愛玉榕小蜂電子傳遞鏈複合基因的加速分化

Abstract

環境溫度會對生物適應能力進行選汰，形成不同地區之間的族群分化。榕屬植物Ficus pumila在台灣有兩個變種，分別為生長在海拔低於500公尺的薜荔 (Ficus pumila var. pumila) 與海拔高於800公尺的愛玉 (Ficus pumila var. awkeotsang)。比較兩個親緣相近的薜荔榕小蜂 (Wiebesia pumilae) 與愛玉榕小蜂 (Wiebesia sp.) 族群，發現兩族群之間粒線體cox1基因有演化速率提高的現象；並且愛玉榕小蜂族群在低溫有較長的半致死時間，可能已適應低溫環境。推測愛玉榕小蜂粒線體基因的演化速率被提高是因應低溫環境下能量需求增加而形成的選汰壓力，需要重新塑造電子傳輸鏈 (Electronic transport chain, ETC) 提高能量產生。由於電子傳遞鏈中所包含的粒線體基因與核基因關聯緊密，前者受環境壓力所產生的置換可能導致後者跟著產生補償性置換。本論文假設愛玉榕小蜂族群的電子傳遞鏈基因演化速率會被提高，以補償粒線體基因加快的演化速率。使用兩個榕小蜂族群的轉錄組，比對同義與非同義置換比率、置換發生的方向性，以及其餘核基因是否在特定功能群有富集 (enrichment) 現象。發現電子傳遞鏈基因的非同義置換率 (0.0025) 相較於其餘核基因 (0.0011) 高出約兩倍，同義置換率值兩者相似 (前者為0.0091，後者為0.0098)。並且發生在愛玉榕小蜂支系中的電子傳遞鏈基因非同義置換位點 (26.5) 顯著高於薜荔榕小蜂支系 (17)，顯示愛玉榕小蜂族群可能受到選汰影響而有適應性演化發生。同義與非同義置換比率較高的其餘核基因列表中糖解作用與細胞膜相關基因皆有富集現象，顯示愛玉榕小蜂的基因分化可能並非僅由粒線體基因帶動，而是受到能量需求改變影響使得能量產生相關的基因皆受到選汰壓力而被提高演化速率。

Temperature may cause selection to adaptation ability of lining things, promoting divergence between populations. Ficus pumila has two variants in Taiwan, called creeping-fig (Ficus pumila var. pumila) and jelly-fig (Ficus pumila var. awkeotsang). The two variants grow at different latitude, creeping-fig < 500 m and jelly-fig > 800 m. Between two closely related pollinating wasps, creeping- (Wiebesia pumilae) and jelly-fig wasps (Wiebesia sp.), previous study found accelerated and perhaps adaptive evolution in their mitochondrial cox1 gene. Also, jelly-fig wasps have longer LT50, showing that jelly-fig wasps may have adapted clod environment. Suppose that the accelerated amino acid changes in the mitochondrial genome of jelly-fig wasps may result from increased energy demands due to cold adaptation which would need a remolding of the electronic transport chain (ETC). Furthermore, because the functions of mtDNA and nDNA encoded ETC genes are tightly linked, mutations in the former may cause compensatory changes in the latter. We hypothesize that evolutionary rate of nDNA encoded ETC genes in jelly-fig wasps are elevated to compensate for the accumulation of amino acid changes in mtDNA-encoded ETC genes. We thus sequenced the transcriptome of two fig wasps. While the Ka (0.0025) of ETC genes are two times higher than the rest of genes (0.0011), their Ks are similar (0.0091 in the former and 0.0098 in the latter). More importantly, most of these non-synonymous changes of ETC complexes are in the lineage leading to jelly-fig wasps (26.5), suggesting their role in cold adaptation. We also found that glycolysis function genes and membrane-related genes has been enriched in the rest of nuclear genes with higher Ka/Ks, suggesting that gene differentiation in the lineage leading to jelly-fig wasps is not only driven by mitochondrial genes but also nuclear genes related to energy production.