Abstract
摘要:扁形動物門包含多種重要的寄生蟲及漁業害蟲以及當代最受歡迎的再生生物學研究模式生物渦蟲,其中鏈渦蟲 (catenulids) 為扁形動物門中最早分支出來的系群。一般教科書對扁形動物的描述為身體構造簡單,缺乏體腔,故傳統上它們被視為原始兩側對稱動物的代表。我們的初步觀察顯示鏈渦蟲疑似有體腔,因此我們將以鏈渦蟲為材料研究體腔的演化。過去二十年的分子親緣研究顯示扁蟲事實上並非最原始的兩側對稱動物,另一方面又顯示體腔可能有多重起源。這個出乎意料複雜的情況衍生出一些問題,如扁形動物的祖先是否有體腔、體腔如何演化出來、又如何失去等等,這些問題至今未曾被研究者以現代生物學方法深入探究。本計劃將以多種螢光分子標記來重建鏈渦蟲的立體結構,以探究體腔構造的細節,並與無體腔的渦蟲比較,藉此評估體腔演化的可能方式。再者,鏈渦蟲的分裂無性生殖方式與環節動物較接近而與其他扁蟲不同,我們將進一步探究其生長與無性生殖的關係,並透過標記幹細胞基因於無性生殖過程中的表現動態來了解扁蟲特有的neoblast幹細胞的演化。因neoblast通常分布於由中胚層構成的間質中,了解鏈渦蟲幹細胞的分布動態及功能也能側面幫助我們探究體腔演化。
Abstract: Platyhelminthes (flatworms) is one of the larger animal phyla that includes many pest and parasite species that are important for aquaculture and medicine, as well as one of the most popular experimental model for regeneration research, the planarians. The flatworm body plan exhibited some seemingly primitive features, e.g. a lack of coelom, single-opening digestive chamber, and simple protonephridia etc., and therefore, it has been traditionally billed as a representative of early-branching bilaterians. However, recent progress in molecular phylogeny has challenged this view by revealing that flatworms are members of Spiralia, a clade that includes annelids and molluscs – each full-blown coelomate protostomes exhibiting a collection of phylogenetically advanced traits. Thus, are the ‘primitive traits’ of the flatworms really primitive, or are they in fact ‘degenerated’ from their complex predecessors. These questions had never been explicitly addressed with modern technical approaches. In this study, we will use a species belonging to Catenulida, a basally-branching clade of Platyhelminthes, to address this issue. We will focus on two separate, but interlinked, questions by taking a ‘molecular anatomy’ approach to characterize the anatomical and morphological organization of catenulid flatworms in detail. The first concerns the evolution of coelom organization. Suspected coelomic spaces were observed in the catenulid specimens, and this is a significant violation of the general definition to Phylum Platyhelminthes. We will characterize cellular organization and extracellular matrix in the body wall and gut wall by immunostaining and staining for molecular markers and render 3-dimensional reconstruction of catenulid flatworm anatomy from confocal imaging data to determine whether the observed open space can be characterized as a coelom. Aside from the suspected coelom, catenulids also differ from other flatworms in that they undergo asexual reproduction by paratomy, which is otherwise most frequently observed in annelids. We will characterize how the specific parameters in body growth relate to the specification of zooids. We will also characterize the stem cell population dynamics during paratomic asexual reproduction of catenulids by looking at the expression patterns of genes that are known to be expressed by the neoblast stem cells of other flatworms. These data can be used to infer the evolutionary origin of the neoblasts and the extraordinary regenerative capability of flatworms and may establish the evolutionary connection between paratomy (catenulids) and fragmentary regeneration (other flatworms). Further, since neoblasts are distributed in the parenchymal tissue that takes up the space between ectoderm and endoderm in other flatworms, probing stem cell localization in catenulid coelom may allow us to gain further insight into the evolution of coloem in Platyhelminthes.
Keyword(s)
扁形動物門
體腔
無性生殖
螢光顯微鏡技術
Platyhelminthes
Coelom
Asexual Reproduction
Fluorescence Microscopy