Response to multi-trait selection for harvest weight, overall survival, and resistance to white spot syndrome virus (WSSV) in Penaeus (Litopenaeus) vannamei

The family based selective breeding program for P. vannamei initiated in Colombia in 1998 has focused on improving harvest weight (HW), overall pond or tank survival (PS) and, since 1999, resistance to White Spot Syndrome Virus (WSSV). Base population breeders were collected from commercial stocks in eight countries. As of January 2006, three sub-populations (lines) have been selected for four (Line 2 and Line 3) or five (Line 1) generations. The protocol applied for inbreeding control in the program has limited the accumulation of inbreeding: the average rate of inbreeding per generation in the target lines ranges from 0.55 to 1.2%. Each line is derived from a breeding nucleus of approximately 70 full-sib and 35 paternal half-sib families that are reproduced on average every 16 months. Genetic ties between lines were established by reuse of a number of breeders in successive batches of nucleus families to facilitate simultaneous analysis of all data. Offspring were performance tested for growth and general survival in semi-intensive commercial ponds and intensively reared concrete tanks, and for resistance to WSSV in experimental challenge tests in which animals were exposed to the virus either by oral minced tissue or individual oral infections. Multi-trait index selection has been carried out in all lines. The mean relative economic weights in the aggregate selection index across lines were 0.59 for HW, 0.15 for PS and 0.23 for WSSV. These correspond to average selection differentials of 1.0 additive genetic standard deviation for HW, 0.6 for PS and 0.5 for WSSV. The complete data set, including records from approximately 250,000 animals representing 971 full-sib families (440 half-sib families), was analyzed with mixed model methodology. Estimated heritabilities were 0.53 ± 0.04 for HW, 0.07 ± 0.01 for PS and 0.03 ± 0.01 for WSSV. Genetic trend analyses, based on mean BLUP EBVs within generation, demonstrated an average selection response per generation of 4.2% for HW, 5.7% for PS and 1.7% for WSSV. The most consistent response between lines and generations was for HW. In spite of the existence of an unfavorable, negative genetic correlation of intermediate magnitude between fast growth and WSSV tolerance (""check: Gitterle et al., 2005), family based multi-trait selection provided simultaneous genetic gains for all traits. However, the gain in resistance to WSSV was small even after several generations of selection and selection for a single trait, such as HW, would likely have a higher rate of improvement for this trait, but might also lead to a correlated increase in susceptibility to WSSV.