A fascinating island: 2n=4

Among Angiosperms only six species are reported possessing the lowest identified chromosome number 2n=4. These plants are the monocotyledons Zingeria biebersteiniana, Colpodium versicolor, Ornithogalum tenuifolium, Rhynchospora tenuis, and the dicotyledons Haplopappus gracilis, Brachyscome dichromosomatica. The low chromosome number may be supposed cytogenetically derived from a different ancestral basic number, characteristic of the each genus, by different processes, including tandem fusion, unequal reciprocal translocations with loss of centric fragments. All these plants possess low nuclear 4C DNA contents, ranging from 1.56 (R. tenuis) to 8.20 (H. gracilis) pg, they display generally a similar size of chromosomes and the position of nucleolus organizing region, that is often located in the terminal or subterminal region of the small chromosome pair, evidencing that these characteristics could be a consequence of common adaptative mechanisms. Peculiar characteristics within these karyotypes are the holocentric chromosomes of R. tenuis and the presence of B chromosomes in the karyotype of B. dichromosomatica. Plants with very low chromosome number may be considered a fascinating “island of interest” comprehending simple systems helpful for the examination of the structural organisation and evolution of Angiosperm chromosomes.