Age determination of gonad maturation and puberty onset in the transparent casper zebrafish juvenile

The transparent double mutant zebrafish, casper, allows direct visualization of gonad development and maturation (White et al. 2008). In this study juvenile casper were imaged at weekly intervals beginning at 3 weeks post fertilization. Both sides of the fish were imaged with transverse, reflected illumination and the images captured through a dissecting microscope fitted with a digital camera. Swimbladder development, bifurcation and fat deposition occurred prior to gonad appearance. Females were noted to have an amorphous golden coloration in the area of subsequent gonad development, while males had a milky-white cast to the mature gonad. Once the golden color in the presumptive female gonad area appeared, well developed ovarian follicles formed within a week in some females. This suggests that the vitellogenesis timeframe may be less than one week in these pubescent animals. Puberty onset in six female juveniles occurred between 87 and 122 dpf and averaged 104 dpf as assessed by attainment of 400 μm diameter by the largest ovarian follicle size class. Puberty in four casper males occurred between 99 and 159 dpf and averaged 139 dpf as assessed in vivo by the opaque milkywhite appearance of the testis. The transparent casper zebrafish allows direct imaging of the gonad and provides a remarkable model system to study in vivo gonadal dynamics