Recognition of self and nonself is important for outcrossing organisms, and different mating types establish the barrier against self-mating. In the unicellular ciliate T. thermophila, mating type determination requires complex DNA rearrangements at a single mat locus during conjugation to produce a type-specific gene pair (MTA and MTB) for 1 of 7 possible mating types. Surprisingly, we found that decreased expression of the DNA breakage-repair protein Ku80 at late stages of conjugation generated persistent selfing phenotype in the progeny. DNA analysis revealed multiple mating-type gene pairs as well as a variety of mis-paired, unusually arranged mating-type genes in these selfers that resemble some proposed rearrangement intermediates. They are found also in normal cells during conjugation and are lost after 10 fissions but are retained in Ku mutants. Silencing of TKU80 or TKU70-2 immediately after conjugation also generated selfing phenotype, revealing a hidden DNA rearrangement process beyond conjugation. Mating reactions between the mutant and normal cells suggest a 2-component system for self–nonself-recognition through MTA and MTB genes.

自我和非自我的识别对于跨界生物很重要，并且不同的交配类型建立了防止自我交配的障碍。在单细胞纤毛虫牛逼。嗜热，交配类型确定需要在一个单一的复杂DNA重排垫轨迹缀合过程中，以产生类型特异性基因对（MTA和MTB），适用于7种可能的配对类型。出乎意料的是，我们发现在结合的后期DNA断裂修复蛋白Ku80的表达降低，在子代中产生了持久的自交表型。DNA分析显示这些自交者中有多个交配型基因对以及各种错配，排列异常的交配型基因，类似于一些拟议的重排中间体。它们在结合过程中也存在于正常细胞中，经过10次裂变后丢失，但保留在Ku突变体中。使TKU80或TKU70-2静音结合后立即产生自发表型，揭示了结合以外的隐藏的DNA重排过程。突变细胞与正常细胞之间的交配反应提示了通过MTA和MTB基因进行自我-非自我识别的两组分系统。