作 者 ：魏宇昆, 高玉葆
期 刊 ：植物生态学报 2008年 32卷 2期 页码：512-520
Keywords:grass, endophyte, genetic diversity, genetic combination, symbiotic interactions,
摘 要 ：近百年的禾草内生真菌研究历经了由浅入深的过程，从最初的家畜中毒事件认识到是一种共生内生真菌存在的缘故，到如今利用分子生物学技术揭示其共生机制，人类发现这类植物内生真菌并非想象中的对生态系统无足轻重。Epichloë及其无性型Neotyphodium与禾本科植物是系统发生的互利共生关系，尤其是Neotyphodium可提高宿主抵抗环境胁迫的能力和抵御动物的取食，增强植物的竞争力。禾草内生真菌有3种生活史：有性生活史、无性生活史和兼性生活史，后者表明真菌在不同的宿主及环境下既能营有性生殖也可营无性繁殖，是一种更灵活而有效的生活史对策。对内生真菌分子系统学、生活史以及与宿主禾草协同进化的研究发现，Neotyphodium起源于禾草致病真菌Epichloë的某些种，或是Epichloë与Neotyphodium的种间杂交后代。植物和内生真菌各异的生活史策略，真菌的种间杂交，两者的协同进化亦或种群间基因流的差异，都促成了共生体多样化的基因组合（Genetic combination ），也是其共生关系多样化的根源。内生真菌对宿主的有益作用只在特定基因型真菌、宿主和一定环境条件下才起作用，自然生态系统的共生关系要比农业系统复杂得多，是一个从互利共生至寄生关系的连续系统。未来对于更多共生体的遗传背景和基因与环境相互作用的阐明将有助于对禾草内生真菌共生关系本质更加深入的认识。
Abstract:Endophytes, especially asexual and systemic endophytes in grasses, are generally viewed as plant mutualists based on the action of their alkaloids. Enhanced drought tolerance is a well-known benefit of endophytic infection in tall fescue (Festuca arundinacea) and perennial ryegrass (Lolium perenne), and increased tolerance to other environmental stresses like heat, low light and low soil fertility has also been reported. Three endophyte life histories have been recognized: symptomatic of life cycle where the fungus horizontally transmits by meiotic ascospores which induce sterilization of the host, asymptomatic life cycle where the fungus remains internal and there is vertical transmission by plant seeds throughout the season and a mixed mechanism of life cycle, which can be plastic. Neotyphodium endophytes are closely related to sexual Epichloë species, which are the grass choke pathogen, and likely evolved either directly from sexual Epichloë species or by interspecific hybridization of distinct lineages of Epichloë and Neotyphodium. In vertical transmission, only one fungal genotype is transmitted to the seed progeny, which are usually produced by outcrossing in the host. The same fungal genotype is present in seeds that are genetically variable, and the high level of genetic specificity is probably tied to genetic incompatibility constraining the diversity of successful genotype-genotype combinations of the systemic seed-borne endophytes and the host grasses. The defensive mutualism depends on a certain grass-endophyte genotype combination and environmental conditions. Recent studies have suggested that there is a mutualism-parasitism continuum for the symbiosis between asexual endophytes and grasses and that the symbiosis existing in native grass-endophyte symbionts has a more complex mechanism than in agricultural ecosystems. The host-specific endophyte, with negligible biomass, may alter plant community structure, reduce plant diversity and control food-web structure by disrupting the transfer of energy from plants to upper trophic levels. Future studies should focus on how ecology and genetics interact to shift fungal life history traits between the extremes of sexuality and asexuality and antagonism and mutualism. These questions require a more comprehensive understanding of the genetic basis and phenotypic plasticity of traits of the grass-endophyte interactions.
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