3.5　The AT-rich Region

The 1473bp AT-rich region，with an AT content of 88.60%，is located between the rrnS and trnI genes and is the longest AT-rich region of the sequenced Thitarodes mt genomes to date.Nine 112bp copies and one partial copy of a 55bp sequence were found in the AT-rich region in this moth mt genome(Fig.5).Conspicuous macro-repeat units(＞50bp long)have been found in most of the available Hepialidae mt genomes，including those of T.pui(119bp)，H.xiaojinensis(118bp)，T.gonggaensis(112bp)，T.renzhiensis(113bp)and T.yunnanensis(107bp)(Cao et al.，2012；Shi et al.，2015a；Yi et al.，2016).These repeat sequences account for some of the variations in mt genome length(Fig.5).An explanation for the origin of these repeat sequences is slipped-strand mispairing during mtDNA replication(Levinson and Gutman，1987；Moritz and Brown，1987).

The microsatellite-like repeat(TA)6 was found to be preceded by an ATTTA motif in the AT-rich region in Thitarodes sp.，whereas it is commonly found as ATTTA+(AT)n in other lepidopteran species(Yin et al.，2014；He et al.，2015；Shi et al.，2015b；Wang et al.，2016).However，this microsatellite-like repeat is not found in H.xiaojinensis，T.pui or T.renzhiensis in Hepialidae.Therefore，it might not be a common feature in the AT-rich region in Hepialidae as it is in other Lepidoptera.Additionally，another widely conserved structure that includes the motif“ATAGA”and a 16～22bp poly-T stretch has also not been found in Hepialidae mt genomes.Combining these several features，including the rearrangement of the tRNA gene，the absence of the conserved elements in the AT-rich region，and the absence of the intergenic spacer upstream of nad2，the Hepialidae mt genome displayed mt genome characteristics unique in Lepidoptera.

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Fig.5　Schematic representation of the AT-rich region in the sequenced Hepialidae

All currently determined Hepialidae species have the ancestral gene order，which is different from previously sequenced Lepidoptera.There are 2.4 repeat units(A-B with 119bp per unit)in T.pui，4.4 repeat units(a—d with 118bp per unit)in H.xiaojinensis，4.9 repeat units(α δwith 107bp per unit)in A.yunnanensis，6.5 repeat units(①-⑥with 112bp per unit)in T.gonggaensis，8.5 repeat units(Ⅰ-Ⅷwith 113bp per unit)in T.renzhiensis and 9.5 repeat units(i- ix with 112bp per unit)in the host insect of O.sinensis(Thitarodes sp.).Similar repetitive sequences are absent from N.hunanensis and E.signifer.