Comprehending sex determination mechanism is a first step for developing sex control breeding biotechnologies in fish. Pseudobagrus ussuriensis, one of bagrid catfishes in Bagridae, had been observed to have about threefold size dimorphism between males and females, but its sex determination mechanism had been unknown. In this study, we firstly used the amplified fragment length polymorphism (AFLP)-based screening approach to isolate a male-specific DNA fragment and thereby identified a 10,569 bp of male-specific sequence and a 10,365 bp of female-related sequence by genome walking in the bagrid catfish, in which a substantial genetic differentiation with 96.35 % nucleotide identity was revealed between them. Subsequently, a high differentiating region of 650 bp with only 70.26 % nucleotide identity was found from the corresponding two sequences, and three primer pairs of male-specific marker, male and female-shared marker with different length products in male and female genomes, and female-related marker were designed. Significantly, when these markers were used to identify genetic sex of the bagrid catfish, only male individuals was detected to amplify the male-specific marker fragment, and female-related marker was discovered to produce dosage association in females and in males. Our current data provide significant genetic evidence that P. ussuriensis has heterogametic XY sex chromosomes in males and homogametic XX sex chromosomes in females. Therefore, sex determination mechanism of P. ussuriensis is male heterogametic XX/XY system.
The objective of this study was to investigate the optimal developmental time to perform sex reversal in Ussuri catfish Tachysurus ussuriensis, to develop monosex breeding in aquaculture. Systematic observations of gonadal sex differentiation of P. ussiriensis were conducted. The genital ridge formed at 9 days post fertilization (dpf) and germ cells begin to proliferate at 17 dpf. The ovarian cavity began forming on 21 dpf and completed by 25 dpf while presumptive testis remained quiescent. The primary oocytes were at the chromatin nucleolus stage by 30 dpf, the peri-nucleolus stage by 44 dpf and the cortical alveoli stage by 64 dpf. The germinal vesicle migrated towards the animal pole (polarization) at 120 dpf. In presumptive testis, germ cells entered into mitosis and blood vessels appeared in the proximal gonad on 30 dpf. The efferent duct anlage appeared on 36 dpf and formation of seminal lobules with spermatogonia and lobules interstitium occurred at 120 dpf. Therefore, gonadal sex differentiation occurred earlier in females than in males, with the histological differentiation preceding cytologic differentiation in T. ussuriensis. This indicates that undifferentiated gonads directly differentiate into ovary or testis between 17 and 21 dpf and artificial induction of sexual reversal by oral steroid administration must be conducted before 17 dpf.
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