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THE DISTRIBUTION OF HETEROCHROMATIN IN THE GENUS NICOTIANA (SOLANACEAE)

A B S T RA CT A survey of the species of the genus Nicotiana was carried out to determine the distribution and the cytological characteristics of heterochromatin in this genus. All examined species of the genus possess knob-type heterochromatin, which is defined as spherical, densely staining regions of the pachytene chromosomes. These knobs are most frequently located near the centromere, the nucleolar organizer, and the ends of the chromosomes. Block-type heterochromatin, defined as any longer-than-broad heterochromatic segment seen at pachytene, was found in three species of the section Paniculatae, three species of the section Tomentosae, and two species of the section Noctiflorae. Three categories of the block-type heterochromatin, corresponding to the three subgenera, were found to differ with respect to overall size, staining properties, and location of the blocks. The distribution of these three types of block heterochromatin is discussed in the light of the latest taxonomic treatments of the genus.