The timing and extent of ‘juvenile ovary’ phase are highly variable during zebrafish testis differentiation

Abstract

Gonad differentiation in male zebrafish involves a transformation from a ‘juvenile ovary’ to testis. In our study, this phenomenon was analysed using a stable transgenic line (vas::egfp) created by Krovel & Olsen (Mechanisms of Development, 116: 141–150, 2002). Our data obtained by histological and immunohistochemical methods show that the onset of enhanced green fluorescent protein (EGFP) expression marks the conversion of the bipotential gonad into a ‘juvenile ovary’, and its subsequent decrease in the males indicates transformation into a testis. Furthermore, during the ‘juvenile ovary’ stage, a high level of variation was observed among individuals not only in the starting point and duration of EGFP expression but also in its intensity. The intensity of reporter expression in the males ranged from a high level of expression – similar to that observed in females – to invisible expression and correlated with the number of oocytes, the shape of ‘juvenile ovary’ and its lumen. Therefore, the authors conclude that although all males develop ‘juvenile ovaries’, they differ vastly in the extent of their commitment toward femaleness during this stage before testis differentiation. The differentiating zebrafish testis shows morphological features similar to those of the transforming gonad of protogynous hermaphrodites, and therefore, it could be a good model for the study of the process of female‐to‐male sex change in adult teleosts.

Anti‐Müllerian hormone and 11 β‐hydroxylase show reciprocal expression to that of aromatase in the transforming gonad of zebrafish males

“…5B). The former were shown earlier to develop normal (i.e., nontransforming) ovaries, whereas the latter were undergoing "ovary-to-testis" transformation (Wang et al, 2007). In the "normal" ovaries with increasing EGFP fluorescence cyp19a1a was expressed in the somatic cells (presumptive granulosa cells) surrounding the healthy oocytes (Fig.…”

“…During gonadal differentiation of zebrafish, males temporarily develop a "juvenile ovary" that is later degenerated and transformed into a testis (Takahashi, 1977;Maack and Segner, 2003;Wang et al, 2007). Although histological analysis showed the presence of degenerating oocytes in these transforming gonads and apoptosis was implicated to be involved with the process (Uchida et al, 2002), the molecular mechanism of this "juvenile ovary-totestis" transformation has never been addressed.…”

“…Individuals at a particular stage were selected based on the gonadal expression of enhanced green fluorescent protein (EGFP) in a vas::egfp transgenic line as described earlier (Wang et al, 2007). The data indicate that down-regulation of cyp19a1a, possibly by amh, might be one factor leading to "juvenile ovaryto-testis" transformation.…”

Heat-induced masculinization in domesticated zebrafish is family-specific and yields a set of different gonadal transcriptomes

“…Sexing of the juvenile zebrafish by examining external phenotypes at this age is not possible. Therefore, we included in the hierarchical clustering samples from the Tg(vasa:vasa-egfp) transgenic line (47) collected at similar age (35 and 42 dpf), because it allows for early sexing from 16 dpf onward (25). This way, we were able to infer the sex of the two juvenile groups at control temperature (JCT1 and JCT2).…”

“…Then, during the 21-to 35-dpf period, in the future males the juvenile ovaries undergo apoptosis and are transformed into testes, with interindividual variation in the timing and duration to complete this process. In contrast, ovarian differentiation continues in the future females (25), with an increase in cytoplasm volume of oocytes at around 60 dpf. Transcriptomic analysis of zebrafish sex differentiation has shown that apoptosis (24), activated by the p53 signaling pathway (26), is required for testis differentiation, whereas up-regulation of the canonical Wnt and NF-κB signaling pathways is needed for ovarian differentiation (27,28).…”