The geographic distribution of diploid and
tetraploid cytotypes and
cpDNA haplotypes throughout the entire range of Aucuba japonica was investigated. We measured relative genome size using flow cytometry and sequenced two
cpDNA intergenic regions, atpB-rbcL and psbA-trnH (GUG). Two haplotypes include both diploids and
tetraploids; four others are all
tetraploids. Based on the combination of these haplotypes and cytotypes, eight "haplo-cytotypes" can be defined, which show a distinct geographic structure. Two diploid haplo-cytotypes are distributed in the southwestern part and six
tetraploid ones in the northeastern part of the Japanese archipelago. Diploid and
tetraploid haplo-cytotypes with the same haplotype are, in one case, disjunctively distributed, and in another case, in contact. The phylogenetic relationships of haplo-cytotypes indicate that the traditional circumscription of A. japonica is paraphyletic. One lineage consists of four
tetraploid haplo-cytotypes and another of diploid and
tetraploid haplo-cytotypes plus A. chinensis. Tetraploidization occurred independently at least three times, once at the base of the former lineage and twice in the latter. Taking the geographic, cytological, and phylogenetic evidence into account, the formation of the present geographic differentiation pattern of haplo-cytotypes through postglacial expansion from glacial refugia is discussed.