COLD-SENSITIVE CHROMOSOME REGIONS AND THEIR RELATION TO CONSTITUTIVE HETEROCHROMATIN IN CESTRUM-PARQUI (SOLANACEAE)

The structural cause for cold-induced undercondensation of certain chromosome segments is still unknown, and no consistent cytochemical correlations have emerged as yet. Cestrum parqui belongs to one of the few plant genera having chromosomes with such cold-sensitive regions. In the present study, four heterochromatin classes were distinguished in the karyotype of this species by sequential fluorochrome staining with chromomycin A3, DAPI, and D 287/170, and C-banding, N-banding, and silver impregnation. (i) Cold-sensitive regions (CSRs): Banding behaviour indicates AT-rich constitutive heterochromatin. Silver impregnation occurs after cold-treatment from interphase to metaphase. CSRs are subject to polymorphisms. Quantitatively they are only a minor component of the band endowment. (ii) Nucleolar organizers: Two pairs were identified. Banding behaviour indicates GC-rich constitutive heterochromatin. The nucleolar constrictions proper are morphologically indistinct at mitotic metaphase and lose silver impregnation capability at that stage. (iii) Non-nucleolar CMA-positively fluorescent C-bands: There is only one polymorphic paracentromeric band in this class. (iv) Low-fluorescence N-bands. These show quenched fluorescence with all fluorochromes and occur as clusters of small bands scattered over all chromosome arms. This class of bands is quantitatively predominating. Dull fluorescence with both the ''GC-specific'' chromomycin and the ''AT-specific'' DAPI is an anomaly but not completely unknown in plants.