Aquaculture plays an important role in economic development and food security in many countries in the world. World aquaculture production in 2006 was 51.7 million tonnes with an estimated value of US$ 78.8 billion (FAO, 2009). World production will need to increase however by 30-40 million tonnes from its current production level by 2030 to meet growing global demand for fish. In this context, aquaculture in Vietnam has developed rapidly over the past decade and the fisheries sector ranked fourth in terms of export value in 2008 (Vietnamnet, 2008). Total fisheries production in Vietnam in 2007 was 4.149 million tonnes, of which fisheries production from catch and aquaculture were 2.064 and 2.085 million tonnes, respectively. A variety of aquatic species are cultured in Vietnam, but shrimps (mainly Black Tiger shrimp Penaeus monodon, and Pacific white shrimp Litopenaeus vannamei) and ‘tra’ or ‘basa’ catfish are the most common species used in aquaculture. The giant freshwater prawn (GFP), Macrobrachium rosenbergii, is one of the most important crustacean species in inland aquaculture in many countries across the world where this species is either native or exotic. GFP is suitable for culture in a variety of farming systems, including monoculture or polyculture in ponds, pens, and integrated or rotational rice-prawn culture models. The GFP industry worldwide relies totally on wild or unimproved stocks, a practice that threatens the long-term sustainability of GFP farming due to low productivity and vulnerability of farmed stocks to disease. The current status of GFP aquaculture highlights the need for initiation of a systematic stock improvement program for the species to improve economically important traits. Large-scale selective breeding programs have been instigated for some finfish, salmonids and GIFT tilapia for example, and some selective breeding trials have been conducted on crustacean species, namely marine penaeid shrimp and freshwater crayfish. Examples of selective breeding programs on aquatic species have demonstrated that significant genetic gains can be achieved for growth rates with gains of around 10-20% per generation. While a selective breeding program is an option for GFP stock improvement, an alternative approach to improving GFP productivity, potentially with more immediate effect and one that is less expensive, is crossbreeding which may produce heterosis or hybrid vigour in crossbred offspring. Therefore, a crossbreeding strategy was trialed in the current study as a starting point for a stock improvement program for the GFP industry in Vietnam. The current study assessed the growth performance of three GFP strains (two wild Vietnamese strains from the Dong Nai and Mekong rivers, and a single domesticated Hawaiian strain) and their reciprocal crosses in a complete 3x3 diallel cross, i.e. three purebred and six crossbred strains. The diallel cross was carried out over two consecutive generations (G1 and G2). Juveniles for the experiments were produced using single-pair matings. Juveniles from each strain combination were stocked into three replicate hapas for 15 weeks. Growth data (body weight, carapace length, standard length) from the G1 and G2 were pooled for all subsequent analyses as there was no effect of generation on growth traits. Results showed that the Hawaiian strain performed best among purebred strains, and crosses with the Dong Nai or Mekong strains as dams and the Hawaiian strain as sires grew significantly faster than did the purebred Dong Nai or Mekong strains. These results suggest potential for heterosis among some crosses. Growth data were analyzed in depth by partitioning the strain combination (cross) effect into three components: strain additive genetic effects, heterotic effects, and strain reciprocal effects. Strain additive genetic and reciprocal effects were significant sources of variation for all growth traits measured. Strain additive genetic effects were highest for the Hawaiian strain and lowest for the Mekong strain for all growth traits. Reciprocal effects influenced negatively on growth rate of crosses with the Hawaiian (H) strain as dams and the Dong Nai (D) or Mekong (M) as sires compared with their reciprocal crosses (DH and MH). Heterotic effects for all growth traits were small and not significantly different from zero (P > 0.05). These results indicate that a crossbreeding approach based on the strains evaluated here provides only limited potential for improving growth rates based simply on heterotic outcomes and that a likely more productive option would be to trial artificial selection on a diverse synthetic stock. The current study also employed genetic markers (microsatellites) to characterize levels and patterns of genetic diversity in three purebred strains of GFP that originated from the diallel cross above. All three purebred strains showed relative high levels of genetic diversity in terms of allele number and individual heterozygosity across the six marker loci screened. Levels of genetic diversity present in the three purebred strains combined into a single stock were compared with that from a combination of three wild river stocks to assess the impact of domestication on genetic diversity of a ‘synthetic’ population. Results demonstrated that there was no significant loss of genetic diversity in the three purebred strains combined compared with a reference set containing the three wild populations. Therefore, a synthetic population formed from these purebred strains successfully captured the majority of genetic variation present in the wild broodstock. This synthetic population provides a potential stock for a future selective breeding program for GFP in Vietnam. The current study was also the first attempt to identify single nucleotide polymorphisms (SNPs) in key growth genes in GFP. Two key candidate genes were targeted, actin and crustacean hyperglycemic hormone (CHH), that are potentially linked to growth performance in GFP. The study screened SNPs in GFP females only, because growth performance of GFP males is influenced strongly by social rank. The study identified four SNPs in intron 3 of the CHH gene that were significantly correlated with individual body weight at harvest, while no SNPs detected in the actin gene were associated with growth traits in GFP. This finding however, needs to be confirmed using larger sample sizes and other GFP lines. The current study has produced important basic knowledge relevant to implementation of an effective stock improvement program for GFP in Vietnam. Results indicate that a selective breeding strategy rather than a crossbreeding approach is likely to be the best strategy for improving GFP culture stocks in Vietnam. In addition, the study demonstrates that application of modern molecular genetic technologies can be efficient in developing a genetically diverse, synthetic population for stock improvement and for identifying potential markers correlated with important commercial traits in GFP. Integration of DNA techniques with traditional breeding practices can facilitate GFP stock improvement in Vietnam and accelerate the industry development when improved lines are available. Some limitations of the current study and recommendations for further work are discussed.
| Identifer | oai:union.ndltd.org:ADTP/279309 |
| Creators | Thanh Nguyen |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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