Variation in mtDNA haplotypes suggests a complex history of reproductive strategy in Cannabis sativa

Cannabis is one example in angiosperms that appears to have a recent origin of dioecy and X/Y sex chromosomes. Several evolutionary explanations for this transition have been proposed, with the most parsimonious beginning with a mitochondrial mutation leading to cytoplasmic male sterility (CMS). Our study utilized 73 Cannabis sativa whole genome shotgun libraries to reveal eight different mtDNA haplotypes. The most common haplotype contained 60 of the 73 individuals studied and was composed of only dioecious individuals. However, other haplotypes contained a mix of dioecious and monoecious individuals, so haplotype alone does not predict dioecy. From these haplotype groupings we further examined the fully annotated mitochondrial genomes of four hemp individuals and looked for genetic variation affecting reproductive strategy (e.g., monoecious vs. dioecious strategies). Specifically, we searched for markers associated with CMS and for gene rearrangements within these mitochondrial genomes. Our results revealed highly syntenic mitochondrial genomes that contained approximately 60 identifiable sequences for protein coding genes, tRNAs and rRNAs and no obvious rearrangements or chimeric genes. We find no clear evidence that the different reproductive patterns are due to easily identifiable CMS mutations. Our results refute the simplest hypothesis that there was a single recent origin of dioecy in a monoecious ancestor. Instead, the story of the evolution of dioecy is likely much more complex. Further exploration of the nuclear and mitochondrial genomes and their interaction is required to fully understand Cannabis mating strategies and the possible existence of CMS in this species.