ABSTRACT
Polytene chromosomes are a special structure of polyploid interphase nuclei. This kind of chromatin organization originates by repeated endoreduplication cycles, i.e., cell cycles in which DNA replication is not followed by mitosis. As polytene chromosomes occur in terminally differentiated cells of high and specialized function, i.e., in plants particularly in tissues related to embryo nutrition, they offer also unique possibilities for studying the process of early differentiation. Although polytene chromosomes are best known from dipteran salivary glands, essentially the same nuclear organization is found in many different taxa throughout the plant and animal kingdoms, e.g., in angiosperms. Although plant polytene chromosomes fully correspond to those of dipters in respect to their development and genetic organization, their morphology frequently is different, because their banding pattern is poorly developed, their condensation level is rather high, and they are normally not somatically paired. Fluorochromes displaced bright field stains for the study of polytene chromosomes in a sense.
