Higher plant mitochondrial genomes are much larger and more complex than those of other organisms. They vary in size from about 200 kb in Brassica species to 2500 kb in watermelon, which is substantially larger and more variable than mammalian (15-18 kb) or fungal (18-78 kb) mitochondrial (mt) DNAs
Animal mitochondrial DNA is a small, extrachromosomal genome, typically ∼16 kb in size. With few exceptions, all animal mitochondrial genomes contain the same 37 genes: two for rRNAs, 13 for proteins and 22 for tRNAs.
Why are animal mitochondrial genomes so conserved and so small in comparison to those of plants?