Multiple genetic lineages of the invasive brown widow spider (Araneae: Theridiidae) from inland areas in the Kanto region, Japan.

The brown widow spider, Latrodectus geometricus Koch, 1841, is a medically important invasive species that possesses neurotoxic venom capable of causing latrodectism in humans. This species has recently been found in inland areas of the Kanto region in Japan, raising concerns about its ongoing range expansion and associated public health risks. Here, we analyzed partial sequences of mitochondrial cytochrome c oxidase subunit I (cox1) and 16S ribosomal DNA from 99 individuals (3 males and 96 females) collected from multiple sites in Tokyo and Kanagawa prefectures, evaluating their phylogenetic positions and introduction history. The analysis revealed two major lineages, with six cox1 haplotypes (LGc1-LGc6) and five 16S rDNA haplotypes (LGs1-LGs5). One lineage comprised previously unreported basal haplotypes, including LGc5 and LGc6 in cox1 and LGs5 in 16S rDNA. The other lineage included four haplotypes (LGc1-LGc4 and LGs1-LGs4), which have been reported from South Africa and various invaded regions globally. In Sagamihara City (Kanagawa), where numerous individuals were consistently detected in previous surveys, both lineages co-occurred, suggesting multiple introductions and/or high propagule pressure. To clarify the dispersal routes and source populations of each lineage and haplotype, more comprehensive genetic data from both its native range in Africa and globally established populations are needed.