Abstract
Two recently introduced fungal plant pathogens (Ceratocystis lukuohia and Ceratocystis huliohia) are responsible for Rapid ‘ōhi‘a Death (ROD) in Hawai‘i. Despite being sexually incompatible, the two pathogens often co-occur in diseased ‘ōhi‘a sapwood, where genetic interaction is possible. We sequenced and annotated 33 mitochondrial genomes of the two pathogens and related species, and investigated 35 total Ceratocystis mitogenomes. Ten mtDNA regions [one group I intron, seven group II introns, and two autonomous homing endonuclease (HE) genes] were heterogeneously present in C. lukuohia mitogenomes, which were otherwise identical. Molecular surveys with specific primers showed that the 10 regions had uneven geographic distribution amongst populations of C. lukuohia. Conversely, identical orthologs of each region were present in every studied isolate of C. huliohia regardless of geographical origin. Close relatives of C. lukuohia lacked or, rarely, had few and dissimilar orthologs of the 10 regions, whereas most relatives of C. huliohia had identical or nearly identical orthologs. Each region included or worked in tandem with HE genes or reverse transcriptase/maturases that could facilitate interspecific horizontal transfers from intron-minus to intron-plus alleles. These results suggest that the 10 regions originated in C. huliohia and are actively moving to populations of C. lukuohia, perhaps through transient cytoplasmic contact of hyphal tips (anastomosis) in the wound surface of ‘ōhi‘a trees. Such contact would allow for the transfer of mitochondria followed by mitochondrial fusion or cytoplasmic exchange of intron intermediaries, which suggests that further genomic interaction may also exist between the two pathogens.
Highlights
Mobile introns and homing endonuclease (HE) genes are diversity-generating elements that contribute to the size and diversity of mitochondrial genomes in fungi and can potentially facilitate horizontal gene transfer between fungal species
Some mitochondrial introns were unique to the Latin American Clade” (LAC) or Australian Clade” (AAC), but most appeared to be orthologs present in both LAC and AAC species, suggesting shared histories of vertical descent (Supplementary Table 1)
Ten mitochondrial regions (SPAMs) found in some but not all isolates of C. lukuohia were found in all isolates of C. huliohia with 100% identity, apparently as a result of horizontal transfer
Summary
Mobile introns and homing endonuclease (HE) genes are diversity-generating elements that contribute to the size and diversity of mitochondrial genomes in fungi and can potentially facilitate horizontal gene transfer between fungal species. Mitochondrial introns can be classified based on splicing mechanism, structure, and intron encoded proteins (IEPs) into either “group I” or Mobile Introns in Rapid ‘Ohi‘a Death “group II,” both of which can be transferred either vertically or horizontally (Belfort et al, 2002; Lang et al, 2007; Hausner, 2012; McNeil et al, 2016; Zubaer et al, 2018). Mobile group I introns use intron encoded HEs to recognize large (∼20– 30 bp) target sites in DNA (Chevalier and Stoddard, 2001) and invade intron-negative alleles (“intron homing”) (Dujon, 1989; Belfort and Perlman, 1995; Stoddard, 2011). In addition to conferring mobility to group I, or in some instances to group II introns, intron encoded homing endonuclease genes (HEGs) can move independently of their intron partners, and such autonomous HEGs can catalyze and direct their own homing mobility to HEG-minus target sites (Sellem and Belcour, 1997; Belfort et al, 2002; Toor and Zimmerly, 2002; Stoddard, 2011; Hafez and Hausner, 2012; Megarioti and Kouvelis, 2020)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
