Microsatellites revealed no genetic differentiation between hatchery and contemporary wild populations of striped catfish, Pangasianodon hypophthalmus (Sauvage 1878) in Vietnam
Introduction
Domestication selection whereby genetic changes occur in hatcheries due to natural selection on fitness and reproductive traits under a human-controlled environment (Doyle, 1983), is an unavoidable process that occurs in hatcheries as a result of captive breeding of aquatic animals for generations. It entails genetic changes caused by either selection, reduction of effective population size (number of broodstocks contributing to a succeeding generation, Ne), inbreeding or combinations of these events (Doyle, 1983). Despite limited documentation, most domesticated aquatic animals have showed changes of various traits towards adaptation to captive conditions (Gjedrem, 2005), for example, improved reproductive success of Nile tilapia, Oreochromis niloticus (Osure and Phelps, 2006). Therefore, domestication is essential for good aquaculture stocks. However, deterioration of traits is also observed, e.g. reduction of survival rate and increasing of abnormality rate of Heterobranchus longifilis after four generations of domestication (Agnèsè et al., 1995), and reduction of growth of Nile tilapia after 50 years of domestication (Brummett et al., 2004), if no rigorous broodstock management is applied.
Loss of genetic variation of hatchery stocks is a common phenomenon which has been reported in many species [e.g., turbot, Scophthalmus maximus (Coughlan et al., 1998); common carp, Cyprinus carpio (Kohlmann et al., 2005); Japanese flounder, Paralichthys olivaceus (Sekino et al., 2002); Atlantic salmon, Salmo salar (Skaala et al., 2004); Kuruma prawn, Marsupenaeus japonicus (Luan et al., 2006)]. This is mainly due to a small founder population and ultimately small effective population size (Ne) (Falconer and Mackay, 1996). As such, it is essential that a proper broodstock management plan is implemented in order to ensure successful domestication.
Striped catfish (Pangasianodon hypophthalmus), also sometimes referred to as sutchi catfish, is a strict freshwater migratory species of the family Pangasiidae and native to the Chao Phraya and Mekong river basins (Rainboth, 1996, Robert and Vidthayanon, 1991). The species is also found in the Ayeyawady basin of Myanmar. Aquaculture of this species has been developed in Thailand for more than 30 years and domesticated stock(s) originated from wild stocks in Chao Phraya River have been established. Aquaculture of the striped catfish in Vietnam has reached commercial scale due to the success of artificial propagation of the Vietnamese stocks of striped catfish in 1996's (Trong et al., 2002, Cacot et al., 2002). The striped catfish aquaculture business has become one of the fastest growing primary food production sectors in the world. Approximately 1.2 million tonnes of this species were produced in 2007 and exported to over 100 countries in the world.
Although currently the supply of striped catfish seed in Vietnam is totally dependent on the operations of hatcheries (270 million Pangasiid fry and fingerlings produced annually) (Van Zalinge et al., 2002, Thang, 2006), issues of genetic management of broodstock have not been addressed (Ha et al., 2008). This poses potential risks associate with losing genetic variation and potential inbreeding, which may hamper the domestication process (Schonhuth et al., 2003). Therefore, the information on genetic diversity of these hatchery and wild stocks is urgently required in order to sustain the quality of broodstock.
Hitherto there are a limited number of studies that addresses the questions relating to genetic variability of the striped catfish. So et al. (2006a) revealed no significant population genetic structure based on mitochondrial DNA-RFLP markers. The follow up study using the hyper-variable marker, microsatellite DNA, revealed three genetically sympatric populations with high level of genetic diversity (So et al., 2006b).
The present study aimed at investigating levels of genetic variation of the stripped catfish of the current wild stocks as well as of selected hatchery stocks in Vietnam using microsatellite DNA markers. Implications for management of broodstock will also be discussed.
Section snippets
Sample collection
Fin clip samples of 391 individuals of P. hypophthalmus were collected from brooder populations reared in four hatcheries located in An Giang and Dong Thap Provinces, two wild populations spawned in 2005 in the Mekong and Bassac Rivers, and one wild population (spawned in 2006) in the Bassac River in Vietnam. Sampling was undertaken between June and August, 2006. Details of sampling localities, sample codes and sample sizes are presented in Fig. 1 and Table 1.
Due to problems associated with
Deviation from Hardy–Weinberg equilibrium (HWE) and linkage disequilibrium
Fisher's exact tests for departures of HWE revealed deviation from HWE towards homozygote excess for all seven populations. As such, the data set was checked for the presence of null alleles which existed in all populations. Thus, the genotype frequencies were adjusted according to the suggestion made by MICROCHECKER. After the adjustment, five populations did not conform to HWE (P < 0.05). However, after sequential Bonferroni correction only PhH4 significantly deviated from HWE, despite a
Genetic diversity within striped catfish populations and an implication on Ne
Genetic variation of striped catfish in Vietnam was characterized by low allele diversity compared to the averages for freshwater fish (A = 9.1 ± 6.1 averaged across 13 species, De Woody and Avise, 2000), marine fish (A = 19.9 ± 6.6 averaged across 12 species, De Woody and Avise, 2000) and for striped catfish populations in Cambodian Mekong River (Ar = 7.9–10.4; sample size = 47.0–59.9; So et al., 2006a). However, the observed and expected heterozygosity was moderate; slightly higher than the average for
Conclusion
In conclusion, the genetic variation within populations of both hatchery and wild striped catfish in Vietnam is moderate as revealed by heterozygosity based on microsatellite markers, which thus implied relatively large population size. However, the allelic diversity was relatively low without evidence for genetic bottleneck. No genetic differences were found between hatchery and the wild stocks. As such, the domestication process is thought to be at a very early stage. The hatchery stocks of
Acknowledgements
Financial support was mainly from Thailand Research Fund awarded to Uthairat Na-Nakorn under the Senior Research Scholar Program 2007 through the project RTA 5080013. The technical assistance of Ms. Srijanya Sukmanomon and Ms. Phinyada Sompuech was greatly appreciated. The authors would like to thank the Vietnam Ministry of Education, Dr. Nguyen Anh Tuan, Rector of Can Tho University (CTU), a leader of Higher Education Project at Cantho University for the granting of a Ph.D. scholarship to Hung
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