Evolution of a pseudogene:: Exclusive survival of a functional mitochondrial nad7 gene supports Haplomitrium as the earliest liverwort lineage and proposes a secondary loss of RNA editing in marchantiidae
Evolution of a pseudogene:: Exclusive survival of a functional mitochondrial nad7 gene supports Haplomitrium as the earliest liverwort lineage and proposes a secondary loss of RNA editing in marchantiidae
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DOI：
10.1093/molbev/msm026
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发表时间：
2007-04-01
影响因子：
10.7
通讯作者：
Knoop, Volker
中科院分区：
文献类型：
--
作者：
Groth-Malonek, Milena;Wahrmund, Ute;Knoop, Volker
基因从线粒体转移到细胞核是内共生假说的必然结果。线粒体核糖体蛋白基因的频繁和独立转移在被子植物中有很多例子，而呼吸链成分基因的转移则是罕见的。一个明显的例外是编码多形地草（Marchantia polymorpha）复合体I亚基7的nad7基因，它作为全长、携带内含子、转录但非剪接的假基因存在于线粒体中，而其功能对应基因是核编码的。为了阐明假基因退化的模式，我们研究了其他12种具有广泛系统发育分布的苔类动物的线粒体nad7位点。我们发现其中11例线粒体nad7基因无功能。然而，假基因退化的模式各不相同：点突变，伴随着单核苷酸索引，主要将停止密码子引入到marchantiid liverworts的阅读框中，而在简单的thalloid和叶状jungermanniids分类群中，较大的索引引入了帧移位。然而，最值得注意的是，在分离的Haplomitrium中，线粒体nad7阅读框似乎是完整的。通过cDNA分析，鉴定了典型的rna编辑事件，以重建保守的密码子身份，并确认了2个肝脏特异性II组内含子的功能性剪接，从而证明了其功能性表达。我们对我们的研究结果进行了解释：1)表明在最早的陆地植物中存在一个功能性线粒体nad7基因，并有力地支持了Haplomitrium在苔类植物中的基础位置；2)表明在后来的苔类分支分支中，假基因退化和线粒体进化的不同模式；3)表明在假定的最古老的陆地植物群体中，一个非功能性基因的维持时间出奇地长。4)支持marchantiid liverworts中rna编辑活性继发性丧失的模型。
Gene transfer from the mitochondrion into the nucleus is a corollary of the endosymbiont hypothesis. The frequent and independent transfer of genes for mitochondrial ribosomal proteins is well documented with many examples in angiosperms, whereas transfer of genes for components of the respiratory chain is a rarity. A notable exception is the nad7 gene, encoding subunit 7 of complex I, in the liverwort Marchantia polymorpha, which resides as a full-length, intron-carrying and transcribed, but nonspliced pseudogene in the chondriome, whereas its functional counterpart is nuclear encoded. To elucidate the patterns of pseudogene degeneration, we have investigated the mitochondrial nad7 locus in 12 other liverworts of broad phylogenetic distribution. We find that the mitochondrial nad7 gene is nonfunctional in 11 of them. However, the modes of pseudogene degeneration vary: whereas point mutations, accompanied by single-nucleotide indels, predominantly introduce stop codons into the reading frame in marchantiid liverworts, larger indels introduce frameshifts in the simple thalloid and leafy jungermanniid taxa. Most notably, however, the mitochondrial nad7 reading frame appears to be intact in the isolated liverwort genus Haplomitrium. Its functional expression is shown by cDNA analysis identifying typical RNA-editing events to reconstitute conserved codon identities and also confirming functional splicing of the 2 liverwort-specific group II introns. We interpret our results 1) to indicate the presence of a functional mitochondrial nad7 gene in the earliest land plants and strongly supporting a basal placement of Haplomitrium among the liverworts, 2) to indicate different modes of pseudogene degeneration and chondriome evolution in the later branching liverwort clades, 3) to suggest a surprisingly long maintenance of a nonfunctional gene in the presumed oldest group of land plants, and 4) to support the model of a secondary loss of RNA-editing activity in marchantiid liverworts.