Extensive variation in nuclear mitochondrial DNA content between the genomes of Phytophthora sojae and Phytophthora ramorum

Abstract

Fragments of mitochondrial DNA (mtDNA) transferred to the nuclear genome are called nuclear mitochondrial DNAs (NUMTs). We report here a comparison of NUMT content between genomes from two species of the same genus. Analysis of the genomes of Phytophthora sojae and P. ramorum revealed large differences in the NUMT content of the two genomes: 16.27 x 10(-3) and 2.28 x 10(-3)% of each genome, respectively. Substantial differences also exist between the two species in the sizes of the NUMTs found in each genome, with ranges of 20 to 405 bp for P. sojae and 19 to 137 bp for P. ramorum. Furthermore, in P. sojae, fragments from the mitochondrial genes rns, rnl, coxl, and nad (various subunits) are found most frequently, whereas P. ramorum NUMTs most often originate from the cox3, rpsl4, nad4, and nad5 genes. The large differences in the presumptive mtDNA insertions suggest that the insertions occurred subsequent to the divergence of the two species, and this is supported by sequence comparisons among the NUMTs and the mtDNA sequences of the two species. P. sojae mtDNA sequences inserted in the nuclear genome appear to have been altered as a result of insertions, deletions, inversions, and translocations and provide insights into active mechanisms of sequence divergence in this plant pathogen. No clear examples were found of NUMTs forming functional nuclear genes or of NUMTs inserted into exons or introns of any nuclear gene.