Laboratory of Genetics, Wageningen University, 6708 PB Wageningen, The Netherlands.
Proc Natl Acad Sci U S A. 2012 Jun 26;109(26):10432-7. doi: 10.1073/pnas.1205333109. Epub 2012 Jun 12.
Prions are infectious proteins that cause fatal diseases in mammals. Prions have also been found in fungi, but studies on their role in nature are scarce. The proposed biological function of fungal prions is debated and varies from detrimental to benign or even beneficial. [Het-s] is a prion of the fungus Podospora anserina. The het-s locus exists as two antagonistic alleles that constitute an allorecognition system: the het-s allele encoding the protein variant capable of prion formation and the het-S allele encoding a protein variant that cannot form a prion. We document here that het-s alleles, capable of prion formation, are nearly twice as frequent as het-S alleles in a natural population of 112 individuals. Then, we report a 92% prevalence of [Het-s] prion infection among the het-s isolates and find evidence of the role of the [Het-s]/het-S allorecognition system on the incidence of infection by a deleterious senescence plasmid. We explain the het-s/het-S allele ratios by the existence of two selective forces operating at different levels. We propose that during the somatic stage, the role of [Het-s]/HET-S in allorecognition leads to frequency-dependent selection for which an equilibrated frequency would be optimal. However, in the sexual cycle, the [Het-s] prion causes meiotic drive favoring the het-s allele. Our findings indicate that [Het-s] is a selected and, therefore, widespread prion whose activity as selfish genetic element is counteracted by balancing selection for allorecognition polymorphism.
朊病毒是一种传染性蛋白质,能导致哺乳动物致命疾病。真菌中也发现了朊病毒,但对其在自然界中作用的研究很少。真菌朊病毒的拟议生物学功能存在争议,从有害到良性甚至有益不等。[Het-s]是一种真菌 Podospora anserina 的朊病毒。het-s 基因座有两个拮抗等位基因,构成了一种同种异体识别系统:het-s 等位基因编码能够形成朊病毒的蛋白质变体,而 het-S 等位基因编码不能形成朊病毒的蛋白质变体。我们在这里记录到,在一个由 112 个个体组成的自然种群中,能够形成朊病毒的 het-s 等位基因的频率几乎是 het-S 等位基因的两倍。然后,我们报告了 het-s 分离株中[Het-s]朊病毒感染的患病率为 92%,并找到了[Het-s]/het-S 同种异体识别系统在有害衰老质粒感染发生率上的作用证据。我们通过两种在不同水平起作用的选择力的存在来解释 het-s/het-S 等位基因比例。我们提出,在体细胞阶段,[Het-s]/HET-S 在同种异体识别中的作用导致了频率依赖性选择,其中平衡频率是最佳的。然而,在有性周期中,[Het-s]朊病毒导致减数分裂驱动有利于 het-s 等位基因。我们的研究结果表明,[Het-s]是一种被选择的、因此广泛存在的朊病毒,其作为自私遗传元件的活性被同种异体识别多态性的平衡选择所抵消。