Metallothioneins in Antarctic fish: Evidence for independent duplication and gene conversion

In the present paper, we examine eight species of Antarctic fish belonging to the suborder Notothenioidei, using reverse-transcriptase polymerase chai n reaction, to investigate the presence of mRNAs encoding metallothionein ( MT) isoforms. A total of 168 bp from the coding region and the complete (13 3-165 bp) 3' untranslated region (UTR) was obtained for all species (for th ree of them, we also sequenced the full-length cDNA, including the 5' UTR). Phylogenetic analyses carried out on the MT-coding region suggest monophyl y for Antarctic fish MTs with respect to other teleost MT genes. Analyses a lso revealed that notothenioid MTs can be divided into at least two groups of paralogy, MT-1 and MT-2. These results indicate that notothenioid MT iso forms arose from at least one gene duplication event occurring in the ances tral lineage of the Notothenioidei. This duplication occurred independent o f the one which gave origin to two metallothionein isoforms in the rainbow trout. In addition, an instance of gene conversion was observed between MT- 1 and MT-2 genes in Norothenia coriiceps. Analyses of the 5' UTR, combined with quantitative assay of differential expression of MT-I and MT-2, indica te that only the 3' UTR underwent a gene conversion event in the mentioned species. These findings, together with the observation of a differential pa ttern of expression for the two MT isoforms, disclose an unexpected complex ity in the evolution and function of notothenioid MTs; as in most teleost s pecies examined (apart from the rainbow trout), a single MT form is present .