Puberty in sea bass: Environmental control and endocrine aspects

Abstract

Puberty is the process by which an immature animal acquires the ability to reproduce for the first time and its onset occurs soon after sexual differentiation and is characterized by the beginning of gametogenesis in both sexes. In fish, the mechanisms underlying puberty and its triggering actors are still largely unknown. In the European sea bass, the recent isolation of genes coding for kisspeptins and their receptors allowed for expression studies that are clarifying something more the general endocrine picture of puberty of this species. We present here new insights on when and how the onset of male sea bass puberty occurs, its dependence of a critical size and how it can be controlled by the photoperiod. The suppression of plasma temporal patterns of key reproductive hormones in fish exposed to continuous light, revealed the existence of a photolabile period in September. Moreover, delayed puberty in male sea bass is an important economic issue because marketing time coincides with its puberty onset. Constant long photoperiod was highly effective in delaying puberty, which was linked to a phase difference in the rhythms of hormones that regulate gametogenesis. Indeed, the study of hormone daily rhythms revealed that its core values had equivalence with the seasonal rhythms so that the daily could be considered as the functional units of the seasonal rhythms. Regarding size, apparently only large fish attain the ability to carry out gametogenesis while the small ones do not succeed. Could this imply that to initiate and conclude puberty fish need to achieve an optimal threshold in hormone production? Studies performed with fish of different size demonstrate that small fish exhibit lower plasma hormonal levels than large fish confirming this assumption. Besides, size-photoperiod combined studies demonstrate a differential role of kisspeptins; apparently while Kiss1 is more linked to the photoperiodic signaling, Kiss2 is mostly involved in the reproductive processes. Is it necessary to accomplish a critical body size/fat stores/energy level in order to launch puberty attaining the hormonal signal able to initiate gametogenesis? Preliminary data show that a long-term restricted feeding regime did not prevent the onset of puberty. Besides, hormonal analysis revealed an upregulation of the kisspeptin system and Fsh, suggesting that the fish is able to maintain the reproductive function even at the expense of other functions.