Abstract
1. Iris japonica Thunb. (Japanese name “Syaga”) is an allotriploid (2n=54), but neither autotriploid nor hypertriploid. It contains 2 different karyotypes; the chromosome constitution may be described as AAB.
2. I. japonica has been probably derived from a hybrid between 2 species which had different karyotypes, probably A and B, and one set (A) of the parent chromosomes was doubled. It might have originated partly in its present habitat and survived there, while the parent species have become extinct.
3. A Chinese species allied to I. japonica is a diploid plant having 36 somatic chromosomes. Its karyotype (C) is different from A and B. Neither this species nor SIMONET's I. japonica is to be considered as the direct parent of our I. japonica Thunb.
4. I. formosana Ohi is also a diploid plant having 28 somatic chromosomes. Not only its karyotype (D) but also the number of the chromosomes are different from those of other species. This type may be intimately related to the ancestral type of other species. The absence of terminal-constriction chromosomes in this karyotype and their presence in other species suggests that the increase of the chromosome number in the latter is mostly due to the fragmentation of the chromosomes which occurred at the attachment constriction.
5. I. japonica and its allied species show an aneuploid series, 2n=28, 34, 36, and an allotriploid 2n=54. They all show different karyotypes.
6. Aneuploidy may appear occasionally in any plant, but in that which propagates by seeds only, the aberrant chromosomes will be easily eliminated through meiosis. Therefore the survival of the aneuploid series in nature is probably due to the capability of vegetative propagation in these plants.
