A fascinating island: 2n = 4

Abstract

Among Angiosperms, only six species are known to possess the lowest identified chromosome number, i.e. 2n = 4. These plants are the monocotyledons Zingeria biebersteiniana, Colpodium versicolor, Ornithogalum tenuifolium and Rhynchospora tenuis, and the dicotyledons Haplopappus gracilis and Brachyscome dichromosomatica. The low chromosome number may be cytogenetically derived from a different ancestral basic number, characteristic of each genus, by different processes, including tandem fusion or unequal reciprocal translocations with loss of centric fragments. All these plants possess low nuclear 4C DNA contents, ranging from 1.56 pg (R. tenuis) to 8.20 pg (H. gracilis), and they generally display a similar chromosome size and a similar position of the nucleolus organising region (NOR), that is often located in the terminal or subterminal region of the small chromosome pair. All 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 B. dichromosomatica. Plants with a very low chromosome number may be considered to constitute a fascinating “island of interest”; moreover, they represent simple systems helpful for the examination of the structural organisation and evolution of Angiosperm chromosomes.