Author summary Plant mitochondrial genomes are commonly depicted in research articles and textbooks as circular molecules that are the size of the genome. Although research on mitochondrial DNA (mtDNA) over the past few decades has revealed that genome-sized circles are exceedingly rare and that alternative forms of mtDNA are more common, many biologists still perceive circular maps as representing one or more physical chromosomes. This misconception can lead to biases in how mitochondrial genomes are assembled and misinterpretation of their evolutionary relationships, synteny, and histories. In this study, we present an assembly methodology that uses short- and long-read sequencing data to determine the mitochondrial genome structures of three lettuce species. We show that these mitochondrial genomes are fluid and dynamic, with multiple sequence arrangements of the genome coexisting within individuals of the same species. Differences in sequence arrangements between species can be explained by rare recombination…