The pattern of geographic variation in karyotypes and mitochondrial DNA in chromosomal hybrid zones of Nelima nigricoxa (Arachnida, Opiliones)

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To reveal the nature and formation process of chromosomal hybbrid zones in Nelima nigricoxa, we analyzed karyotypes, allozymes, and mtDNA for various populations of the species and the closely related congeners. A total of seven karyotypes were recongnized in Nelima nigricoxa : 2n=20 (Kyushu, Shikoku, and Chugoku district westward of the Asahi River), 2n=18a (West of Mt.Daisen, Tottori Pref.and Kinki district), 2n=18b (north of Okayama city), 2n=16 (Chugoku mountains eastward from the Asahi River), 2n=22a (Izu-Hakone), 2n=22b (Miura Peninsula), 2n=22c (Mt.Kiyosumi, Chiba Pref.). Of these, each of the three adjoining karyotype pairs (2n=20/18a, 2n=18a/16, and2n=16/18a) forms a hybrid zone with a width less than 1-3 km. In a population (Shimimu-dani) in a hybrid zone between 2n=16 and 18a in Sekigane-cho, Tottori Prefecture, the ratio of individuals with 2n=17 chromosomes was significantly lower than the expected ratio from the Hardy-Weinberg theorem (<0.02). The relative fitness of indi … More viduals with 2n=17 was 0.4 when taking 2n=16 equals 1. The figure was much lower than the relative fitness of heterozygotes, 0.7 obtained in a population (Kawadoko) in a hybrid zone (2n=18/20) in Mt.Daisen. Difference in the size of chromosomes that pertain to the chromosome rearrangement may explain this result.We analyzed sequences of about 1 kilobase pairs of CO I gene coded by the mtDNA for several population of N.nigiricoxa and some populations of the closest relatives. The phylogeny obtained was : (nigricoxa Mt.Kiyosumi, Chiba, 2n=22, genufusca Sapporo, 2n=22) (genufusca Tokusen Falls, Yamaguchi, 2n=22 (nigricoxa Tokusen Falls, Yamaguchi, 2n=20 (nigricoxa Mt.Senjo, 2n=18 (nigricoxa Makiyaha Mt.Daisen 2n=20, nigricoxa 2n=20 Makihara Mt.Daisen)))) and the bootstrap probability for each branch was 100% without exception. It was confirmed that grographic differentiation can be detected also in mtDNA level among some morphologically non-differentiatied populations of Nelima nigricoxa in the Chogoku district. Less