Selection for improved white spot syndrome virus resistance increased larval survival and growth rate of Pacific Whiteleg shrimp, Liptopenaeus vannamei.

Outbreaks of contagious diseases, including White spot syndrome virus (WSSV), occur more frequently due to environment changes and as commercial shrimp production becomes intensified. The over-arching aim of this study was to examine new traits to improve disease resistance of Whiteleg shrimp, Liptopenaeus vannamei, to WSSV. Specifically, we made a compressive evaluation of the breeding population to determine a suitable selection criterion for improved WSSV resistance. To achieve this objective, we analysed five traits (viral titre, WSSV resistance, larval survival, body weight and standard length) recorded for 120,000 individual shrimps that were offspring of 228 sires and 300 dams produced over two generations of selection in 2017 and 2018. Our restricted maximum likelihood mixed model analysis showed that there is additive genetic variation in viral copy number (or viral titre, viral load) with the heritability that equals 0.18 ± 0.02. Viral titre displayed a moderate and negative genetic correlation with WSSV resistance (rg = -0.55). These results suggest that viral titre can be used as a selection criterion to improve WSSV resistance, but selection for decreased viral titre (i.e., increased resistance) may not capture all genetic expression in WSSV resistance. In addition to the estimation of population genetic parameters, we evaluated direct response to selection for increased WSSV resistance, which was measured as differences in estimated breeding values between the high and low resistant lines. The direct genetic gain achieved for WSSV resistance averaged 12.9% after one generation of selection in this Whiteleg shrimp population. The selection program also resulted in positive impacts on growth and larval survival by 7% and 17%, respectively. There is abundant genetic variation in WSSV resistance (h2 = 0.19-0.27), suggesting that the tested Whiteleg shrimp population will continue to respond to future selection. Collectively, the results obtained in our study provide important information to assist the design and implementation of genetic improvement programs for disease traits in aquaculture species, including L. vannamei.