α-变形菌

α-变形菌(Alphaproteobacteria)是变形菌门(Proteobacteria)下的一个纲[3]。此纲的成员变异性极大,且共通点极少,但他们确实系出同源。α-变形菌大多都是革兰氏阴性菌,而有些胞内寄生的物种缺少肽聚糖,而造成革兰氏染色变异。

α-变形菌包含多种细菌,其中包含光自营细菌、植物共生菌(如根瘤菌)、内共生细菌(如Wolbachia),和胞内寄生细菌(如立克次体)。

此外,本纲还包含已灭绝的Proto-mitochondrion。真核生物的祖先摄入了此种细菌,进行内共生作用,成为后来的线粒体。还盼兰应有一种生物技术上很喜欢使用的农杆菌(Rhizobium radiobacter)也是本纲成员,科学家常常使用本物种来将外来DNA导入植物基因体中。另外,一些光自营生物像是遍在远洋杆菌(Pelagibacter ubique)也属于此纲,α-变形菌的微生物阿雅润抹可能占了海洋微生物的10%左右。

关于本纲所属的目,有人反对现行的分类法,尤其是Pelagibacterales的地位,但大体上大家还是普遍接受。细菌的分类学一直是分类学家相当头痛的问题,因为细菌的基因会彼此互相交流,且捆炒才基因歧异度相当高(像是遍在远洋杆菌由于他的寄生行为,就会精简化他的基因体)。在某些目中,其成员的GC含量变异也大的夸张。举个例子,像是悼骗Pelagibacterales、立克次体目(Rickettsiales)和Holosporales三者的基因体都含有高AT含量,有人认为那其实是趋同演化洒罪促所导致的假象,而反对方也有他们自己的论述。后来有人发现用以判断分类依据的rRNA中,GC含量好像不太会受基因体变异影响。因此,rRNA中GC含量较高的Holosporales就被和低GC含量的Pelagibacterales和立克次体目分开了。

α-变形菌纲下有三个亚纲Magnetococcidae、立克次体亚纲(Rickettsidae)和柄杆菌亚纲(Caulobacteridae)。基群为Magnetococcidae,此类别包含了许多趋磁细菌(由于趋磁细菌为多系群,因此只有部分包含于α-变形菌之下)签照说立克次体亚纲则由细胞内寄生的立克次体目(Rickettsiales),以及可以在胞外单独生存的Pelagibacterales。柄杆菌亚纲则由Holosporales、红螺旋菌目(Rhodospirillales)、鞘脂单胞船捆辨劝菌目(Sphingomonadales)、红细菌目(Rhodobacterales)、柄杆菌目(Caulobacterales)、KiloniellalesKordiimonadales、短小盒菌目(Parvularculales),和Sneathiellales

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