Expression and phylogenetic analysis of the ninth complement component (C9) in rainbow trout

In order to characterise grass carp MHC class I (Ctid-MHC I) sequences, 26 Ctid-MHC I genes were cloned from 12 individuals and their α domain lineages were analysed. Simultaneously, a quantitative reverse transcription-polymerase chain reaction (Q-RT-PCR) assay was developed to detect Ctid-MHC I tissue-specific expression. The results suggested that Ctid-MHC I could be divided into eight lineages (Ctid-NA–Ctid-NH). Based on whether they contained the motif of eight key amino acids (YYRTKWYY), Ctid-MHC I lineages were divided into two groups [Ctid-MHC I (8+) and Ctid-MHC I (8−)]. The expression analysis showed that the Ctid-MHC I locus/loci appeared in the kidney, gill, intestine, heart, spleen, liver, and brain. A GenBank homology BLAST was performed independently with each α domain, and Ctid-MHC I α1, α2, and α3 were categorised into two (V and IX), five (II, IV–VII), and four (IV–VII) domain lineages, respectively. Based on the alphabetic labelling system created in our earlier studies, one α1 (IX), four α2 (IV–VII), and unique α3 (V–VII) domain lineages were observed in grass carp and across the teleostean species.