Many ladybird species are known to have an elytral colour polymorphism, which indicates geographical variation. The ladybird beetle Cheilomenes sexmaculata (Fabricius) exhibits elytral colour polymorphism and has expanded its distribution from 33°N to 36°N in Japan over 100 years since 1900. The mitochondrial COI gene haplotypes were integrated into two haplotype groups, with one group existing at higher frequencies in lower latitudes, the other group appearing at higher frequencies in higher latitudes. In addition, the dark morph types of this species increase with latitude, whereas the light types appear at higher relative frequencies in lower latitudes.

In the present study, we first determined the morph types of individuals and examined the mitochondrial DNA COI gene. Second, we investigated the relationship between the genetic population structure based on the mitochondrial DNA COI gene and the morph types’ geographical variation. Results indicated that the mtCOI genetic structure was associated with the morph types by latitude; specifically, the haplotype group existing at higher frequencies in lower latitudes tended to be light morph types. In contrast, the haplotype dominant in higher latitudes more frequently exhibited dark morph types, indicating that dark morph types in the higher latitude genetic group may have led the distributional expansion toward higher latitudes since 1900 rather than the lower latitude light morph types.

已知许多瓢虫物种具有鞘翅颜色多态性，这表明地理变异。瓢虫甲虫Cheilomenes sexmaculata (Fabricius) 表现出鞘翅颜色多态性，自 1900 年以来的 100 多年间，其在日本的分布范围从 33°N 扩展到 36°N。线粒体 COI 基因单倍型被整合为两个单倍型群，其中一个群存在于较低纬度的频率较高，另一组出现在较高纬度的较高频率。此外，该物种的暗变体类型随着纬度的增加而增加，而光变体在低纬度地区以较高的相对频率出现。

在本研究中，我们首先确定了个体的形态类型并检查了线粒体 DNA COI 基因。其次，我们研究了基于线粒体 DNA COI 基因的遗传种群结构与形态类型的地理变异之间的关系。结果表明，mtCOI遗传结构与纬度的形态类型相关；具体而言，在低纬度地区以较高频率存在的单倍型群体往往是轻变型。相比之下，在高纬度地区占主导地位的单倍型更频繁地表现出暗变体类型，表明自 1900 年以来，高纬度遗传组中的暗变体类型可能导致了向高纬度地区的分布扩展，而不是低纬度光变体类型。