Institute of Integrative Biology (IBZ), Forest Pathology and Dendrology, ETH Zurich, 8092 Zürich, Switzerland.
BMC Evol Biol. 2011 Sep 30;11:282. doi: 10.1186/1471-2148-11-282.
Fungi are asexually and sexually reproducing organisms that can combine the evolutionary advantages of the two reproductive modes. However, for many fungi the sexual cycle has never been observed in the field or in vitro and it remains unclear whether sexual reproduction is absent or cryptic. Nevertheless, there are indirect approaches to assess the occurrence of sex in a species, such as population studies, expression analysis of genes involved in mating processes and analysis of their selective constraints. The members of the Phialocephala fortinii s. l. - Acephala applanata species complex (PAC) are ascomycetes and the predominant dark septate endophytes that colonize woody plant roots. Despite their abundance in many ecosystems of the northern hemisphere, no sexual state has been identified to date and little is known about their reproductive biology, and how it shaped their evolutionary history and contributes to their ecological role in forest ecosystems. We therefore aimed at assessing the importance of sexual reproduction by indirect approaches that included molecular analyses of the mating type (MAT) genes involved in reproductive processes.
The study included 19 PAC species and > 3, 000 strains that represented populations from different hosts, continents and ecosystems. Whereas A. applanata had a homothallic (self-fertile) MAT locus structure, all other species were structurally heterothallic (self-sterile). Compatible mating types were observed to co-occur more frequently than expected by chance. Moreover, in > 80% of the populations a 1:1 mating type ratio and gametic equilibrium were found. MAT genes were shown to evolve under strong purifying selection.
The signature of sex was found in worldwide populations of PAC species and functionality of MAT genes is likely preserved by purifying selection. We hypothesize that cryptic sex regularely occurs in the PAC and that further field studies and in vitro crosses will lead to the discovery of the sexual state. Although structurally heterothallic species prevail, it cannot be excluded that homothallism represents the ancestral breeding system in the PAC.
真菌是无性和有性繁殖的生物体，可以结合两种繁殖模式的进化优势。然而，对于许多真菌来说，其有性周期在野外或体外从未被观察到，目前尚不清楚有性繁殖是不存在还是隐藏的。尽管如此，还是有一些间接的方法可以评估一个物种是否存在性繁殖，例如种群研究、参与交配过程的基因的表达分析以及对其选择限制的分析。Phialocephala fortinii s. l.-Acephala applanata 种复合体（PAC）的成员是子囊菌，是定殖木本植物根部的主要暗隔内生真菌。尽管它们在北半球的许多生态系统中都很丰富，但迄今为止尚未确定其有性状态，对其生殖生物学知之甚少，也不知道它如何塑造其进化历史以及如何为其在森林生态系统中的生态角色做出贡献。因此，我们旨在通过包括参与生殖过程的交配型（MAT）基因的分子分析等间接方法来评估有性繁殖的重要性。
该研究包括 19 种 PAC 物种和超过 3000 株菌株，代表了来自不同宿主、大陆和生态系统的种群。虽然 A. applanata 具有同宗交配（自育）MAT 基因结构，但所有其他物种的结构都是异宗交配（自交不育）。观察到相容交配类型比随机预期更频繁地共同出现。此外，在超过 80%的种群中发现了 1:1 的交配型比例和配子平衡。MAT 基因显示出在强烈的纯化选择下进化。
在 PAC 物种的全球种群中发现了性的特征，MAT 基因的功能可能通过纯化选择得到保留。我们假设 PAC 中经常发生隐性性，并且进一步的野外研究和体外杂交将导致发现有性状态。尽管结构上异宗交配的物种占主导地位，但不能排除同宗交配代表 PAC 中的原始繁殖系统。
