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Crossbreeding between wild and domesticated strains of important aquaculture species to determine potential for heterosis and heterobeltiosis is of interest to aquaculturists. The present study was aimed to evaluate broodstock strain effects and heterosis for growth and survival rates from larvae to grow out stages of bighead catfish (Clarias macrocephalus) crossed from one domesticated (Can Tho, CT) and two wild (Ca Mau, CM and Hau Giang, HG) populations. Nine crossbreeds were reared for 40 days at the larval stage in static tanks, 60 days for juveniles, and 90 days for grow out in recirculating systems. At harvest, pure CT exhibited the highest growth (final body weight BW = 154.1 ± 61.3 g), while HG × CT and HG × CM crossbreeds had the lowest (57.0 ± 17.0 g and 68.0 ± 28.5 g, respectively) and the other crossbreeds had intermediate values (mean BW ranged from 93.4 g to 113.1 g) (P = 0.01). The three stages showed consistently that the domestic CT strain grew faster than wild CM and HG strains. The growth performance of offspring was affected by dam strains, but not sire strains. Parental crosses generally had better growth rates compared to crossbreeds of the same dam source. However, crossbreeds did not differ in feed conversion ratios and survival rates from parental crosses. None of the crossbreeds had better growth than the better parent (heterobeltiosis), suggesting that crossbreeding among strains in the Mekong Delta may not be an effective genetic improvement program for bighead catfish.

The widespread farming of catfish (genus Clarias ) hybrids between bighead catfish ( C. macrocephalus , Cm ) and the introduced North African Catfish ( C. gariepinus , Cg ) in the Mekong Delta, where flooding is common, has raised concerns of introgression of Cg into the native catfish due to backcrossing by escaped hybrids with wild Cm . This study employed novel PCR-RFLP analyses of a mitochondrial ( Cytochrome c oxidase subunit I , COI) and two nuclear ( Rhodopsin and Tropomyosin ) markers and six microsatellite loci to differentiate hybrid individuals from their parental species, and evaluate whether introgression has occurred in wild and cultured Cm populations in the region. Results of marker screening showed that sizes and frequencies of microsatellite alleles differed between the two parental species. Four loci exhibited fixed species-specific differences in allele frequency (monomorphic in Cg and highly polymorphic in Cm ). Levels of sequence variation varied at the three genes examined, ranging from 3 to 8% (sequence length 652 bp for COI, 802 bp for Rhodopsin and 925 bp for Tropomyosin ). Based on variable sites identified that were not shared between the two species, 3 restriction enzymes ( Spe I, Xcm I and PflMI ) were selected to digest PCR products at species-specific sites within the COI, Rhodopsin and Tropomyosin genes, respectively. Results of PCR-RFLP analyses confirmed that Cm is the maternal lineage of cultured hybrids, while nuclear genes ( Rhodopsin and Tropomyosin ) and microsatellite loci revealed that hybrids possessed admixed multi-locus genotypes relative to the two parental species. Analyses of six microsatellite loci and PCR-RFLP of two nuclear genes in 473 individuals collected from 11 wild and three cultured populations revealed one F1 hybrid in the wild but no evidence of widespread introgression into native bighead catfish. These results differ from findings in Thailand where intensive catfish culture and deliberate species hybridization is practiced.