Comparative evolutionary analysis of organellar genomic diversity in green plants

The mitochondrial genome (mitogenome) and plastid genome (plastome) of plants vary immensely in genome size and gene content. They have also developed several eccentric features, such as the preference for horizontal gene transfer of mitochondrial genes, the reduction of the plastome in non-photosynthetic plants, and variable amounts of RNA editing affecting both genomes. Different organismal lifestyles can partially account for the highly diverse organellar genomes across the tree of green plants. For example, endosymbiotic and parasitic lifestyles can dramatically affect the genomic architectures of plant mitochondria and plastids. In this study, the organellar genomes of several green plants with atypical lifestyles were investigated and compared with the breadth of organelle genomic diversity within green plants. Next-generation sequencing and comparative evolutionary analyses were performed on organellar genomes of parasitic plants in Orobanchaceae and endosymbiotic algae in Chlorellaceae. Comparative organellar genomic analysis from endosymbiotic green algae provided no evidence for genome reduction; instead the endosymbiont genomes are generally larger in genome size and richer in intron content. Similarly, facultative hemiparasitic species in Orobanchaceae revealed minimal organellar genome degradation, but some evidence for several horizontal transferred genes. In both groups, the lack of genomic reduction may be attributed to the retention of photosynthetic ability. In addition, the extent of RNA editing was examined in the mitogenome of Welwitschia, a xerophytic plant. RNA editing sites in Welwitschia are extremely reduced compared with other gymnosperms, and may be caused by retroprocessing. Taken together, these results demonstrated that atypical lifestyle does not necessarily lead to the production of unusual genomic features and exhibited the convergence and divergence in green plants organelle genomes