頭索動物文昌魚Vasa/Nanos基因及生殖發育之研究

Expression of the germline markers Vasa/Nanos and germline development in the amphioxus Asymmetron lucayanum

彭立穎(Li-Ying Peng)

指導教授 ： 游智凱

國立臺灣大學/理學院/海洋研究所/碩士(2014年)

https://doi.org/10.6342/NTU.2014.00841

摘要

生殖細胞在多細胞動物產生下一代時扮演著重要角色，而在發育過程中也和一般體細胞有所差異。先前的研究已把動物生殖前驅細胞特化之方式歸類成兩種，其一為藉由母源生殖細胞質 (germ plasm)驅動生殖細胞特化的預成 (preformation)機制，也被稱為親代遺傳 (maternal inheritance)機制。另一種則是藉由相鄰細胞所釋放誘導訊息來驅動特化的後生 (epigenesis)機制，也被稱為合子誘導 (zygotic induction)機制。頭索動物的生殖細胞特化方式在早期的研究中被分類為後生機制，然而近年從頭索動物文昌魚科 (Branchiostoma floridae、Branchiostoma belcheri、Branchiostoma japonicum)的研究結果則是支持著預成機制。本研究中我利用生殖基因vasa和nanos來當作生殖世系的標定 (germline markers)，探討另一類群的頭索動物-偏文昌魚科之側殖文昌魚 (Asymmetron lucayanum)的生殖細胞特化機制，偏文昌魚科的形態特徵與文昌魚科最大的不同在於:其生殖腺只在身體右側形成，文昌魚科則是身體兩側皆有生殖腺。我的結果顯示: 在側殖文昌魚受精卵之靠近植物極的細胞質中，偵測到vasa與nanos mRNA的聚集現象，此聚集物質於卵裂時期將會不對稱地傳遞至其中一顆囊胚細胞 (blastomere)中，而此群攜帶聚集物質的細胞在神經胚形成時 (neurulation)開始移動至後端的生長區域 (growth zone)，此觀察結果與先前在文昌魚科的結論一致，因此在側殖文昌魚同樣也支持著頭索動物的生殖細胞特化應為預成機制。此外，我們發現從神經胚形成晚期開始，表現vasa基因的細胞將在體腹側呈現兩側分佈，而在變態 (metamorphosis)之後，表現vasa基因的細胞只存在於右側之未來生殖腺形成處，因此我們認為在其左側發生了未知的負調控，此調控導致生殖細胞在左側被抑制而未形成生殖腺，未來我們將試著了解左右不對稱的調控與側殖文昌魚只有右邊會產生性腺的關係。

關鍵字

生殖細胞 ； Vasa基因 ； Nanos基因 ； 頭索動物 ； 側殖文昌魚 ； 演化

並列摘要

Germline cells are segregated from somatic cells during multicellular animal development and provide the essential genetic materials for the next generation. Previous studies have categorized the mechanisms of germline specification into two major categories, with one uses localized germ plasm (preformation/maternal inheritance) and the other uses inductive signals (epigenesis/zygotic induction). In cephalochordates, it had long been thought the primordial germ cells (PGCs) are specified by epigenesis. Recent studies in Branchiostoma floridae, Branchiostoma belcheri, and Branchiostoma japonicum have challenged this traditional view by showing the existence of maternal germ plasm that contains conserved germline markers. In this research, I investigated the process of PGC formation in another distantly related cephalochordate species, Asymmetron lucayanum, by examing the expression patterns of germline markers Vasa and Nanos. Asymmetron represents one of the two major genera in Cephalochordata. Unlike Branchiostoma, which forms gonads on both sides of the body, Asymmetron forms gonads only on the right side of its body. I found that Asymmetron germline markers are localized in granular cytoplasm near the vegetal pole of the fertilized eggs. The granules were inherited asymmetrically by a single blastomere during the following cleavages, and this germline marker-positive cell migrated to the posterior growth zone during neurulation. These results are similar to the observations in B. floridae, suggesting that Asymmetron also specifies its PGCs using preformation mechanism. Furthermore, in Asymmetron, I found symmetric distribution of the germline marker vasa under both sides of the somites from the late neurula to the late larva stage. However, after metamorphosis, PGCs became asymmetrically localized only on the right side of the juvenile. This transition indicates that between the late larva and the juvenile adult of A. lucayanum, unknown negative regulator(s) may inhibit PGCs from forming gonads on the left side of the body. Further studies on the left-right differential regulation during gonad development will shed light on the source of the right-sided gonad of A. lucayanum.

並列關鍵字

Germline Cell ； Vasa ； Nanos ； Cephalochordate ； Asymmetron lucayanum ； Evolution

參考文獻

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