COLD-SENSITIVE CHROMOSOME REGIONS AND HETEROCHROMATIN IN CESTRUM (SOLANACEAE) - C-STRIGILLATUM, C-FASCICULATUM, AND C-ELEGANS

Cold-induced mitotic under-condensation of certain chromosome segments is a rare phenomenon in plants. There are about 11 genera of monocotyledons and only 3 of dicotyledons, where species are known to have such cold-sensitive regions (CSRs). The molecular causes of cold-induced undercondensation are not clear, and no consistent cytochemical characteristics of CSRs are known. Recently we have presented a chromosome banding analysis on CSRs and their relation to constitutive heterochromatin in Cestrum parqui (Solanaceae), a species of sect. Cestrum. The present study is concerned with a similar analysis in C. strigillatum of sect. Cestrum, and in C. fasciculatum and C. elegans of sect. Habrothamnus. Chromomycin/DAPI fluorescent double staining, sequential C-banding, and sequential silver impregnation were applied. The species differ in detail but are similar qualitatively. Four classes of heterochromatin can be discriminated. (1) CSRs, with banding properties indicating AT-rich constitutive heterochromatin. After cold-treatment CSR heterochromatin can be silver-impregnated from interphase, as chromocentres, to metaphase, as undercondensed segments. CSRs are subject to frequent heteromorphy. (2) Nucleolar organizers. Two pairs were identified in the karyotypes. Banding properties indicate GC-rich heterochromatin. The nucleolar organizing regions are less evident and their silver-reducing capability reduces during metaphase. (3) Non-nucleolar CMA-positively fluorescing bands. These are minute, polymorphic, positively C-stained, and restricted to one or a few sites in the karyotypes. (4) Indifferently fluorescing, positively C-stained bands. They occur on centromeres, some chromosome ends, and clustered over the chromosome arms. They are mostly very delicate and do not resist harsh banding treatments. - The species investigated here and C. parqui resemble each other qualitatively in heterochromatin classes (1), (2), and (3), but differ much in banding properties of class (4). Therefore, heterochromatin characteristics in the genus are not so uniform as the present results in C. strigillatum, C. fasciculatum, and C. elegans appear to show.