研究線蟲尾部的形態發生

Abstract

形態發生（morphogenesis）是多細胞生物個體發育出完整外觀樣貌的重要步驟。在線蟲（Caenorhabditis elegans）中，環繞蟲體的表皮組織發育為形態發生的主要環節，如蟲體的尾部即由四個表皮細胞，hyp 8, 9, 10及 11所構成。在線蟲的胚胎發育時期，有一對短暫存活的tail spike cells提供尾部發育的骨架。這對細胞在2-fold之前細胞質會融合為一，延伸出一條細胞突起至蟲體最末端，並被雙核的hyp 10所包覆；在3-fold時期tail spike cells會進行計畫性細胞死亡（programmed cell death），留下細長的細胞突起做為線蟲尾部發育的支架。先前的研究中已發現cdh-3/cadherin及eff-1/fusogen的突變株有尾部末端發育的缺陷（tail tip morphogenesis defect），顯示細胞接合（cell junction）以及細胞融合（cell fusion）有參與線蟲尾部的形態發育。在本篇論文中，我們挑選了兩個尾部形態異常的突變株tp11及tp108進行遺傳性狀分析以及基因定位實驗（genetic mapping）。利用AJM-1::GFP作為細胞邊緣的標記，我們發現tp108的尾部異常主要發生在hyp 8及hyp 11。根據此一性狀進行基因定位實驗的結果顯示tp108為已知基因vab-9的突變株，而VAB-9在先前的研究已被發現作用於細胞接合處（junction protein）。而tp11的尾部異常座落於尾部尖端，主要影響hyp 10的細胞形狀及大小。利用四維顯微鏡技術，我們發現tp11的尾部異常在3-fold時期之前就已發生，顯示此一性狀出現在胚胎發育的早期，而tp11可能是參與線蟲尾部發育的新基因。tp11的基因定位實驗結果顯示其為未知基因K02E10.4的突變株，而我們將繼續研究K02E10.4在線蟲尾部形態發生中所扮演的角色。

Morphogenesis is an essential process for a multi-cellular organism to develop into its characteristic body shape. In C. elegans, this process is largely controlled by the development of epidermis, a single epithelial layer that surrounds the worm. Four hypodermal cells, hyp 8, 9, 10 and 11 form the posterior tip in the tail. A pair of short-lived tail spike cells provides a scaffold for tail development during embryogenesis. They fuse and extend a single posterior process to the tail tip, over which a binuclear hyp 10 cell is wrapped by at the 2-fold stage. The tail spike cells undergo programmed cell death at the 3-fold stage, leaving behind a narrow spike of microtubule-based structure that is still wrapped by hyp 10. Previous studies showed that cdh-3/cadherin and eff-1/fusogen mutants have an abnormal tail tip, suggesting that cell-cell interaction and cell fusion are important for the formation of this structure. However, not much is known about the morphogenesis of tail tip. Here we report the characterization of two mutants, tp11 and tp108, which were isolated in a screen for mutants with tail defects. We used AJM-1::GFP, a marker for adherens junctions, to study the tail defect of the tp108 allele and found that most hyp 8 and some hyp 11 cells have a bulged shape. tp108 turns out to be a vab-9 allele which encodes a claudin-like junction protein, suggesting that mutation of junction results in tail morphology defect. We used AJM-1::GFP to label the boundaries of tail hypodermal cells, and performed time-course fluorescence and Nomarski microscopy analysis of wild-type and tp11 embryos. The result showed that the abnormality of the tp11 tail predominantly resides in hyp 10. The position and shape of hyp 10 appear normal at the 2-fold stage, but hyp 10 fail to elongate properly during later embryogenesis. tp11 turns out to be a K02E10.4 allele, and further studies of which could shed light on the tail tip morphogenesis in C. elegans.