| Albinism occurred frequently in hatchery-reared turbot with extremely high proportion in China. The high albinism rate has become a serious problem for the aquaculture of flatfish, decreasing the market value and increasing the risk of predation for juveniles released to enhance wild stocks. To find the mechanism of albinism in turbot we presently research the tyrosinase activities in different skins of turbot, obverse the microsturcture of melanophores and clone the differently expressed genefragments in normal and albino skins.To elucidate the mechanism of albinism, we compared the biochemicaland histochemical activity of tyrosinase in the skins of normal and albino turbot using substrate L-DOPA. It was found that: (1) tyrosinase activity existed in all the skin extracts tested including pigmented and non-pigmented skins from ocular or blind side of normal and albino turbot, and the tyrosinase activity of the ocular skin extracts was significantly higher than that of the blind skin extracts within one individual for both normal and albino turbot. Unexpectedly, the tyrosinase activity in the extracts of albino skin of ocular side of albino turbot (termed AOA skin) was about 56% higher than that of pigmented skin of ocular side of normal turbot (termed PON skin); (2) temperature and salinity have similar effects on the tyrosinase activity between ocular pigmented and albino skin extracts, and the optimal temperature was 55~60 and optimal salinity 26%; however, different pH values had different effects on the tyrosinase activity between them, and the optimal pH value of ocular pigmented skin extracts was 7.0 and ocular albino skin extracts 8.0; all the three activators tested (SDS, trypsin and zymosan) can increase the activity of tyrosinase in both ocular pigmented and albino skin extracts of turbot. But the level ofactivation on the ocular albino skin extracts of albino turbot was significantly higher than that on ocular pigmented skin extracts of normal turbot.Histochemical staining showed that tyrosinase activity was present only in the PON skin but not in the AOA skin, and the white skin of blind side of albino (termed WBA skin) and normal turbot (termed WBN skin). A large amount of positive black granules was formed in the epidermal cells of PON skin but no black granules were formed in the skins of AOA, WBA and WBN.The ultrastructure of the albino and pigment skins in turbot was performed using the Transmission Electron Microscope (TEM). The pigment cells in the PON skin of adult turbot consist of melanophores and iridophores. The melanophores in the pigment skins of normal turbot distribute closely and include many melanin granules. The content and structure of melanophores in the albino turbot was dramatically different from those in pigment turbot. The results show that the melanophores degenerated in albino skin on ocular side of albino turbot.To investigate the mechanism of albinism in turbot on the molecular level, relevant subset of differentially expressed gene of interest were identified and cloned by the Selective Subtraction Hybridization. Selecting 16 cDNA clone fragments from cDNA in AOA skin, one clone shares 25% and 26.5% identities with the tyrosinase gene of Medaka fish and Fugu fish; and a group of clones sharing 100% homologous with the genes coding by the mitochondrion genome, such as cytochrome c oxidase II gene, NADH dehydrogenase 4 gene, tRNA-Val gene, ATP synthase FO subunit6 gene.Selecting 19 cDNA clone fragments from cDNA in PON skin, 16 clones similar to the differential expressed gene fragment of malignant melanoma, which include two homologous conservative sequences about 20bps. |
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