臺灣鐵線蟲生物多樣性及寄生對於寄主形態發育的操控

Abstract

鐵線蟲在生態上的獨特性使其在寄生蟲的研究中佔有一定的地位。儘管牠在應用上的價值不高，長久以來的傳說以及對鐵線蟲的誤解推動了大部分的相關研究。鐵線蟲的生活史橫跨水陸兩域，幼蟲在水中孵化並寄生在水生的保幼寄主體內形成包囊。受感染的保幼寄主被陸生最終寄主攝食後胞囊得以進入最終寄主體內發育，並在成熟後鑽出回到水中繁殖。在我們的採集中，臺灣目前發現三種鐵線蟲，分別是臺灣索鐵線蟲 (Chordodes formosanus)，臺灣尖尾鐵線蟲 (Acutogordius formosanus n. sp.)，原鐵線蟲 (Gordius sp.)，並在水生保幼寄主與淡水魚體內各別發現一種形態與上述種類均不相符的鐵線蟲。三種已知的鐵線蟲除了原鐵線蟲的最終寄主尚不明瞭之外，臺灣索鐵線蟲的主要寄主為斧螳 (Hierodula spp.)，而少部分個體也曾在花螳螂科 (Hymenopodidae) 及螽斯科 (Tettigoniidae) 的體內發現；臺灣尖尾鐵線蟲則廣泛的被發現寄生在直翅目 (Orthoptera) 的不同科之中。在臺灣索鐵線蟲的發育過程中，牠會影響最終寄主的形態發育並造成形態上的異速生長與間性。這兩個現象可能有利於寄生蟲提高對寄主資源掠奪的效率。間性現象也廣泛的被發現存在不同的寄生關係中，但至今其機制仍然不明瞭。昆蟲的性徵分化一直以來被認為不會受到細胞外的因子所調控，這包含寄生蟲的影響在內。因此藉由幼年化的方式阻斷寄主的性徵發育，使其停留在分化之前被認為是造成昆蟲間性的原因。然而在受到臺灣索鐵線蟲感染的寬腹螳螂 (H. patellifera) 身上，寄生蟲造成的影響卻不支持幼年化的假說，反而偏向寄主的雌雄二型性性分化過程受到干擾。儘管目前我們尚未收集到足夠多的樣本來支持這個假說，鐵線蟲的研究卻很有可能在當代的昆蟲性徵發育領域中帶來新的觀點。

The horsehair worm is the parasite which is famous for its unique properties instead of its applicability in human lives. Its life cycle typically goes through aquatic and terrestrial environment, with two parasitic phases in aquatic paratenic host and terrestrial definitive host. There are three known horsehair worm species in Taiwan (Chordodes formosanus, Acutogordius formosanus n. sp., Gordius sp.) and two undetermined species, respectively, detected from aquatic paratenic hosts and a fresh water fish. Except Gordius sp. whose definitive host is still unknown, the host range of C. formosanus, which is specific to the Hierodula mantids (despite it can be found to parasitize Hymenopodidae mantids and katydids), is much narrower than that of the Acutogordius formosanus n. sp. which is frequently found in different family of the orthopteran hosts. During the definitive host phase, C. formosana manipulates the morphological development of its mantid host for increasing the efficiency of resource exploitation, and consequently cause the morphological allometry and intersexuality on the infected host. Mechanism of the parasitic intersexuality is unclear. Since the insect sexual differentiation has long been believed to be the cell-autonomous process which lacks regulation outside a cell, it is generally believed that the parasitic intersexuality is caused by the inhibition of ontogeny, which is known as juvenilization. However, our study of the horsehair worm effect on H. patellifera is likely to conflict to the hypothesis of juvenilization and suggests the alternative hypothesis of the interference in insect sexual differentiation. Despite the sample size is still less to confidently support this conclusion, studies of the horsehair worms might provide the new light in the understanding of the developmental process of the insect sexual differentiation.