台灣鱸鰻族群遺傳結構之長期變化

Abstract

鱸鰻 (Anguilla marmorata) 為廣泛分布在印度─太平洋區的熱帶性淡水鰻。綜合前人研究，目前認為鱸鰻至少可分為四個遺傳種群：西北太平洋群、南太平洋群、印度洋群及關島地區群。台灣的鱸鰻則屬西北太平洋群，其產卵場在馬里亞納群島附近 (12–17˚N, 131–143˚E)，和日本鰻 (A. japonica) 的產卵場 (12–17˚N, 137–143˚E) 有部分重疊。目前已知日本鰻在時間和空間上均無遺傳分化的現象，西北太平洋鱸鰻所處為同一洋流系統，其中是否有時間上的分化則需進一步證實。本研究以5組微衛星多型性DNA分析從1994-2012年間台灣地區之鱸鰻標本，輔以體長測量，欲了解台灣鱸鰻族群結構之長期演變過程，以及其族群結構與生活史的關聯性。實驗結果發現，玻璃鰻的平均體長有色素階段、年間及月間之差異性。等位基因豐富度 (Allele richness from 6.95 to 7.73) 以及異型合子觀測值 (observed heterozygosity from 0.43 to 0.92) 皆顯示鱸鰻族群結構依舊維持高度基因多樣性，且近親交配程度低 (FIS= -0.006)。以時間尺度來看，各年間並無顯著分化 (FST= 0.014)。評估19年間之有效族群大小，顯示台灣地區鱸鰻的遺傳組成呈現高度的異質性 (Average Ne= 540.18)。台灣與爪哇地區之樣本比較則有空間上的遺傳分化現象 (FIS= 0.108)。台灣地區之鱸鰻過去可能曾發生過瓶頸效應 (Mean M-ratio= 0.46)，但就過去19年而言，其族群之長期結構相當穩定。

Anguilla marmorata is a tropical freshwater eel that widely distributed in the Indo-Pacific Ocean. Previous studies suggested that the species composed of several populations throughout its range, inclusive of the western North Pacific, South Pacific, Indian Ocean and Guam. A. marmorata in Taiwan belongs to the western North Pacific population, whose spawning area (12–17˚N, 131–143˚E) is overlaping with that of A. japonica (12–17˚N, 137–143˚E). In A. japonica, recent molecular population genetic studies have shown no genetic difference in temporal and spatial scales. The western North Pacific population of A. marmorata utilize the same current systems with A. japonica for larval transportation. Whether the genetic differentiation among annual recruits of A. marmorata should be further investigated. We used 5 polymorphic microsatellite DNA loci to examine the genetic structure of A. marmorata in Taiwan from 1994 through 2012. To better understand the long term change of A. marmorata in Taiwan, we also investigated temporal variation of the total length (TL) of glass eels. Analysis of TL revealed that there were significant differences with stages, months and years. The results showed that A. marmorata in Taiwan still have high genetic diversity in terms of allelic richness (6.95 to 7.73) and observed heterozygosity (0.43 to 0.92). Inbreeding index was low (FIS= -0.006), and there was no genetic differentiation among annual cohorts (FST= 0.014). Estimated effective population sizes revealed that the population maintain genetic variation over longer periods of time (Average Ne= 540.18). While comparing the samples from Taiwan with the samples from Java, we found genetic differentiation in spatial scales (FIS= 0.108). A. marmorata in Taiwan might experience bottleneck in the past (Mean M-ratio= 0.46), but no evidence of genetic bottleneck is found in 19 annual cohorts. The conclusions of this study showed that the long term population genetic structure of A. marmorata in Taiwan over the past 19 years was quite stable.