西北太平洋兩棲洄游蝦虎魚日本禿頭鯊的族群遺傳結構之研究

Abstract

日本禿頭鯊是兩棲洄游性魚類。其成魚雖然在淡水中產卵，但其仔魚卻有很長的海洋浮游期 (PLD, 125-186 days)，而長海洋浮游期可能會抑制族群的遺傳分化。為了驗證此假說，本實驗於2007至2009年在台灣東部的福隆、南澳以及花蓮採集剛溯河之日本禿頭鯊後期仔魚120尾並分析其粒腺體DNA控制區(465bp)的遺傳變異。此外，基因庫中來自日本三個不同地點的75尾日本禿頭鯊的相同基因片斷亦用於分析，以期能了解整個西北太平洋日本禿頭鯊的族群遺傳結構。聚類分析樹(neighbor-joining tree)及分子變異分析(AMOVA, ΦST = 0.00401, P>0.05) 結果顯示，此八個樣本間並沒有顯著的遺傳分化。但成對ΦST 的分析確顯示花蓮秋季樣本和其他五個樣本間有明顯遺傳差異，顯示台灣秋季的樣本在東北季風的吹拂下可能限制了魚苗向北的輸送。另一方面，其他樣本間缺乏遺傳分化可能和海流漂送仔稚魚造成族群間基因交流有關。日本禿頭鯊的平均仔稚魚浮游期在日本 (208 + 22 days) 要比台灣 (163.72 + 12.79 days) 多45天，這段時間剛好足以讓日本禿頭鯊的仔魚從台灣經由黑潮輸送到日本。因此，即使日本禿頭鯊的成魚是分布在相互隔離的淡水系統中，仍然可以經由仔稚魚長距離的散布來增加族群間基因的流動，進而抑制了族群遺傳的分化。因此，長海洋仔稚浮游期在日本禿頭鯊族群的散布距離及族群結構上扮演非常重要的角色。

The goby Sicyopterus japonicus is an amphidromous species that spawns in freshwater and has a long marine pelagic larval duration which may diminish population genetic differentiation. To prove this hypothesis, a total of 465 base pairs in the control region of the mitochondrial DNA in S. japonicus were analyzed from 119 specimens collected from 3 different river systems in Fulong, Nan-ao and Hualien in eastern Taiwan during 2007 to 2009. Additional 74 mtDNA sequences from Japan were also included in the analysis to have a general view of its population genetic structure in the northwestern Pacific. Neighbor-joining tree and AMOVA analyses indicated that the eight populations has no significant differentiation (ΦST = 0.00401, P>0.05). The number of migrants per generation (Nm), on the other hand, ranges from 9.25787 (between HW-AUT and KOC) up to infinity. Infinite value of Nm indicates that an extremely high gene flow has occurred between the Taiwan and Japan populations particularly during spring. Significant genetic differences however were observed for specimens collected in Taiwan during autumn and winter seasons in the Pairwise ΦST test, indicating that unfavorable environmental condition (e.g. cold water temperature) might have restricted dispersal of the goby larvae during these seasons and affected the survival and the recruitment of the goby larvae in Japan. On the other hand, the absence of population genetic differentiation found in spring is probably due to gene flow that occurs during the dispersal and transport of the larvae from the south to the north. Meanwhile, the difference in mean pelagic larval duration (PLD) between Japan (208 + 22 days) and Taiwan (163.72 + 12.79 days) is approximately 45 days, which allows the larvae of this species to disperse from Taiwan to Japan by the Kuroshio Current. Although the adults of this species are distributed among isolated freshwater systems, gene flow and larval dispersal can take place to restrict population genetic differentiation. Accordingly, it is no doubt that long marine PLD and oceanic current seem to play an important role in determining the larval dispersal and subsequently, the population structuring of this goby species.