在自然演化歷程中酵母菌的聚集行為(clustering)為生存策略的一種，用以因應環境的變化提高生存力。母子細胞分離不全之聚集表型(clumping phenotype)為子細胞與母細胞間的隔膜(septum)在細胞分裂時無法順利分裂所形成，而AMN1基因便是影響著酵母菌細胞分離之重要角色；同時AMN1基因具有單一核甘酸多型性(Single Nucleiotide Polymorphism；SNP)的特徵，因此胺基酸序列的不同也將使得野生酵母菌株RM(聚集表型)與實驗室菌株BY(不聚集表型)發展出不同的細胞表型。此外，AMN1基因除了影響酵母菌細胞的聚集表型，同時也作為反式調控因子來間接調控其他下游基因的表現，其中包含了受壓力環境所誘導表現的相關基因，進而可能影響壓力環境耐受性。在本篇論文中，我利用具有野生菌株RM(聚集表型)，與三種非聚集表型菌株(AMN1基因剔除菌株#6、定點突變菌株#6-1、實驗室菌株BY)的共培養，來觀察AMN1基因表現程度如何影響細胞在生長環境下的生存，同時給予多樣的生長環境以模擬自然中可能面對之環境壓力。在我的結果當中可觀察到不同環境對於酵母菌細胞型態(聚集或非聚集表型)與生存有著不同程度的影響：野生菌株RM(聚集表型)因其經常面臨多變的環境而發展出快速適應之生存力與多元抗壓機制，在與實驗室菌株BY的競爭當中除了高鹽壓力環境外，皆顯示了野生菌株更具競爭優勢的結果；實驗室菌株(非聚集表型)因人擇的篩選使其在碳養分的利用更有效率，同時由於其基因多型性所造就之獨特的基因表現，使其在特定的壓力環境下(如鹽類環境)生存。而在野生菌株RM與基因剔除菌株#6的競爭當中，我更觀察到AMN1基因的剔除反而可能提高菌株的生存與競爭力，在五種培養環境(不含37oC與酒精環境)皆顯示基因剔除菌株#6具有更好的競爭優勢。因此我認為在細胞當中，聚集表型對於野生菌株RM在壓力環境當中的生存應是有貢獻的；同時AMN1基因的存在與否也將會影響到酵母菌細胞對於壓力環境的適應性。

AMN1 is the key gene to mediate the cell separation and then affect the phenotype of strain. The strain carried an Amn1p368D sequence would perform as a clumping phenotype, while carried an Amn1p368V sequence performed as a single-cell phenotype. Besides, AMN1 can also plays a role as trans regulatory element (TREs) to affect different gene expression, which including the expression of stress-related genes under stress environment. To figure out whether the expression of AMN1 affect the cells’ fitness under different stress, I co-cultured the wild strain RM, which cells express clumping phenotype, with three single-cell phenotype strains like the lab strain BY, the mutant strain #6-1, and the AMN1 gene knock-out strain #6 in several different stress environment, respectively. After ten days culturing, I found that the wild strain RM shows a better fitness comparing to the lab strain BY in every stress environment except for salt stress environment, and that the gene knock-out strain #6 also shows a better fitness comparing to the wild strain RM in most environment: general environment, 23oC permissive temperature environment, osmotic stress (sorbitol and salt), and oxidative stress.
The results that RM shows a better fitness indicated the clumping phenotype might has a positive effect on cells’ survival, besides the different evolvement between wild strain and lab strain; and that a better fitness of #6 indicated the expression of AMN1 also has the effect on cells’ fitness to stress environment.

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