Characterization of WSSV resistance in selected families of Litopenaeus vannamei

Disease outbreaks caused by the white spot syndrome virus (WSSV) have resulted in high economic losses in the shrimp-culture industry worldwide. After three generations of selective breeding for WSSV resistance, 29 families of Litopenaeus vannamei (G₃) were selected and divided into three groups: relatively high-resistance, moderate-resistance, and susceptible families, which had average survival rates of 22.7±12.30, 9.08±1.46, and 0.78±1.70%, respectively, after WSSV infection. Appearance of the mortality peak in the relatively high-resistance families was postponed by 2–3days compared to the same in susceptible families and unselected shrimps. Using real-time quantitative polymerase chain reaction, the copy number of WSSV DNA in the muscles of surviving shrimps from the relatively high-resistance family 14 was significantly lower than that of the moderate-resistance family 17, the susceptible family 4, and the unselected family of shrimps at 36hourspostinfection (hpi) and thereafter; for example, at 48hpi, the DNA copy number was (23.87±2.56)×10⁶copiesg⁻¹ in the high-resistance families against (40.03±2.54)×10⁶, (65.70±3.44)×10⁶, and (47.87±3.95)×10⁶copiesg⁻¹ body weight, respectively, in the other groups. The WSSV copy number in the muscles of dead shrimps from the relatively high-resistance family 14 was significantly higher than that of the moderate-resistance family 17, the susceptible family 4, and the unselected family of shrimps at 48hpi and thereafter; for example, at 72hpi, the DNA copy number was (54.1±3.9)×10⁶copiesg⁻¹ compared to values of (31.5±2.6)×10⁶, (11.6±2.9)×10⁶, and (25.4±2.7)×10⁶copiesg⁻¹ body weight in the other groups, respectively. The results indicated that shrimps from the relatively high-resistance families could inhibit the replication of WSSV and therefore had apparent WSSV resistance. These results provide a direction for the future selective breeding of shrimps for WSSV resistance.