Breeding for disease resistance of Penaeid shrimps

Diseases are a major constraint on the intensive production of shrimps. Conditions in production ponds favour disease development, and epidemics of several previously unreported diseases have occurred and caused severe losses. When elimination, eradication or cultural control is difficult, selective breeding for host resistance to the pathogen may be an attractive option for disease control. However, host resistance is not a panacea and should only be considered when (a) the disease causes severe damage (b) there are no other existing simple cost effective control measures and (c) there is demonstrable genetic variation in resistance and this is not coupled with an excessive level of negative associations with other desirable characteristics. Shrimp have only recently been domesticated and breeding for resistance only began in the mid 1990s; there is limited experience with shrimp breeding in particular and crustaceans in general. Consequently, the principles and concepts behind breeding programmes are based largely on experiences with other species in both the plant and animal kingdoms. Commercial growers now seed ponds with shrimp populations selected for resistance to Taura Syndrome Virus with excellent results, whilst up to now development of White Spot Syndrome Virus resistant populations has been an elusive goal. The original TSV resistant populations were developed using simple mass selection techniques (Colombia). In later generations family based selection has been applied on populations, which initially had survival rates of about 30%, with care taken to reduce inbreeding and loss of genetic variation. This suggests that when the original populations have a reasonable level of resistance, and straightforward, effective selection protocols exist, it is relatively simple to breed for resistance. With catastrophic diseases, such as WSSV, which cause mortalities of 98% or more the frequency of resistance is low and it is suggested that for theoretical reasons single gene, rather than polygenic, resistance is likely to develop. The low frequency of resistance genes in breeding populations may cause genetic bottlenecks which will greatly reduce the genetic variation in the populations. In order to maintain the genetic variation the genes from the small numbers of survivors should be introgressed into populations with broader genetic variability. Furthermore, in order to minimize the probability of breakdown of resistance pyramiding of resistant genes on different loci would be advantageous. Genetic variation in resistance may be encountered either in the initial base populations or may spontaneously arise due to mutations or new recombinants. With extremely prolific species such as shrimps, millions of animals can readily be screened for survival and hence resistant mutants or recombinants may be identified. Once genetic variation has been detected the most appropriate breeding methodology will depend on the nature of both the resistance and the disease or diseases that are of interest to the producers.