落叶松八齿小蠹与长喙壳真菌间的种特异性伴生关系

The species specific associations between Ips subelongatus and ophiostomatoid fungi

小蠹虫与长喙壳类真菌(Ophiostomatoid fungi)在自然界中形成稳定的伴生关系,是森林生态系统内一种普遍的生态学现象。已有研究表明欧亚大陆的齿小蠹属(Ips)昆虫与多种长喙壳类真菌形成广泛的伴生关系,其中部分真菌是重要的针叶树病原菌。随着借助于DNA信息特征的系统发育分析,揭示出形态特征和亲缘关系十分接近的3种齿小蠹属昆虫,云杉八齿小蠹(I.typographus),欧洲落叶松八齿小蠹(I.cembrae)和亚洲落叶松八齿小蠹(I.subelongatus)确定为不同种之后,相应地与之稳定伴生的长喙壳类真菌Ceratocystis polonica也由过去一个种揭示为3个种的复合体,各自与3种小蠹虫稳定伴生,形成密切的种特异性伴生关系。小蠹虫与真菌的种特异性伴生被认为是处于同一森林生态系统内的生物协同进化的结果。通过对我国东北地区落叶松八齿小蠹虫体、坑道标本上伴生真菌菌株的采集、分离和生理学、形态学特征观察,以及基于ITS、β-tubulin、MAT-2 HMG box多位点DNA序列的系统发育分析,首次确定了长喙壳真菌Ceratocystis fujiensis在我国东北地区异域分布的3种落叶松林内普遍存在,与落叶松八齿小蠹形成稳定的伴生关系。作为亚洲落叶松八齿小蠹伴生的主要真菌,也是伴生菌区系中的先锋种和致病力最强的病原菌,C.fujiensis在我国落叶松人工林的广泛分布值得高度重视,将为制定防治病虫复合危害的有效策略和措施提供科学基础。研究结果进一步支持了齿小蠹属昆虫与长喙壳真菌间的种特异性伴生假说。同时,多基因序列特征表明落叶松八齿小蠹与C.fujiensis在亚洲范围内的不同地理种群存在着显著的遗传多样性,预示特异性伴生在不同种群间发生的可能,可以为种特异性伴生假说和小蠹虫-真菌间共生关系的研究提供良好的模式材料。

The bark-beetles and ophiostomatoid fungi formed a stable association relationship in nature. It is a universal ecology phenomenon occurring frequently in forest ecological systems. The association between the beetles and fungi are regarded to some extent as a symbiosis in a broad sense. They have been extensively and intensively documented and illustrated from all over the world since it was first realized. Some research showed that Ips beetles spread over Eurasia were extensively associated with a number of ophiostomatoid fungi species. Among which, some species are important pathogens of conifers. Although the morphology and ecology characteristics of three Ips beetles, I. typographus, I. cembrae, and I. subelongatus were almost identical, these beetles were evidenced in a series studies as distinct species based on the phylogenetic analysis of DNA information and critical morphological comparison. In parallel, an ophiostomatoid fungus, Ceratocystis polonica previously recognized as a closely associated fungus with these beetles was also separated into three distinct entities resided in the Ceratocystis coerulescens species complex, and stably associated with three kinds of bark beetles, respectively, where a species specific association between bark beetles and ophiostomatoid fungi was established. The species specific association was understood as a result of co-evolution in forests where both partners lived together in a common habitat. The samples collected in this study from three allopatric Larix hosts which lps subelongatus attacked seriously. The ophiostomatoid strains were isolated from the body surface of Ips subelongatus, blue-stain tissue or the galleries bark-beetles attacked. Observations of the colony, physiology and morphology characteristic, and comparisons of anamorph and teleomorph structures of all Ceratocystis strains, confirmed that the strains are indistinguishable from Ceratocystis fujiensis M.J. Wingf. The phylogenetic analysis based on sequences derived from the ITS regions of the rDNA operon, the partial 15-tubulin gene and the MAT-2 HMG box gene, further determined a total of nineteen fungal strains as species Ceratocystis fujiensis. The study first determined Ceratocystis fujiensis extensively existed in three allopatric larch forests in northeast and north China, which formed a stable association relationship with Ips subelongatus under such ecological conditions. Ceratocystis fujiensis was proved as a dominant fungus associated with lps subelongatus in Japan, and a pioneer species and the strongest pathogenic fungus in all the bark-beetles associated fungi. Because C. fufiensis widely infesting Larix spp. plants in China, inevitably, this phenomenon should be paid high attention. The study provides scientific basis to make the policies and measures for the government to prevent and control the composite damages of diseases and pests effectively. The result in this paper provides support to the hypothesis of species specific association between lps beetles and ophiostomatoid fungi. Meanwhile, the muhi-gene sequences characteristic indicates that I. subelongatus and C. fufiensis exist significant genetic diversity in different geographic populations, which probably predict the possibility of the species specific association occurred under species levels. The associations between Ips beetles and ophiostomatoid fungi exhibit an ideal insight into completely understanding the symbiotic relationship between the bark beetles and ophiostomatoid fungi, and species specific association as well.