces aggression in receiver males, and whether the pregnanetriol 3-glucuronates also reduce male-male aggression. Males were allowed to fight their mirror image when exposed to either: i) water control or a chemical stimulus; ii) dominant male urine (DMU); iii) C18-solid phase (C18-SPE) DMU eluate; iv) C18-SPE DMU eluate plus filtrate; v) the two pregnanetriol 3-glucuronates (P3Gs); or vi) P3Gs plus DMU filtrate. Control males mounted an increasingly aggressive fight against their image over time. However, DMU significantly reduced this aggressive response. The two urinary P3Gs did not replicate the effect of whole DMU. Neither did the C18-SPE DMU eluate, containing the P3Gs, alone, nor the C18-SPE DMU filtrate to which the two P3Gs were added. Only exposure to reconstituted DMU (C18-SPE eluate plus filtrate) restored the aggression-reducing effect of whole DMU. Olfactory activity was present in the eluate and the polar filtrate in electro-olfactogram studies. We conclude that P3Gs alone have no reducing effect on aggression and that the urinary signal driving off male competition is likely to be a multi-component pheromone, with components present in both the polar and non-polar urine fractions.

Lack of evidence for a role of olfaction on first maturation in farmed sea bass Dicentrarchus labrax.

Chemical communication is widespread in the animal kingdom and olfaction constitutes a powerful channel for social and environmental cues. In fish, olfactory stimuli are known to influence physiological processes, including reproduction. Here we investigate the effects of olfaction on puberty in European sea bass Dicentrarchus labrax males. Intact sea bass coming to first maturity (puberty) are able to smell conspecific odours. However, induced anosmia during most of the spermatogenesis period had no effect on the sex ratio, gonad maturation state or gonado-somatic index at the time of reproduction. Furthermore anosmia decreased mRNA expression of brain KISS2 and pituitary LHb and FSHb, but not brain GnRH1 and GnRH3. Thus, although anosmia seems to modify gene expression of key reproduction related genetic factors, it seems to be insufficient to stop or delay growth or gonadal development and maturation.

Changes in the gene expression profiles of the brains of male European eels (Anguilla anguilla) during sexual maturation.

BACKGROUND: The vertebrate brain plays a critical role in the regulation of sexual maturation and reproduction by integrating environmental information with developmental and endocrine status. The European eel Anguilla anguilla is an important species in which to better understand the neuroendocrine factors that control reproduction because it is an endangered species, has a complex life cycle that includes two extreme long distance migrations with both freshwater and seawater stages and because it occupies a key position within the teleost phylogeny. At present, mature eels have never been caught in the wild and little is known about most aspects of reproduction in A. anguilla. The goal of this study was to identify genes that may be involved in sexual maturation in experimentally matured eels. For this, we used microarrays to compare the gene expression profiles of sexually mature to immature males. RESULTS: Using a false discovery rate of 0.05, a total of 1,497 differentially expressed genes were identified. Of this set, 991 were expressed at higher levels in brains (forebrain and midbrain) of mature males while 506 were expressed at lower levels relative to brains of immature males. The set of up-regulated genes includes genes involved in neuroendocrine processes, cell-cell signaling, neurogenesis and development. Interestingly, while genes involved in immune system function were down-regulated in the brains of mature males, changes in the expression levels of several receptors and channels were observed suggesting that some rewiring is occurring in the brain at sexual maturity. CONCLUSIONS: This study shows that the brains of eels undergo major changes at the molecular level at sexual maturity that may include re-organization at the cellular level. Here, we have defined a set of genes that help to understand the molecular mechanisms controlling reproduction in eels. Some of these genes have previously described functions while many others have roles that have yet to be characterized in a reproductive context. Since most of the genes examined here have orthologs in other vertebrates, the results of this study will contribute to the body of knowledge concerning reproduction in vertebrates as well as to an improved understanding of eel biology.

Environmental modulation of androgen levels and secondary sex characters in two populations of the peacock blenny Salaria pavo.

Morphology and endocrinology were studied in two populations of the peacock blenny Salaria pavo, with different regimes of sexual selection imposed by differences in nest site availability. The peacock blenny is a small, sexually dimorphic benthic fish that presents exclusive paternal care of the clutch and inhabits rocky shores of the Mediterranean and adjacent Atlantic areas. In a population from the Gulf of Trieste (Northern Adriatic sea) inhabiting rocky shores where nest sites are abundant, male-male competition for nests is low, males court females and a low frequency of alternative reproductive tactics (small, parasitic female-mimicking sneaker males that change tactic into nest holders in subsequent breeding seasons) occurs. Conversely at Ria Formosa, a coastal lagoon in Southern Portugal, where nest sites are scarce and highly aggregated, male-male competition for nests is very high, there is sex-role reversal with female courtship and a high frequency of alternative reproductive tactics is observed. Concomitantly, at Ria Formosa nest holder males are larger and present more developed secondary sex characters and higher levels of 11KT than at the Gulf of Trieste. However, the gonads of nest holders and parasitic males were larger in the Gulf of Trieste population. Competition for nests at Ria Formosa seems to promote more developed secondary sex characters in nest site scarcity conditions, while competition for females at the Gulf of Trieste seems to be spurring sperm competition among males in populations where nest sites are more abundant. 11KT was thus associated with the development and expression of secondary sex characters in contrasting environments. These results exemplify how the modulation of behavioral plasticity and secondary sex characters by the social environment can be mediated by androgens.