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Phaseolus polytene chromosomes

Nagl, W., Dept. of Biology, University of Kaiserslautern, Aug 1996

Polytene chromosomes

Polytene chromosomes represent a special structural organization of polyploid interphase nuclei, which originate by repeated endoreduplication cycles. A prerequisite for polytenization is the absence of any kind of mitotic chromosome condensation during cell growth. Independent of their species-specific structure, polytene chromosomes are characteristic of terminally differentiated cells of high and specialized synthetic activity. Although polyteny is best known from larval salivary glands and other tissues in dipters, essentially the same nuclear organization can be found in many different taxa throughout the animal and plant kingdoms (for reviews and further information see below).

Although plant polytene chromosomes correspond to those of dipters in respect to their development, genetic organization and function, their morphology normally is somewhat different. Their banding pattern is poorly developed, their degree of condensation varies considerably, and they are - in most cases - not somatically paired. Their general appearance differs between species, what can be primarily understood on the basis of different species-specific structures of the diploid interphase nuclei (Nagl et al., 1983; Nagl, in press). For reviews and further information see below.

Polytene chromosomes in Phaseolus

In Phaseolus coccineus and P.vulgaris (and sometimes in different other species), polyteny occurs in the basal giant cells of the embryo-suspensor (review: Nagl, 1974). The suspensor, a short-lived differentiation of the embryo proper, deals both as transfer system for nutrients and as an organ of high sythetic activity, including phytohoromes (Nagl, 1985). It develops early in embryogenesis and undergoes autolysis before the seed ripens.

In allmost all cells, the polytene chromosomes display a granular euchromatic structure with some fuzzy bands, and distinct heterochromatic regions. A clear banding pattern is found very rarely. It particularly occurs under conditions of inhibiting transcription. Fig. 1 shows a banded polytene chromosome of P.vulgaris in comparison with mitotic chromosomes. Except the nucleolus-organizing chromosomes, the Phaseolus mitotic and polytenic chromosomes are hardly distinguished.

Picture of Phaseolus polytene and mitotic metaphase chromosomes Fig. 1. a) An in part banded polytene chromosome of Phaseolus vulgaris cv. Hilds Marona. The plants were chilled bevore harvesting to respectively 12°C and 8°C at a day/night regimen of 13/11 hrs for two weeks (Nagl, 1969). Fixed with ethanol/acetic acid (3:1), phase contrast, 1500 x. b) Mitotic chromosomes from a root tip after colchicine treatment. Feulgen, 1500 x.
Foto kindly provided by Prof. W. Nagl 1996.


Therefore, various additional methods, such as fluorescence in situ hybridization, are now used to establish a polytene karyotype (Nenno et al.,1993, 1994). Fig. 2 shows the idiogram of the preliminary polytene karyotype and the localization of the ribosomal RNA genes (rDNA) and phaseolin genes used as molecular markers.

Preliminary polytene karyotype

Fig. 2
Idiogram of the preliminary polytene karyotype of Phaseolus coccineus. Hybridization sites of DNA probes for the 18S-5.8S-25S ribosomal RNA genes (R) and phaseolin genes (P) used as molecular markers. Foto kindly provided by Prof. W. Nagl 1996.


Additional references

Reviews and further information