Mapping and characterization of polymorphism in mtDNA of Cryphonectria parasitica: evidence of the presence of an optional intron.

The mitochondrial DNA (mtDNA) of the filamentous ascomycete Cryphonectria parasitica is large and polymorphic so, to better understand the nature of the polymorphisms within populations, a small collection of Italian strains of the fungus was examined. Known mtDNA polymorphisms were mapped and found to cluster in four regions of the mtDNA molecule, particularly in the RFLP region 2 where five different mtDNA haplotypes out of 13 strains were identified. This region included an area of 8.4kbp which was entirely sequenced in strain Ep155 showing the presence of two introns. An internal 3.2kbp portion was sequenced also in six additional strains. Sequence comparison of the C. parasitica mitochondrial intronic ORFs revealed similarities to known endonucleases such as those of Podospora anserina and Neurospora crassa. DNA sequence analysis showed that three polymorphisms of this mtDNA region within this population of 12 strains were due to the optional presence in the ND5 gene of an intron and of an intervening sequence within the intron. Evidence was also found within this population of mixed mitochondrial types within a single strain.