Abstract
In order to study the evolution of mitochondrial genomes in the early branching lineages of the monocotyledons, i.e., the Acorales and Alismatales, we are sequencing complete genomes from a suite of key taxa. As a starting point the present paper describes the mitochondrial genome of Butomus umbellatus (Butomaceae) based on next-generation sequencing data. The genome was assembled into a circular molecule, 450,826 bp in length. Coding sequences cover only 8.2% of the genome and include 28 protein coding genes, four rRNA genes, and 12 tRNA genes. Some of the tRNA genes and a 16S rRNA gene are transferred from the plastid genome. However, the total amount of recognized plastid sequences in the mitochondrial genome is only 1.5% and the amount of DNA transferred from the nucleus is also low. RNA editing is abundant and a total of 557 edited sites are predicted in the protein coding genes. Compared to the 40 angiosperm mitochondrial genomes sequenced to date, the GC content of the Butomus genome is uniquely high (49.1%). The overall similarity between the mitochondrial genomes of Butomus and Spirodela (Araceae), the closest relative yet sequenced, is low (less than 20%), and the two genomes differ in size by a factor 2. Gene order is also largely unconserved. However, based on its phylogenetic position within the core alismatids Butomus will serve as a good reference point for subsequent studies in the early branching lineages of the monocotyledons.
Highlights
The mitochondrial genomes of embryophytes or land plants are renowned among eukaryotes for their astonishing complexity, and evolutionary plasticity seems to prevail among angiosperms in particular
The majority of mitochondrial genomes can be mapped to a single circular molecule, except in Silene and Cucumis where they were mapped to several individual chromosomes [3,4]
As we did not perform similar searches with the remaining intergenic regions of the mitochondrial genome we cannot rule out the possibility that some of these may include sequences of nuclear origin, but we assume that the general content of nuclear DNA in the Butomus mitochondrial genome is very low
Summary
The mitochondrial genomes of embryophytes or land plants are renowned among eukaryotes for their astonishing complexity, and evolutionary plasticity seems to prevail among angiosperms in particular. Phylogenetic Analyses Sequences of mitochondrial genes from Butomus and seed plant species, for which the complete mitochondrial genome is available, were extracted from GenBank (see Table 1). As we did not perform similar searches with the remaining intergenic regions of the mitochondrial genome we cannot rule out the possibility that some of these may include sequences of nuclear origin, but we assume that the general content of nuclear DNA in the Butomus mitochondrial genome is very low .
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