Heterochromati in American and Japanese Species of Trillium

Abstract

In both Japanese and American species of Trillium, H-sgements 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. 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. The visible degree of 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. 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. 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.