Heterochromatin in American and Japanese Species of Trillium

I. Fusion of chromocentres and the distribution of H-segments

Abstract

1. Heterochromatin segments, of similar size and position, appear the same, after identical treatments, in American and Japanese species. The segments differ in distribution, being entirely proximal in Japanese species, and proximal, median or distal in American species. They differ also in the amount of intercalated euchromatin. The H-patterns of T. erectum and T. kamtschaticurn are, in a few chromosomes, the same.
2. The distribution of H-segments in a species is not related to that of nucleolar organisers, but heterozygosity for H-pattern does reflect heterozygosity for the adjoining euchromatin as well.
3. The chromocentres of resting nuclei, in the absence of any fusion, correspond with the H-segments in size and number. The fusion of H-segments to form fewer chromocentres increases i) when the nuclear volume is small, ii) when the chromosome number is low, and iii) when the segments are localised near the centromeres. These factors increase the likelihood of segments being adjacent at telophase when the chromocentres are forming.