Determining the infection history of living organisms is essential for understanding the evolution of infection agents with their host, particularly for key aspects such as immunity. Viruses, which can spread between individuals and often cause disease, have been widely examined. The increasing availability of fish genome sequences has provided specific insights into the diversity and host distribution of retroviruses in fish. The shortspine spurdog (Squalus mitsukurii ) is an important elasmobranch species; this medium-sized dogfish typically lives at depths of 100~500 m. However, the retroviral envelope polyprotein in dogfish has not been examined. Thus, the aim of the present study was to identify and analyze the retroviral envelope polyprotein in various tissues of dogfish. The 1334-base pair full-length novel cDNA of dogfish envelope polyprotein (dEnv) was obtained by 3' and 5'-rapid amplification of cDNA end analysis from S. mitsukurii. The open reading frame showed a complete coding sequence of 815 base pairs with a deduced peptide sequence of 183 amino acids that exhibited 34~50% identity with other fish and bird species. It was also expressed according to reverse transcription and real-time polymerase chain reaction in the kidney, liver, intestine, and lung, but not in the gill. This distribution can be assessed by identifying and analyzing endogenous retroviruses in fish, which consists of three main genes: gag, pol and env. Dogfish envelope polyprotein sequence is likely important in evolution and induces rearrangements, altering the regulatory and coding sequences. This is the first report of the identification and molecular characterization of retroviral envelope polyprotein in various tissues of S. mitsukurii.

Introduction
Materials and Methods
    1. Experimental fish
    2. Experimental fish
    3. RNA extraction
    4. Molecular cloning of dogfish envelope polyprotein
    5. Rapid amplification of cDNA ends analysis
    6. Sequence analysis
    7. RT-PCR expression analysis
    8. Quantitative PCR expression analysis
Results and Discussion
    1. Cloning and sequencing of dogfish envelopepolyprotein
    2. mRNA expression of dogfish envelope polyproteinin various tissues

• Soo Cheol Kim(Department of Biomedical and Electronic Engineering, Chonnam National University, Yeosu 59626, Korea)
• Kanij Rukshana Sumi(Department of Fisheries Science, Chonnam National University, Yeosu 59626, Korea)
• Myeong Rak Choe(Department of Biomedical and Electronic Engineering, Chonnam National University, Yeosu 59626, Korea)
• Kang Hee Kho(Department of Fisheries Science, Chonnam National University, Yeosu 59626, Korea) Corresponding author