線蟲捕捉菌中高保守性但具可塑性之獵物感受訊息傳遞系統

Abstract

線蟲捕捉菌會產生由菌絲所特化之捕捉構造捕捉線蟲，線蟲與線蟲捕捉菌在野外中普遍存在，但線蟲捕捉菌與線蟲之間之交互關係卻很少人研究。本研究透過實驗室演化與基因剔除試圖探討線蟲捕捉菌與養分之間的關係與真菌是如何感知線蟲的存在。Mitogen-activated protein kinase (MAPK)訊息傳遞系統在真菌界中高度保守，且有許多研究結果證明在動植物病原真菌與致病性相關。線蟲捕捉菌會透過感知線蟲費洛蒙之訊號而產生陷阱，但其中之分子機制尚未明瞭，為了知道MAPK訊息傳遞系統相關基因是否參與線蟲感知與陷阱的產生，將G蛋白訊息傳遞系統上之β次單元gpb1進行基因剔除後，發現線蟲捕捉菌在加入線蟲及線蟲費洛蒙均不會產生陷阱，而高度保守之費洛蒙感知系統上游的G蛋白偶聯受體ste2的基因剔除株所產生的陷阱數量與野生型並沒有差別，顯示線蟲捕捉菌中有不同的G蛋白偶聯受體與線蟲的感知相關。另外，Cell wall integrity (CWI) MAPK訊息傳遞系統上的bck1基因剔除株在生長、產孢與產陷阱上均有缺陷。營養缺乏如缺氮被認為是推進線蟲捕捉菌演化的動力之一，為了要對線蟲捕捉菌捕捉能力的演化有更多的了解，將線蟲捕捉菌培養在富含營養、缺乏營養、缺乏營養但給與線蟲等三種環境下，想知道線蟲捕捉菌是否會因環境中富含營養而喪失捕捉線蟲之能力。在富含營養環境下，一株獨立演化株出現不同於親代的表現型，該突變株可以產生更多的氣生菌絲與能被模式線蟲Caenorabditis elegans誘導出更多的陷阱，這說明了在營養豐富的環境下，線蟲捕捉菌的捕捉能力是具有可塑性的。綜合以上的研究結果顯示營養豐富的環境與兩個高度保守之MAPK訊息傳遞系統會影響線蟲捕捉菌感知線蟲與產生陷阱的能力。

Nematode-trapping fungi develop trapping structures to capture and kill nematodes in response to nematode signals. To this day, the molecular mechanisms and evolutionary origin of nematode-sensing and trap formation in nematode-trapping fungi remain poorly understood. Mitogen-activated protein kinases (MAPKs) are highly conserved in the fungal kingdom and have been shown to be required for pathogenesis in various plant and animal pathogenic fungi. We thus hypothesize that MAPK signaling pathways might be essential for sensing the nematode-associated signals in Arthrobotrys oligospora. To investigate the function of the MAPK genes, we generated loss-of-function mutants for components in the evolutionary-conserved MAPK-mediated pheromone response pathway and cell wall integrity pathway through homologous recombination. Deletion of the G protein β subunit gpb1 of the pheromone response pathway abolished trap formation upon Caenorhabditis elegans nematode and ascarosides, which are nematode pheromones known to trigger trap morphogenesis in nematode-trapping fungi, induction. Intriguingly, an A. oligospora strain lacking ste2, G-protein coupled receptor (GPCR) that in other fungi participates in pheromone sensing, showed identical sensitivity towards C. elegans. This observation suggested multiple GPCRs in A. oligospora are likely involved in sensing nematodes. In addition, bck1 mutant of the other conserved cell wall integrity MAPK pathway resulted in severe defects in growth, conidiation, and trap formation. Nutrient deprivation, such as nitrogen limitation, has long been hypothesized as the selection force that drives the evolution of nematophagous fungi. In order to gain insights into the evolutionary trajectory of nematode-trapping ability in fungi, the model nematode-trapping fungus A. oligospora was experimentally evolved in three conditions with differences in nutrient content: rich medium, low-nutrient medium, and low-nutrient medium supplemented with C. elegans nematodes. Laboratory evolution under rich medium resulted in one independent evolved line that exhibited different amounts of aerial hyphae and trap formation compared to the ancestral strain. This evolved line formed more traps when exposed to C. elegans and ascarosides. These results suggested that prey-sensing is a plastic trait when evolving under nutrient-rich laboratory conditions. In summary, we demonstrated that the environmental nutritional status and two evolutionary-conserved MAPK signaling pathways play essential roles during nematode-sensing and trap morphogenesis in A. oligospora.