基于rDNAITS序列分析Alternaria与相似属的关系及A.leucanthemi的分类地位

对Alternaria,Stemphylium,Ulocladium,Nimbya,Embellisia等五个形态相似属的代表性种进行了5.8S/ITS区段序列测定,连同从GenBank下载的相关有性型Pleospora,Lewia的同项资料,由Neighbor-joining方法构建系统发育树。序列对比结果显示,所有参试的菌株/种的5.8SrDNA序列保守程度较高,ITS序列变化较大,ITS1比ITS2变化更大。由5.8S/ITS构建的系统发育树可以足够将供试菌区分到属;系统发育树显示Alternaria,Ulocladium,Nimbya和Embellisia系统学关系较近,又存在一定的分化;Stemphylium聚在相对较远的分支上,与上述四属关系较远,相对独立进化;有性型的Pleospora与无性型的Stemphylium被聚到一个分支,这种不同阶段菌株在遗传基础上的一致,充分证明所用方法及所选测序的rDNA区段对本类群真菌的分类很有价值。Alternarialeucanthemi位于系统发育树的外侧,较为独立进化,与Alternaria的其它种亲缘关系较远,却与Stemphyliumspp.和Bipolarisspp.关系较近,对其目前的分类地位提出质疑。

Phylogenetic relationships of Alternaria and related genera and taxonomic status of A. leucanthemi inferred from ITS rDNA sequence analysis

In order to further elucidate the relationships among Alternaria, Ulocladium, Stemphylium, Nimbya and Embellisia and their teleomorphic Pleospora and Lewia, nuclear internal transcribed spacer (ITS) and 5.8SrDNA from these genera/species are determined and compared. Alignment of the 5.8S/ITS of the tested isolates shows that 5.8SrDNA are highly conserved. The ITS sequences, especially ITS1, are very variable in both base composition and length. Phylogenetic analysis of the 5.8S/ITS sequence data sets by NJ (Neighbor-joining) method clearly separated these tested fungi on generic level. The results indicate that Alternaria spp., Ulocladium spp., Nimbya spp. and Embellisia spp. are placed together in a large Alternaria-complex clade and are closely related, they are relatively differentiated to some extent; Stemphylium spp. are clustered in a distant clade and phylogenetically distinct from the other four genera, showing a relative independent evolution. The anamorphic Stemphylium spp. and its teleomorphic Pleospora spp. are clustered in a clade. Such a phylogenetic consistency on different genetic phase of strains strongly testifies that using these methods and different analysis regions of rDNA selected are very valuable in studying systematics of these fungi. Based upon 5.8S/ITS sequence identities, Alternaria leucanthemi is obviously different from most Alternaria spp.. It is closely related to Stemphylium spp. and Bipolaris spp., and its present taxonomic status is doubtful.