Genetic diversity in natural populations of Japanese Potamoid crabs

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Japanese Potamoid consists of 15 species divided into 3 genea (Geotholphusa, Candidiopotamon and Ryukyum). Among the species, only G. dehaani inhabits widely through Honsyu to Kagoshima and the distributions of the other species are restricted in the Ryukyu Islands. We studied genetic variation in natural populations for each species or genetic differentiation among species mainly by using mtDNA sequence determination.In natural populations of G. dehaani, extensive genetic divergence was observed. The genetically differentiated populations are usually distributed parapatrically and very narrow hybrid zones are formed at where they come into contact with each other. Detailed genetic structure at the hybrid zones were investigated in relation to the development of reproductive isolation between genetically differentiated parental populations. From the results we discussed the factors that are responsible to maintain the very narrow hybrid zones.Based on nucleotide sequence variation in ND2 gene of mtDNA (1,008-1,011 bp) phylogenetic relationships were inferred for 15 species of Japanese Potamoid. Our results supported some previous taxonomic aspects from morphological studies. The genus Candidiopotamon, however, was not shown as a robust assemblage in the molecular tree. Moreover, phylogenetic relationships among species of Geothelphusa were not consistent with that of deduced from morphological or ecological studies. Phylogenetic relationships revealed in the present study suggests that the major evolutionary lineages of Japanese Potamoid have occurred at 200-250 mya when the Ryukyu region was thought to be separated into some land blocks.