Department of Plant Pathology, University of Stellenbosch, Matieland 7602, South Africa.
Fungal Biol. 2011 Feb;115(2):157-68. doi: 10.1016/j.funbio.2010.11.005. Epub 2010 Dec 4.
Pythium vexans fits into the internal transcribed spacer (ITS) clade K sensu Lévesque & De Cock (2004). Within clade K, P. vexans forms a distinct clade containing two enigmatic species, Pythium indigoferae and Pythium cucurbitacearum of which no ex-type strains are available. In South Africa, as well as in other regions of the world, P. vexans isolates are known to be heterogeneous in their ITS sequences and may consist of more than one species. This study aimed to investigate the diversity of South African P. vexans isolates, mainly from grapevines, but also citrus and apple using (i) phylogenetic analyses of the ITS, cytochrome c oxidase (cox) I, cox II, and β-tubulin regions and (ii) seven biometric oogonial parameters. Each of the phylogenies clustered P. vexans isolates into a single well-supported clade, distinct from other clade K species. The β-tubulin region was phylogenetically uninformative regarding the P. vexans group. The ITS phylogeny and combined cox I and II phylogenies, although each revealing several P. vexans subclades, were incongruent. One of the most striking incongruences was the presence of one cox subclade that contained two distinct ITS subclades (Ib and IV). Three groups (A-C) were subjectively identified among South African P. vexans isolates using (i) phylogenetic clades (ITS and cox), (ii) univariate analysis of oogonial diameters, and (iii) multivariate analyses of biometric oogonial parameters. Group A is considered to be P. vexans s. str. since it contained the P. vexans CBS reference strain from Van der Plaats-Niterink (1981). This group had significantly smaller oogonial diameters than group B and C isolates. Group B contained the isolates from ITS subclades Ib and IV, which formed a single cox subclade. The ITS subclade IV isolates were all sexually sterile or produced mainly abortive oospores, as opposed to the sexually fertile subclade Ib isolates, and may thus represent a distinct assemblage within group B. Although ITS subclade Ib included the P. indigoferae ex-type sequence, this group was considered to be P. vexans since South African isolates in this clade produced globose sporangia. Group C contained four apple isolates that were related to, but distinct from P. cucurbitacearum. Although P. vexans groups A-C might be distinct species, they are not described here as such due to (i) these groups only representing some of the known diversity in P. vexans, (ii) conflicting gene tree phylogenies preventing phylogenetic species identification, and (iii) sexually sterile isolates preventing the broad application of biometrical data.
水霉 vexans 属于内部转录间隔区 (ITS) 分支 K sensu Lévesque & De Cock（2004）。在分支 K 内，水霉 vexans 形成了一个独特的分支，包含两个神秘物种，即 Pythium indigoferae 和 Pythium cucurbitacearum，但其无性型菌株均不可用。在南非以及世界其他地区，水霉 vexans 分离株在其 ITS 序列上表现出多样性，可能由不止一个种组成。本研究旨在通过（i）ITS、细胞色素 c 氧化酶（cox）I、cox II 和 β-微管蛋白区的系统发育分析，以及（ii）7 种生物测量卵孢子参数，调查南非水霉 vexans 分离株的多样性，这些分离株主要来自葡萄，但也来自柑橘和苹果。每个系统发育树将水霉 vexans 分离株聚类到一个单独的、由支持的分支中，与其他分支 K 物种明显不同。β-微管蛋白区在水霉 vexans 组中没有系统发育信息。ITS 系统发育树和合并的 cox I 和 II 系统发育树虽然都揭示了几个水霉 vexans 亚群，但并不一致。最显著的不一致之一是存在一个 cox 亚群，其中包含两个不同的 ITS 亚群（Ib 和 IV）。使用（i）系统发育分支（ITS 和 cox）、（ii）卵孢子直径的单变量分析和（iii）生物测量卵孢子参数的多变量分析，对南非水霉 vexans 分离株进行了主观分组（A-C）。组 A 被认为是水霉 vexans s. str.，因为它包含了 Van der Plaats-Niterink（1981）的水霉 vexans CBS 参考菌株。该组的卵孢子直径明显小于组 B 和 C 分离株。组 B 包含来自 ITS 亚群 Ib 和 IV 的分离株，它们形成了一个单独的 cox 亚群。ITS 亚群 IV 分离株均为性不育或主要产生败育卵孢子，而不是可育的 ITS 亚群 Ib 分离株，因此可能代表组 B 内的一个独特组合。尽管 ITS 亚群 Ib 包含了 P. indigoferae 无性型序列，但由于（i）该组只代表了水霉 vexans 已知多样性的一部分，（ii）基因树系统发育不一致，无法进行系统发育种鉴定，以及（iii）性不育分离株阻碍了生物计量数据的广泛应用，该组被认为是水霉 vexans。组 C 包含 4 个苹果分离株，它们与 P. cucurbitacearum 有关，但又不同。尽管水霉 vexans 组 A-C 可能是不同的种，但由于（i）这些组仅代表了水霉 vexans 已知多样性的一部分，（ii）基因树系统发育不一致，无法进行系统发育种鉴定，以及（iii）性不育分离株阻碍了生物计量数据的广泛应用，因此并未在此处将其描述为不同的种。
