Effects of selection for fast growth on survival rate during grow-out phase in giant freshwater prawn (Macrobrachium rosenbergii)

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 (r_g = -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.

Molecular cloning and expression analysis of a prawn (Macrobrachium rosenbergii) juvenile hormone esterase-like carboxylesterase following immune challenge

Methyl farnesoate (MF), the crustacean juvenile hormone (JH), plays critical roles in various physiological processes in crustaceans. The titer of MF is precisely regulated by specific carboxylesterase. Here, we report for the first time that the cloning and expression analysis of a JH esterase-like carboxylesterase from the prawn Macrobrachium rosenbergii (named as MrCXE). MrCXE contained a 1935-bp open reading frame (ORF) conceptually translated into a 644-amino acids protein. MrCXE protein shared the highest identity (36%) with JH esterase-like carboxylesterase from the swimming crab, Portunus trituberculatus and exhibited the typical motifs of JH esterase-like carboxylesterases. MrCXE was most abundantly expressed in hepatopancreas, the major tissue for MF metabolism. MrCXE was expressed at a low level in gut and was not detected in other tissues. Additionally, MrCXE expression was upregulated in hepatopancreas by eyestalk ablation to increase MF level. Furthermore, the mRNA level of MrCXE was significantly increased in the hepatopancreas when challenged by the bacterial pathogens Aeromonas hydrophila and Vibrio parahaemolyticus. To our knowledge, this is the first report that the JH esterase-like carboxylesterase is involved in the innate immune response of the crustaceans.

An 8-Year Breeding Program for Asian Seabass Lates calcarifer: Genetic Evaluation, Experiences, and Challenges

Selective breeding for marine finfish is challenging due to difficulties in reproduction, larval rearing, and on-growth in captive environments. The farming of Asian seabass (Lates calcarifer) has all these problems and our knowledge of the quantitative genetic information (heritability and correlations) of traits necessary for commercial exploitation is poor. The present study was conducted to address this knowledge gap and to provide information that can be applied to sea bass and other aquaculture species. We carried out a comprehensive genetic evaluation for three traits (body weight, total length, and survival) collected from a breeding population for Asian seabass over an eight-year period from 2010 to 2017. Statistical analysis was carried out on 4,567 adult fish at 105, 180, 270, 360, 450, and 570 days post-hatch (dph). The heritabilities (h²) estimated for body weight and length using linear mixed model were moderate to high (0.12 to 0.78 and 0.41 to 0.85, respectively) and they differed between the measurement periods. Survival during grow-out phase was analyzed using threshold logistic and probit models. The heritability estimates for survival rate on the underlying liability scale ( hL2 ) varied from 0.05 to 0.21. When the observed heritability obtained from the linear mixed model was back-transformed to the liability scale, they were similar but not significant. In addition, we examined effects of genotype by environment (G × E) interaction on body traits. The genetic correlation for body weight between tank and sea cage cultures were high (0.91-0.94) in the first and second rearing periods (180 and 270 dph) but the correlation was decreased to 0.59 ± 0.33 at 360 dph. This suggests that the genotype by environment interaction is important for body traits in this population. Furthermore, the genetic correlations of body weights between different measurement periods were moderate but different from one. This suggests that body weights measured at different time points may be different traits and selection for improved early weight may not capture all genetic expressions in subsequent rearing periods in Asian seabass. Selection of the nucleus in sea cages may produce genotypes that do not perform equally well in tanks, although this deserves further studies to determine a suitable selection environment and optimize the breeding program. This paper discusses challenges encountered during implementation of the selection program for L. calcarifer.