Patterns of the disposition of chromosomal hybrid zones in several species of harvestmen (Arachnida, Opiliones) in the Kinki District and Shikoku

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We surveyed number of chromosomes of several species of harvestmen including Gagrellula ferruginea, Nelima nigricoxa, and N.genufusca in the Kinki District and adjacent areas of Honshu, and Shikoku to obtain information on geographical positions of chromosomal hybrid zones of these species. Most notable result was available in Gagrellula ferruginea in Shikoku. The results obtained from the survey were as follows : The diploid number of chromosomes increases from 2n=16 to 20 in the southern part of Takamatsu City, Kagawa Pref., through intermediate populations polymorphic for the number, although populations in eastern part of the prefecture including Is. Shodo invariably showed 2n=16. Populations in the western part of Kagawa Prefecture were polymorphic (2n=12/13/14) or monomorphic (2n=12) and the distributional range of the 2n=12 population overlaps that of the 2n=20 populations in Mt. Ryuo, without any indication of hybridization. This means that the 2n=12 population is reproductively fully isolated from the 2n=20 population. However, both the forms intergrade one another through populations with intermediate numbers (2n=14,16,18), making narrow zones of contact in the areas where two neighboring populations with different chromosome numbers abut in Sanyo side of Chugoku District (Okayama and Hyogo Prefectures), Honshu. Thus, the overlap of the distributional ranges of 2n=12 and 2n=20 populations with a series of intermediate populations connecting both ends can be considered a case of circular overlap that arose from successive increase (or decrease) of chromosome numbers.