Analyses of foreign plastid sequences in plant mitochondria

MARIA VIRGINIA SANCHEZ PUERTA; CAROLINA LIA GANDINI

BAHÍA BLANCA

Congreso; 6th Argentinian Conference on Bioinformatics and Computational Biology; 2015

A2B2C

Background: Horizontal gene transfer (HGT) is defined as the exchange of genetic material between species, and is now considered to have a significant role in land plant evolution. In contrast to plastids, plant mitochondrial genomes (mtDNA) present exceptionally high rates of HGT. A fusion-compatibility model, in which entire mitochondria are transferred and fused with the native ones, has been recently postulated to explain the mechanisms underlying the HGT process in plant mitochondria. In recent years, the number of HGT events between mitochondria of unrelated species has increased considerably. Most cases involved foreign mitochondrial sequences, although a few sequences of plastid origin were reported. We hypothesize that foreign plastid sequences are initially acquired by the native mtDNA by intracellular gene transfer (IGT) and then horizontally transferred to a distantly related plant following the fusion-compatibility model, rather than directly from a foreign plastid to the mitochondrial genome. To test this hypothesis, we looked for foreign mtDNA fragments flanking the horizontally-transferred plastid sequence. Materials and methods: Mitochondrial genomes containing foreign plastid sequences were obtained from GenBank. Homologous sequences were identified using NCBI-BLASTN and aligned with MUSCLE v3.7. To infer the donor lineages, maximum-likelihood phlylogenetic analyses were performed with RAxML v.8.0, under the GTR+G model. Blast and phylogenetic analyses of regions flanking the aforementioned plastid sequences were done to identify their origin. Results: We searched public databases and the bibliography for foreign plastid regions within angiosperm mitochondrial genomes, Out of ~60 sequenced angiosperm mitochondrial genomes we identified less than 20 cases of foreign plastid sequences. Phylogenetic analyses confirmed their acquisition by HGT. Analyses of flanking regions revealed the presence of foreign mitochondrial sequences surrounding 8 of the foreign plastid sequences. These results suggest that those sequences were first transferred to the mtDNA within the donor cell and then horizontally transferred to the mitochondria of the recipient plant. The rest of the cases could not be fairly tested because there is no mitochondrial data available from the putative donor lineages. Conclusions: Phylogenetic trees are helpful to identify events of HGT. However, identifying the genome donor of plastid foreign sequences requires the analysis of flanking regions to distinguish between plastid-to-mitochondria or mitochondria-to-mitochondria transfers. To date, there is no unambiguous evidence for plastid to mitochondria HGT among angiosperms.