Assembly and phylogenomic analysis of cotton mitochondrial genomes provide insights into the history of cotton evolution

Cotton is a major crop that provides the most important renewable textile fibers in the world. Studies of the taxonomy and evolution of cotton species have received wide attentions, not only due to cotton’s economic value but also due to the fact that Gossypium is an ideal model system to study the origin, evolution, and cultivation of polyploid species. Previous studies suggested the involvement of mitochondrial genome editing sites and copy number as well as mitochondrial functions in cotton fiber elongation. Whereas, with only a few mitogenomes assembled in the cotton genus Gossypium, our knowledge about their roles in cotton evolution and speciation is still scarce. To close this gap, here we assembled 20 mitogenomes from 15 cotton species spanning all the cotton clades (A–G, K, and AD genomes) and 5 cotton relatives using short and long sequencing reads. Systematic analyses uncovered a high level of mitochondrial gene sequence conservation, abundant sequence repeats and many insertions of foreign sequences, as well as extensive structural variations in cotton mitogenomes. The sequence repeats and foreign sequences caused significant mitogenome size inflation in Gossypium and its close relative Kokia in general, while there is no significant difference between the lint and fuzz cotton mitogenomes in terms of gene content, RNA editing, and gene expression level. Interestingly, we further revealed the specific presence and expression of two novel mitochondrial open reading frames (ORFs) in lint-fiber cotton species. Finally, these structural features and novel ORFs help us gain valuable insights into the history of cotton evolution and polyploidization and the origin of species producing long lint fibers from a mitogenomic perspective.