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The aim of this study was to estimate heterosis and heritability for harvest body weight of the Pacific white shrimp (Litopenaeus vannamei) measured at commercial farm conditions. Heterosis and heritability were estimated using a base population from diallel crosses of eight introduced strains. The base population included 9936 shrimp from 207 families that were produced with 188 sires and 172 dams using a nested mating design by artificial insemination. Heterosis was calculated basing on the least squares means (LSM) of harvest body weight. The results showed that most of the hybrids (75%) have positive heterosis for harvest body weight, which ranged from À13.36% (UA2 9 UA5) to 13.80% (UA6 9 UA5) with a mean of 2.41%. The high amount of heterosis manifested in the hybrids indicated the usefulness of these hybrids for improving the growth. Variance components and heritability for harvest body weight were estimated using an animal model. The heritability estimate for harvest body weight was 0.092 AE 0.082 (h 2 ) when genetic groups were excluded from the pedigree, but it was decreased when genetic groups were included in the pedigree (h 2 group = 0.066 AE 0.050), implying that there are strain additive genetic effect and heterosis in the base population. However, the heritability estimates for harvest body weight were significantly different from zero (P < 0.05) and there was no significant difference between h 2 and h 2 group (P > 0.05). The results from this study indicated that significant improvement for growth is possible through crossbreeding and selective breeding in L. vannamei.

Regarding the practical farming of Litopenaeus vannamei, the deterioration of water quality from intensive culture systems and environmental pollution is a common but troublesome problem in the cultivation of this species. The toxicities that result from deteriorating water quality, such as that from ammonia stress, have lethal effects on juvenile shrimp and can increase their susceptibility to pathogens. The toxicity of ammonia plays an important role in the frequently high mortality during the early stage on shrimp farms. However, little information is available regarding the genetic parameters of the ammonia tolerance of juveniles in the early stage, but this information is necessary to understand the potential for the genetic improvement of this trait. Considering the euryhalinity of L. vannamei and the fact that low salinity can increase the toxicity of ammonia stress, we estimated the heritability of ammonia tolerance in juveniles in 30‰ (normal) and 5‰ (low) salinity in this study using the survival time (ST) at individual level and the survival status at the half-lethal time (SS50) at the family level. In the normal and low salinity conditions and for the merged data, the heritability estimates of the ST (0.784±0.070, 0.575±0.068, and 0.517±0.058, respectively) and SS50 (0.402±0.061, 0.216±0.050, and 0.264±0.050, respectively) were all significantly greater than zero, which indicates that the ammonia-tolerance of shrimp can be greatly improved. So it might provide an alternative method to reduce mortality, help to enhance resistance to pathogens and reduce the occurrence of infectious diseases. The significant positive genetic correlation between ST and body length suggested that ammonia is more toxic to shrimp in the early stage. The medium-strength genetic correlations of the ST and SS50 between the two environments (0.394±0.097 and 0.377±0.098, respectively) indicate a strong genotype-by-environment (G×E) interaction for ammonia tolerance between the different salinity levels. Therefore, salinity-specific breeding programs for ammonia tolerance in shrimp should be purposefully implemented.

sets of genetic parameters respectively. The results would provide crucial information in pacific white shrimp breeding programs in China.

A multi-trait selective breeding program of Macrobrachium rosenbergii was initiated in China in 2015. In this program, the M . rosenbergii resources were widely collected from four countries, the origin of the founders was verified with 16 microsatellites and the pedigree was reconstructed, and the optimum contribution selection was used to make the mating design. In this study, we evaluated the genetic parameters and selection response for the harvest body weight (HBW) of M . rosenbergii after being communally reared for 95–109 days. The data were collected from two generations that comprised 25,212 progenies from 150 sires and 198 dams. The residual maximum-likelihood methodology was employed to evaluate the variance components, by fitting an animal model. The accuracy of estimated breeding values increased by 0.38% after pedigree reconstruction using microsatellite markers. The estimated heritability ( h 2 ) for HBW was moderate (0.212 ± 0.049) and the common environmental coefficient ( c 2 ) was low (0.063 ± 0.017) when all the data were used for the analysis. Within generations, h 2 was moderate to high (0.198 ± 0.080 to 0.338 ± 0.049). c 2 could only be estimated in G 1 , which was 0.055 ± 0.030. The average HBW of males was significantly larger than that of females ( P < 0.01). h 2 estimated for female HBWs were higher than that for males within generations, while h 2 estimated for female HBWs were lower than that for males across generations. But they were not significantly different ( P > 0.05). The genetic correlations between sexes were moderate to high within each generation (0.529 to 0.763). Two methods were used to estimate the realized response. One method was calculated from the differences between the least squares means of the selected population HBW and that of control population HBW, which was 14.01%. The other method was calculated from the differences between the EBVs of the selected population HBW and that of control population HBW, which was 11.52%. The predicted responses derived from two sets of genetic parameters acquired from within- and across- generation datasets were 11.68% and 10.67%, respectively. The present study provides valuable information for breeding programs of M . rosenbergii .

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A bstractAmmonia, toxic to aquaculture organisms, represents a potential problem in aquaculture systems, and the situation is exacerbated in closed and intensive shrimp farming operations, expecially for Litopenaeus vannamei . Assessing the potential for the genetic improvement of resistance to ammonia in L . vannamei requires knowledge of the genetic parameters of this trait. The heritability of resistance to ammonia was estimated using two descriptors in the present study: the survival time (ST) and the survival status at half lethal time (SS 50 ) for each individual under high ammonia challenge. The heritability of ST and SS 50 were low (0.154 4±0.044 6 and 0.147 5±0.040 0, respectively), but they were both signifi cantly diff erent from zero ( P <0.01). Moreover, these two estimates were basically the same and showed no signifi cant diff erences from each other ( P >0.05), suggesting that ST and SS 50 could be used as suitable indicators for resistance to ammonia. There were also positive phenotypic and genetic correlation between resistance to ammonia and body weight, which means that resistance to ammonia can be enhanced by the improvement of husbandry practices that increase the body weight. The results from the present study suggest that the selection for higher body weight does not have any negative consequences for resistance to ammonia. In addition to quantitative genetics, tools from molecular genetics can be applied to selective breeding programs to improve the effi ciency of selection for traits with low heritability.