Highly variable mitochondrial chromosome content in a holoparasitic plant due to recurrent gains of foreign circular DNA

Multichromosomal mitochondrial genomes (mtDNAs) in eukaryotes exhibit remarkable structural diversity, yet intraspecific variability and the origin of the individual chromosomes remain poorly understood. We focus on a holoparasitic angiosperm with a mtDNA consisting of 65 chromosomes largely composed of foreign DNA acquired by horizontal gene transfer (HGT) from its mimosoid hosts. The frequency, timing and population dynamics of these HGT events have not been examined. Here we sampled different individuals of the holoparasite Lophophytum mirabile, along with their host plants, to assess mtDNA intraspecific variability and capture recent events that may bring insights into the HGT process. We also gathered mitochondrial data from 43 mimosoids to identify older and recent HGT events and assess precisely the proportion of foreign DNA. Through comparative genomic and evolutionary analyses, we uncovered great intraspecific variability in chromosome content and defined the mitochondrial pangenome of L. mirabile with 105 distinct chromosomes. The estimated foreign content reaches 93.5% of the mtDNA, including 73 fully-foreign chromosomes that support the circle-mediated HGT model as a key mechanism for their acquisition. We inferred recurrent DNA transfers from the host plants leading to new mitochondrial chromosomes that replicate autonomously. Our results emphasize the importance of adopting a pangenomic approach to fully capture the genetic diversity and evolution of multichromosomal mitochondrial genomes. This study shows that HGT can strongly influence the mtDNA content and generate enormous intraspecific variability even in geographically close individuals.