Reverse transcriptase-related enzymes are associated with horizontal chromosome transfer in an asexual pathogen

Supernumerary chromosomes have been shown to transfer horizontally from one isolate to another. However, the mechanism by which horizontal chromosome transfer (HCT) occurs is unknown. In this study, we compared the genomes of 11 isolates comprising six Fusarium species that cause soybean sudden death syndrome (SDS) or bean root rot (BRR), and detected numerous instances of HCT in supernumerary chromosomes. We also identified a statistically significant number (21 standard deviations above the mean) of single nucleotide polymorphisms (SNPs) in the supernumerary chromosomes between isolates of the asexual pathogen F. virguliforme. Supernumerary chromosomes carried reverse transcriptase-related genes (RVT); the presence of long RVT open reading frames (ORFs) in the supernumerary chromosome was correlated with the presence of two or more chromosome copies with a significant number of SNPs between them. Our results suggest that supernumerary chromosomes transfer horizontally via an RNA intermediate. Understanding the mechanism by which HCT occurs will have a profound impact on understanding evolution and applying biotechnology as well as accepting HCT as a natural source of genetic variation.