The occurrence of group II introns in plant mitochondrial genomes is strikingly different between the six major land plant clades, contrasting their highly conserved counterparts in chloroplast DNA. Their present distribution likely reflects numerous ancient intron gains and losses during early plant evolution before the emergence of seed plants. As a novelty for plant organelles, we here report on five cases of twintrons, introns-within-introns, in the mitogenomes of lycophytes and hornworts. An internal group II intron interrupts an intron-borne maturase of an atp9 intron in Lycopodiaceae, whose splicing precedes splicing of the external intron. An invasive, hypermobile group II intron in cox1, has conquered nine further locations including a previously overlooked sdh3 intron and, most surprisingly, also itself. In those cases, splicing of the external introns does not depend on splicing of the internal introns. Similar cases are identified in the mtDNAs of hornworts. Although disrupting a group I intron-encoded protein in one case, we could not detect splicing of the internal group II intron in this 'mixed' group I/II twintron. We suggest the name 'zombie' twintrons (half-dead, half-alive) for such cases where splicing of external introns does not depend any more on prior splicing of fossilized internal introns.

中文翻译：

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一个功能性的双子加速器，“僵尸”双子加速器和一个超机动的II组内含子侵入植物线粒体。

植物线粒体基因组中II组内含子的出现在六个主要陆地植物进化枝之间显着不同，这与它们在叶绿体DNA中高度保守的对应物形成对比。它们的当前分布可能反映出在种子植物出现之前植物早期进化过程中许多古代内含子的得失。作为植物细胞器的一种新颖性，我们在此报告了5例双生子，即内生内含子和内生子。内部的第II组内含子中断了番茄科的atp9内含子的内含子成熟酶，后者的剪接先于外部内含子的剪接。Cox1中的一个侵入性，超机动的II类内含子已经征服了另外9个位置，其中包括一个先前被忽略的sdh3内含子，最令人惊讶的是它本身。在那种情况下 内含子的剪接不取决于内含子的剪接。在金盏花的mtDNA中鉴定出类似的情况。尽管在一种情况下会破坏I组内含子编码的蛋白质，但我们无法在此“混合” I / II组双子加速器中检测到内部II组内含子的剪接。对于外部内含子剪接不再依赖于化石内部内含子的剪接的这种情况，我们建议使用“僵尸”双子加速器（半死，半活）。