Expression Analysis of Zebrafish Gonad Development Reveals Early Divergence

Zebrafish gonad development is a complex process. Using classical histological technique, Takahashi (1977) and Maack & Segner (2003) observed that all zebrafish developed juvenile-ovary at around 10 dpf. Later, Wang et al. (2007) reported that several states of juvenile-ovary exist based on the varying fluorescence intensity observed from vas::egfp transgenic zebrafish. Currently the transition from a primordial gonad to juvenile-ovary then to either testis or ovary is poorly studied. This project aims to enhance the knowledge of zebrafish gonad development by analysing the juveniles' transcriptome.

Using the vas::egfp transgenic line, zebrafish that showed strong (female) or weak (male) vas::egfp fluorescence at 28, 35, 42 and 105 dpf were analysed by a custom-made microarray. By comparing gene expression between the two groups, 9,259 genes were found to have expression level that was significantly different in at least one time point, while only eight genes were significantly different for all time points. These genes and a few candidate genes were analysed by qPCR in younger zebrafish (7, 14 and 21 dpf). Based on qPCR expression profile, the younger zebrafish can be clustered into at least two groups. This indicates diverse gonadal transcriptome of the zebrafish from as early as 7 dpf.

Our data showed that although phenotyically all zebrafish go through the juvenile-ovary stage between 10-40 dpf, two divergent expression patterns potentially indicating early gonadal differentiation between the sexes can be observed as early as 7 dpf. Three clusters of zebrafish expression profiles observed concurred with the various juvenile-ovary states described previously.