Original Article Heavy Metal-Induced Differential Gene Expression of Metallothionein in Javanese Medaka, Oryzias javanicus Seonock Woo, Seungshic Yum, Jee Hyun Jung, Won Joon Shim, Chang-Hoon Lee, Taek-Kyun Lee Southern Coastal Environment Research Division, Korea Ocean Research and Development Institute, Geoje 656 -830, Korea Received: 31 March 2006 / Accepted: 28 May 2006 / Published online: 15 September 2006 Abstract A metallothionein (MT) gene was isolated for the first time from Javanese medaka, Oryzias javanicus, which shows high adaptability from freshwater to seawater. The full-length cDNA of MT from O. javanicus (OjaMT) comprises 349 bp, excluding the poly(A) + stretch, and codes for a total of 60 amino acids. The positions of cysteine residues are highly conserved. The pattern of OjaMT expression in- duced by six heavy metals was analyzed via real- time quantitative polymerase chain reaction (PCR). The level of hepatic OjaMT mRNA was increased in a dose-dependent manner by Ag, Cd, Cu, and Zn after 24 h of exposure. However, after Cr and Ni exposure, a significant decrease in OjaMT levels was observed. Cadmium-induced OjaMT expression was detectable in fishes as young as 3 months. After Cd exposure, OjaMT induction was prominent in intestine and liver and moderate in muscle and gill. OjaMT mRNA levels could represent a good bio- marker for monitoring heavy metals in seawater. Keywords: heavy metals — Javanese medaka — metallothionein — OjaMT — Oryzias javanicus — real-time quantitative PCR Introduction The marine environment has been affected by potential hazards, including various environmental and industrial chemicals, heavy metals, and biolog- ical agents, which are discharged as a result of human activities. These exogenous materials im- pose abiotic stresses on aquatic organisms and affect entire biological processes in terms of development, physiology, and even genetics. Much effort has been devoted to assessing the risk of pollutants and to warning about their dangerous effects on human health and natural ecosystems. Even though marine ecosystems are distinctive from freshwater and terrestrial ecosystems, marine toxicity assessment is not an independent issue, as most pollutants detected in marine environments originate from terrestrial environments through rivers or from industrial complexes located in coastal areas. In this sense, any test organism that can be applied to both freshwater and marine environments will help to deduce consecutive and stable data on the harmful effects of pollutants. However, almost all of the test organisms developed for aquatic environ- ments so far (e.g., daphnids, fathead minnow, and rainbow trout for freshwater; mysids, sheepshead minnow, and silversides for seawater; USEPA, 2002) do not satisfy these requirements. Recently, Javanese medaka, Oryzias javanicus, which is distributed in marine environments around the Indonesian Islands and the Malay Peninsula, have been identified as being capable of surviving in freshwater (Inoue and Takei, 2002, 2003). According to these reports, O. javanicus adults showed 100% survival after direct transfer from seawater (SW) to 50% seawater and freshwater (FW), and no apparent difficulties in spawning and egg hatching in 50% SW and FW were observed. O. javanicus shares the same advantages as the labo- ratory animal Japanese medaka, O. latipes, in terms of its small size, ease of culture in the laboratory, frequent spawning of transparent eggs, and relative- ly short generation span. Moreover, genomic infor- mation in terms of gene transfer and gene knockout techniques are available, as well as physiological and histological tools and data, for Japanese medaka Correspondence to: Seungshic Yum; E-mail: syum@kordi.re.kr 654 DOI: 10.1007/s10126-006-6046-0 & Volume 8, 654–662 (2006) & * Springer Science+Business Media, Inc. 2006