marker-assisted selection in wheat - ictsd

Chapter 16 – Possibilities for marker-assisted selection in aquaculture breeding schemes 311IntroductionAquaculture is an expanding industry,with a total global value of US$61 billion(FAO, 2003). FAO estimates that thereare around 200 species in culture, ofwhich carps and oysters have the largestworldwide production. However, only afew species have ongoing selective breedingprogrammes.MAS is not used in any aquaculturebreeding scheme today. The aim of thischapter, therefore, is to review briefly thecurrent status of aquaculture breedingschemes and to evaluate the possibilities forMAS of aquaculture species.Traits of breeding interestGrowth rate is the most important trait formost aquaculture species under selection. Itis recorded on the selection candidates, andcan easily be improved using mass selection.Sexual maturation is a trait that leadsto reduced growth, reduced feed conversionefficiency and reduced fillet qualityin several aquaculture species. Therefore,selection against early maturation is oftenperformed, i.e. the individuals that becomesexually mature before market size are discardedas selection candidates. In tilapia,late maturity is desirable because of excessivespawning that results in overcrowdingof ponds and reduced size of the fish.For many other traits, however, accuratemeasurement techniques for live individualsare inadequate. Hence, selectionmust be based on information from otherfamily members, e.g. on siblings. MASwould be especially valuable for traits thatare difficult and/or costly to measure onthe selection candidate or for traits thatare measured late in life or at slaughter(Lande and Thompson, 1990; Poompuangand Hallerman, 1997). Examples of theseimportant traits are:• Disease resistance. Challenge tests existfor viral (e.g. white spot syndrome inshrimps and infectious pancreatic necrosisin most marine fishes) and bacterial(e.g. furunculosis and vibriosis) diseases,as well as for parasites (e.g. sea lice).When challenge tests are used in breedingprogrammes, however, survivingindividuals cannot enter the breedingnucleus because of the risk that theywill introduce the disease to the nucleus.Therefore, these individuals cannot beselection candidates and only their sibs,who have no records for these traits, arecandidates.• Fillet quality traits. To this group of traitsbelong colour, texture, gaping, differentfat-related traits (e.g. fat percentage anddistribution) and dressing percentage.Accurate measurements of these traits areavailable only for slaughtered individuals.Techniques for measuring fillet colour onlive fish are under development.• Feed conversion efficiency is a trait thatcan be measured practically only at thefamily level at a young age in the breedingnucleus, but not at the individual levelor in grow-out operations. The valueof such early records is rather limitedbecause of the unknown correlation withfeed conversion efficiency at a later age.Feed intake is, in general, a difficult traitto measure in aquaculture species due tounequal feed intake over days. No activeselection programme for aquaculturespecies selects directly for feed conversionefficiency; rather, indirect selectionis practised by selecting for growth.• Salinity and low temperature toleranceare two traits of interest for tilapia breedingprogrammes. Today, tilapias are producedin freshwater in tropical and subtropicalareas. The purpose of selectingfor salinity and temperature tolerance