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By means of morphological and chromosomal analyses it is considered that the Himalayan species Trillium govanianum is an intergeneric hybrid between Trillium and Daiswa. Trillium govanianum is an allotetraploid (4x=20) that most likely has originated from the 10 GG genome of a plant in the genus Trillium and the 10 DD genome of a plant from the genus Daiswa. Factors concerning the emergence of the polyploid species have been discussed from an evolutionary viewpoint and on a consideration of the paleoclimatics in the Himalayan mountain region.

Chromosome analysis by the cold-induced heterochromatin banding method was carried out for populations of the American species Trillium undulatum. The banded karyotypes of individuals within the species were polymorphic. Furthermore, a comparative study showed that the genome of T. undulatum has a close relationship with the G genome of the Asian species T. govanianum. It is an interesting evolutionarily problem to speculate on how the American and Asian species of Trillium species connected in both continents across the Pacific Ocean.

1) It was shown that megasporogenesis of Trillium kamtschaticum Pall. took place, as in other species of Trillium, after Allium-type.2) A method of analysis of the mode of disjunction of bivalent chromosomes at the first meiotic anaphase was described with “half tetrads” of the EMCs of this plant.3) The “half tetrad” analysis was carried out with ten individuals having heterozygosity in differential segments of chromosomes. There could be found no post-reductional disjunction among thirty-one heterozygous chromosome pairs detected in these individuals

The mechanisms of sterility were investigated with three interspecific hybrid species of Trillium found in Japan.There were three possible mechanisms concerned in sterility operative during microsporogenesis of these plants: 1) univalent formation, 2) non-dis-junction of dicentric and acentric chromatids produced in bivalents formed by a pair of corresponding chromosomes belonging to a pair of different genomes included in the hybrids, and 3) random assortment of chromosomes in dyads due to random disjunction of the bivalents formed by heterogenetic chromosome pair.The first mechanism is effective in hybrids with an odd number of different gametic sets. The latter are operative irrespective of polyploid of hybrids.It was assumed from the above that all possible interspecific hybrids of Japanese Trillium, both living and extinct, are sterile due to the above mentioned mechanisms.The origin of the dicentric chromatids, which are the cause of the second sterile mechanism, was discussed on the basis of comparative examinations of the types and frequencies of the dicentrics in all the fertile and sterile species of Japanese Trillium. The conclusion was reached that a majority of the dicentrics originate from inversion crossing-over at prophase. The simultaneous chromatid breakages at chiasmata in metaphase bivalents and the subsequent illegitimate reunion of the broken ends sometimes deform the configuration of chromatids at chiasmata in such a way as that described by Matsuura (1950). But the deformation seems to be facilitated only when the dicentrics produced by inversion crossing-over pre-exist.

1. In both Japanese and American species of Trillium, H-segments are over-contracted through interphase and up to pachytene of meiosis. They are under-contracted throughout mitosis at low temperatures. At all other stages and in most other conditions, they are indistinguishable from euchromatin. Cold inhibited contraction of H-segments does not recover before late anaphase, even if the temperature is raised.2. There are indications that the euchromatin can be converted to heterochromatin, and that the contraction of the euchromatin associated with H-segments is affected by the presence of heterochromatin.3. The visible degree of allocycly (i.e. differential contraction) of particular segments differs according to the genetically determined nature of the segment, its position and length, the heterochromatin content of the whole nucleus, the extent of the cold-treatment, the fixative used and the degree of squashing.4. The several apparent differences between the heterochromatin of American and Japanese species result from the difference in distribution of the H-segments and of the chiasmata.5. These observations, and those of previous authors, are discussed in relation to the possible nature and function of the H-segments, and their differential contraction.

On the bases of a series of studies on karyotypes, chromosome pairing and external characters, the genomic constitutions of all the Japanese members of the genus Trillium were suggested as follows:Plant Chromosome Genomnumber pairingT. kamtschaticum 2x=10 5II K1K1T. Hagae 3x=15 5II+5I K1K2TT. Hagae 6x=30 15II K1K1K2K2TTT. Tschonoskii 4x=20 10II K2K2TTT. Smallii 4x=20 10II SSxxT. ainabile 6x=30 15II KKSSxxIn the above tabulation, x represents a genom undefined as yet. K1 and K2 are genoms highly homologous, but not identical, at least, chromosome morphologically. These two genoms are comprised by an collective symbol K, when the distinction between them is not necessary in description or is not definitive in the present status of investigation.The writer wishes to express his gratitude to Professor H. Matsuura of Hokkaido University for his valuable suggestions in the course of the work. Expense of the present work was partly defrayed by a Grant in Aid for Fundamental Scientific Research of the Ministry of Education.

1. The degree of differentiation attained after chilling varies from one H-segment to another in Trillium, both between and within the species. These can be grouped into four degrees:1° Slight (e.g. T. stylosum)2° Moderate (e.g. most Trillium segments)3° HIGH (e.g. Paris polyphylla)4° Extreme (e.g. T. undulatum and Japanese species)2. H-segments lose their differential appearance when chilled chromosomes are blocked at metaphase and returned to normal temperatures. This would seem to be due to a synthesis of DNA by H-segments, which are regarded as being retained in an early prophase condition. There is evidence that all H-segments from metaphase to late anaphase are capable of such synthesis.3. Differential reactivity in polyploid cells and plants is identical with that in its diploid counter-part.4. Chemical treatment of Trillium chromosomes has failed to reveal H-segments. This might be due to the chemical treatments being nonspecific for heterochromatin. All the chemical methods suggested in the literature seem to be variable and therefore unreliable in their results.