Polymorphism of Major Histocompatibility Complex Class I alpha cDNA and Resistance against Streptococcosis of Six Strains of Nile Tilapia (Oreochromis niloticus Linnaeus)

Abstract

The partial cDNA encoding major histocompatibility complex class I alpha (MHC class Iα) gene was obtained from a cDNA library of Nile tilapia (Oreochromis niloticus) spleen. 5′ RACE was then conducted to get the full-length of MHC class Iα cDNA. Sequence analysis revealed that this
cDNA consisted of 2,131 bp including a 1,071 bp open reading frame with 5′ untranslated and 3′ untranslated regions (UTR) of 128 bp and 932 bp respectively. The highly polymorphic characteristic of the MHC class Iα gene in six different strains of Nile tilapia (ST1-ST6) was evident. At least 15 different alleles were observed when the specific primers were used to amplify nucleotide sequences of the α1 and α2 domains. Additionally, resistance against streptococcosis of these strains was compared by intraperitoneal injection with viable Streptococcus agalactiae. The results revealed that the ST1 strain showed a significantly higher survival rate than the other five strains (ST2-ST6) with 100±0.0% survival after seven days of injection (P<0.05), while the others had 86.67±5.77, 73.33±5.77, 70.00±0.00, 60.00±0.00 and 43.43±5.77% survival, respectively. Some hematological and immunological parameters were also compared. The findings provide useful information for further investigation to select diseaseresistant
strains of Nile tilapia using MHC class Iα as the molecular marker.