Histological study of gonadal development and sex differentiation in Salvelinus fontinalis under Tasmanian climate conditions

This study describes the developmental process of gonads in brook trout from 0 degree days post-hatch (°dph) until completion of sex differentiation (3354°dph). Gonadal development was divided into undifferentiated (0–2013°dph) and differentiated phases (2769–3354°dph). Fertilised eggs (n = 1000) were incubated at 9.5-10°C until hatching at 66 days post-fertilisation (dpf). A total of 20% of alevins sampled on 0°dph were found with unpaired and undifferentiated gonads, indicating that gonadal development commenced before hatch. Initially, undifferentiated gonads contained stromal tissue and few primordial gonadal cells (PGC) (n = 2–5). During the undifferentiated phase, gonads increased in size and proliferative activity of the PGC increased their number (n = 15–22). The differentiated phase commenced with the appearance of sex differentiation at 2769°dph where gonads could be clearly differentiated as ovaries and presumptive testes. Ovaries were identified by the presence of oogonia while presumptive testes contained spermatogonia, vena comittis and a proximal network of cavities. Both ovaries and testes underwent further differentiation until the end of this phase (3354°dph). Oogonia were transformed into primary oocytes while spermatogonial cysts were observed in testes. However, differentiation of steroidogenic cells could not be observed. Direct sex differentiation was found in this study as undifferentiated gonads directly developed into testes and ovaries with anatomical differentiation preceding cytological differentiation. This study confirms previous studies that the prehatch period should be targeted when attempting to produce future monosex populations via indirect sex reversal using androgen treatment.