3.3　Introns

Notable in the O.sinensis mtDNA，it is the high degree of invasion by mobile DNAelements(groupⅠand groupⅡintrons).The number of introns exhibits remarkable variation in fungal mt genomes(Burger et al.，2003).The largest number of mt introns was documented for O.sinensis(n=54)in Hypocreales in our analysis，while M.chlamydosporia has only one intron(Table 1)(Lin et al.，2015).In O.sinensis，45 introns were found in 11 PCGs and 9 in 2 rRNA subunits，accounting for 84.17%(89725bp)and 15.83%(14，805bp in rnl，2025bp in rns)of the whole intron length，respectively.As shown in other fungi，introns were abundant in cox1，rnl，and cob genes(Lang et al.，2007)，containing 14，8，and 6 introns，respectively，accounting for 94.82%，75.11%and 92.32%of each gene(Table 2).To these，cox2，and nad5 should also be included since 93.18%and 81.15%of each gene were covered by introns，respectively.In contrast，several genes，such as nad3，nad4，and atp8，were intronless in O.sinensis.(https://www.daowen.com)

BLASTx and RNAweasel results showed that 46 GroupⅠintrons and 6 GroupⅡintrons were included in the O.sinensis mt genome，with 2 short introns failure to classify.GroupⅠintrons are reported to be dominant in fungal mt genomes，while group II introns are found more frequently in plant mt genome(Lang et al.，2007).Group I introns encode various homing endonuclease genes(HEGs)with LAGLIDADG or GIYYIG domain motifs，while group IIintrons mostly encode reverse transcriptases(RTs)(Lang et al.，2007).The motifs of HEGs and RTs in introns catalyze the transfer and site-sp ecific integration of the introns into various genes(Lang et al.，2007).