Analyses of mitochondrial genomes of the genus Ammopiptanthus provide new insights into the evolution of legume plants

The genus Ammopiptanthus is mainly distributed in the semi-arid regions of northwest China. It only contains two species: A. mongolicus and A. nanus. They exhibit similar morphological characteristics, but little is known about their genetic differences. We performed de novo sequencing and mitochondrial genome assembly and found that both species contain 36 protein-coding genes, 3 rRNA, and 16 tRNA genes. The comparison of A. mongolicus and A. nanus mitogenomes revealed a 6.5-kb region loss in A. nanus mitogenome, which was replaced by a 1.5-kb different sequence element. Other small variations between the two species include 25 SNPs, 4 deletions, and 2 insertions. Meta-analyses indicated that 34% of the A. mongolicus mitogenome was unique to Ammopiptanthus and has no similarity with any other known legume mitogenomes. These unique sequences are mainly distributed in intergenic regions. Gene gain and loss analysis showed that this mitogenome gained two genes, rps7 and sdh4, during evolution, with rps7 transferred from the chloroplast genome. Selective pressure analysis implied that the atp8 gene was under positive selection. Our study established high-quality reference mitogenomes for the genus Ammopiptanthus, revealed novel genetic differences between A. mongolicus and A. nanus, and provided new insights into the evolution of legume plants.