Maternal Inheritance of U’s Triangle and Evolutionary Process of Brassica Mitochondrial Genomes

Abstract

The sequences and genomic structures of plant mitochondrial (mt) genomes provide unique material for phylogenetic studies. The nature of uniparental inheritance renders an advantage when utilizing mt genomes for determining the parental sources of hybridized taxa. In this study, a concatenated matrix of mt genes was used to infer the phylogenetic relationships of six cultivated <i>Brassica</i> taxa and explore the maternal origins of three allotetraploids. The well-resolved sister relationships between two pairs of diploid and allotetraploid taxa suggest that <i>Brassica carinata</i> (<i>car</i>) possessed a maternal origin from <i>Brassica nigra</i>, while <i>Brassica juncea</i> (<i>jun</i>) was maternally derived from <i>Brassica rapa</i> (<i>cam</i>). Another allotetraploid taxon, <i>Brassica napus</i> (cv. Wester) may have been maternally derived from the common ancestor of <i>B. rapa</i> and <i>Brassica oleracea</i> (<i>ole</i>), and/or have undergone (an) extra hybridization event(s) along its evolutionary history. The characteristics of <i>Brassica</i> mt genomic structures also supported the phylogenetic results. <i>Sinapis arvensis</i> was nested inside the <i>Brassica</i> species, sister to the <i>B. nigra-B. carinata</i> lineage, and possessed an mt genome structure that mostly resembled <i>B. nigra</i>. Collectively, the evidence supported a systematic revision that placed <i>S. arvensis</i> within <i>Brassica.</i> Finally, ancestral mt genomes at each evolutionary node of <i>Brassica</i> were reconstructed, and the detailed and dynamic evolution of <i>Brassica</i> mt genomes was successfully reproduced. The mt genome of <i>B. nigra</i> structurally resembled that of the <i>Brassica</i> ancestor the most, with only one reversion of a block, and the <i>Brassica oleracea</i> underwent the most drastic changes. These findings suggested that repeat-mediated recombinations were largely responsible for the observed structural variations in the evolutionary history of <i>Brassica</i> mt genomes.