ニガナとその近縁属(キク科・タンポポ連)の分類学的研究 : III.CrepidiastrumとParaixerisの果皮の解剖学的および核学的特徴とその分類学的意義  [in Japanese] Biosystematic Studies of the Genus lxeris and its Allied Genera (Compositae-Lactuceae) : III. Fruit Wall Anatomy and Karyology of Crepidiastrum and Paraixeris, and their Taxonomic Implications  [in Japanese]

ニガナ属(キク科タンポポ連)に近縁とされるアゼトウナ属Crepidiastrum並びにヤクシソウ属Paraixerisの植物について,そう果の解剖学的形質並びに染色体数,核型の綿密な比較研究を行った。これら両属に属する植物はいずれも染色体の基本数はx=5,休止核の形態,prochromosome typeで基本核型は同一であること,さらに両群ともに翼をもったそう果をもつことから,これら両属の植物はニガナ属とは独立したグループを構成するが,タンポポ連の中では最も近縁な位置を占めると考えられる。さらに,今回得られた結果を綜合すると,ヤクシソム属はアゼトウナ属の一つの節として取扱うのが妥当であるとの結論に達した。命名上の問題は次報(Pak and Kawano, 1991 in press)で詳細に論ずることとする。

Mature fruit wall structures and chromosome morphology were investigated for all seven species of Crepidiastrum and five species of Paraixeris (Compositae-Lactuceae). Two genera had 10-20 obtuse costae in the transverse section of the fruit, of which five were much thicker than the remainder. Furthermore, the two genera examined agreed with each other in having basically the same costa structures, i.e., 1) sclereid-fibers lacing between the exocarp and the underlying libriform fibers, 2) libriform fiber cells occupying nearly the whole costa, and under lying fiber-sclereid tissue being only 1-3 cell layers thick, and 3) intercosta lacking fiber-sclereid layers. The chromosome morphologies of all seven species of Crepidiastrum and four species of Paraixeris were also investigated. Somatic chromosome numbers and karyotypes of Paraixeris chelidonifolia (2n=10) and P. yoshinoi (2n=10) were reported for the first time. The results of the present study clearly revealed that plants of these two genera are very similar in chromosomal characters: 1) all species are diploid (2n=10) with the same basic number of x=5; 2) the type of resting nuclei is prochromosome type; 3) they have nearly the same total genome length, i.e., 22.3μm-37.1μm in Crepidiastrum and 24.9μm-34.6μm in Paraixeris; 4) the length of each chromosome also falls in a similar size range, i.e., 1.5μm-4.6μm in Crepidiastrum and 1.7μm-4.0μm, in Paraixeris; 5) they possess the same basic karyotype complement, i.e., 2sm+3st, with the one exception of C. ameristophyllum; and 6) they have a characteristic chromosome pair (No. 1) with a secondary constriction at the distal portion of the short arms (with the one exception of P. sonchifolia). Two genera also have many additional common gross morphological features: inner and outer bracts, pappus, and flowering habit. In conclusion, the overall similarity in chromosome morphology as well as fruit wall anatomy of Crepidiastrum and Paraixeris clearly indicates that they belong to a distinct phylogenetic group among the Crepis-series (JEFFERY, 1966). considering all these common characters of Crepidiastrum and Paraixeris, these two genera should be united and referred to a single genus, i. e., Crepidastrum.