The endosymbiotic theory hypothesizes that mitochondria are the result of a symbiosis between a proto-eukaryotic cell and an alpha-proteobacterium. what aspect(s) of mitochondrial dna support this hypothesis?

We can say the endosymbiotic theory is the results of the mitochondria of a symbiosis which is between an alpha-proteobacterium and pro-eukaryotic material-DAN(small genome), but the other organelles of eukaryotic cells do not contain genome which is located in the nucleus for other different processes of life.
Mitochondria as a symbiont have similarities with plastid DNA of bacteria sequence in a phylogenetic tree. In addition, this is a prove that mitochondria originate from a group of bacteria which is known as alphaproteobacteria.