3.1 Genome Organization，Base Composition，and Gene ...
The complete mt genome of this moth(Thitarodes sp.)is a circular DNA molecule of 16，280bp in length(GenBank No.：KX527574；Table 1)，making it the largest mt genome in the Hepialidae to date(Table 2).It is comparable to other lepidopteran mt genomes，which range from 14，535bp in Ostrinia nubilalis(Coates et al.，2005)to 16，499bp in Praysoleae(van Asch et al.，2016).The large mt genome size is primarily attributed to the expansion of the AT-rich region(Fig.1；Table 2)(Cao et al.，2012；van Asch et al.，2016).
Fig.1 Circular map of the mitochondrial genome of Thitarodes sp.
Genes on the H-strand are located on the first circle(from outermost to innermost)，while genes on the L-strand are on the second circle.On the third circle，green represents the locations of tRNAs and rRNAs.The fourth circle shows the GC content，which is plotted using a sliding window(100bp)，as the deviation from the average GC content of the entire sequence.The black color represents below-average GCcontent in the 100bp window，while the deep brick red indicates above-average GC content.The inner circle indicates the GC skew(dark green：G＞C；dark purple：G＜C).
Table 1 Annotation of the Thitarodes sp.mt genome
Continued table
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H and L refer to the heavy and light strands，respectively.
Positive values of intergenic regions indicate gap nucleotides；a negative value indicates overlapped nucleotides.
Table 2 Comparison of the nucleotide composition of mitochondrial genomes of the moths from family Hepialidae
Continued table
The proportions of A，T，C，and G of the H-strand in this moth mt genome indicate a remarkably high A+T content(81.20%)and low G+C content(18.80%)(Table 2)，which is consistent with the AT-bias that is generally observed in other lepidopteran mt genomes(Kim et al.，2006；Niu et al.，2016).The A+T content of Grapholita dimorpha Komai(74.84%)is the lowest reported in Lepidoptera(Niu et al.，2016)，while that of Coreana raphaelis is the highest，at up to 82.7%(Kim et al.，2006).
The AT and GC skew values in the H strand of this moth mt genome(Fig.1，ATskew=0.010，GC-skew=—0.191)indicate a slight A skew and a moderate Cskew.A similar trend has been observed in other lepidopteran mt genomes，with the AT-skew varying from—0.04742(C.raphaelis)to 0.05878(Bombyx mori)and the GC-skew ranging from—0.31769(Ochrogaster lunifer)to—0.15802(C.raphaelis)(Cao et al.，2012).
This moth mt genome contains 13 PCGs，2 r RNAs，22 putative tRNAs，and an AT-rich region(Table 1，Fig.1).More genes(9 PCGs and 14 tRNAs)are located on the heavy strand(H-strand)，whereas fewer genes(4 PCGs，8 tRNAs，and 2 rRNA genes)are located on the light strand(L-strand)，similar reports of other species of Lepidoptera(Park et al.，2016).The gene order is identical to the ancestral gene arrangement but differs from that of other Lepidoptera due to the rearrangement of 3 tRNAs between the AT-rich region and nad2.The ancestral type is IQM arrangement at the AT-rich region and nad2 junction，while most lepidopteran mt genomes show MIQ arrangement(Park et al.，2016).This IQM arrangement is also found in other Hepialidae，such as Thitarodes renzhiensis，Thitarodes yunnanensis(Ahamus yunnanensis)and Hepialus xiaojinensis，as well as in Nepticuloidea(Stigmella roborella)，which is also an ancient，non-ditrysian lepidopteran group(Cao et al.，2012；Timmermans et al.，2014；Chen et al.，2015；Shi et al.，2015a；Yi et al.，2016).