北海道の汽水湖に生息するヌマガレイとカレイ類雑種における皮膚偽腫瘍の発生状況

北海道オホーツク海沿岸のクッチャロ湖からヌマガレイとオショロガレイ(ヌマガレイとスナガレイの雑種)を採集し,体表の皮膚偽腫瘍の発生状況を調べた。本病変はヌマガレイとオショロガレイともに見られ,カレイ類雑種からは初記録である。ヌマガレイでは,体長10cm 未満の小型魚にのみ病変が認められ,魚体長の増加とともに病変の発生率は減少した。この湖にはカレイ類の魚類捕食者が見られず,また病変の退行や病魚の湖外への移動が考えられないことから,病魚は死亡すると推定した。Starry flounder (Platichthys stellatus) and Oshoro flounder, hybrids of starry flounder and stone flounder (Kareius bicolaratus), collected in the brackish-water Lake Kuccharo, Hokkaido, were examined for skin pseudotumors. The disease was found in Oshoro flounder, which represents the first record of skin pseudotumors from pleuronectid hybrids. In starry flounder, only small fish (less than 10 cm in body length) had skin pseudotumors, and prevalence of the disease sharply decreased with fish size. No predatory fish was caught in the lake. It thus seems likely that small starry flounder die due to skin pseudotumors in the lake

Identification and Characterization of Small Molecule Compounds That Modulate Trichothecene Production by <i>Fusarium graminearum</i>

From the RIKEN Natural Products Depository (NPDepo) chemical library, we identified small molecules that alter trichothecene 15-acetyldeoxynivalenol (15-ADON) production by <i>Fusarium graminearum</i>. Among trichothecene production activators, a furanocoumarin NPD12671 showed the strongest stimulatory activity on 15-ADON production by the fungus cultured in a 24-well plate. NPD12671 significantly increased the transcription of <i>Tri6</i>, a transcription factor gene necessary for trichothecene biosynthesis, in both trichothecene-inducing and noninducing culture conditions. Dihydroartemisinin (DHA) was identified as the most effective inhibitor of trichothecene production in 24-well plate culture; DHA inhibited trichothecene production (>50% inhibition at 1 μM) without affecting fungal mass by suppressing <i>Tri6</i> expression. To determine the effect of DHA on trichothecene pathway <i>Tri</i> gene expression, we generated a constitutively <i>Tri6</i>-overexpressing strain that produced 15-ADON in YG_60 medium in Erlenmeyer flasks, conditions under which no trichothecenes are produced by the wild-type. While 5 μM DHA failed to inhibit trichothecene biosynthesis by the overexpressor in trichothecene-inducing YS_60 culture, trichothecene production was suppressed in the YG_60 culture. Regardless of a high <i>Tri6</i> transcript level in the constitutive overexpressor, the YG_60 culture showed reduced accumulation of <i>Tri5</i> and <i>Tri4</i> mRNA upon treatment with 5 μM DHA. Deletion mutants of <i>FgOs2</i> were also generated and examined; both NPD12671 and DHA modulated trichothecene production as they did in the wild-type strain. These results are discussed in light of the mode of actions of these chemicals on trichothecene biosynthesis