Chromosome organization: curiosity in diversity

This article attempts to apprise curiosities attendant in organizational diversity and behaviour of chromosomes from academic and utilitarian prospective, pinpointing certain uncommon but unique features. The chromosomes, comprised of DNA and proteins are the major component of the nucleus in Eukaryotes, wherein about 4 cm long DNA fiber is compacted to form a chromosome of 1 µm in diameter and 10 µm in length. Chromosomes have defined territories in the nucleus and serve as the continuum for sustenance of heredity and variation both at asexual and sexual level realised through mitotic and meiotic division, respectively. Notwithstanding the above commonalities, the plants and animals significantly differ for their chromosome and cellular organization. The animal cell lacks cell-wall but owe dynamically organized well differentiated linkage groups with specific location of repeat families, in clear contrast to plants where cell has a well-defined cell-wall but lacks repeat family-based precise differentiation between the linkage groups. Curiously, the plant systems offer unique in planta opportunities to realize variation through the incidence of polyploidy, intergenomic hybridity and genomic orientation, kinetochore function, de novo somatic diversity, polysomaty, endopolyploidy and polyteny in differentiating tissues, B chromosomes that at times become a source of variation, adaptation and speciation. Drastic variations are encountered in form, function, and number of chromosomes in plants, ranging from n = 2 in Brachyscome dichromosomatica to n = 320 in Sedum suaveolens (Crassulaceae), at family level from n = 4 to 120 in the Brassicaceae, and at genus level from n = 2–45 in Brachyscome (Asteraceae)—in angiosperms; to n = 720 in ferns. However, the cell size does matter to decide the size of chromosomes a cell can accommodate. One of the key realizations in the “genomics era” is that polyploidy is ubiquitous in plants, and that all modern flowering plant genomes are derived from processes set in motion by a history of repeated, episodic whole-genome doubling, or polyploidy. Viewpoints are underway that foresee the dominance of perennial herbs and prominence of auto-polyploids in the coming centuries if the present global trend of climate change continues at the given fast pace in Anthropocene. However, when divergent diploid genomes are combined to form an allopolyploid genome, there are instances of genomic/genetic changes to realize genome/chromosome elimination and/or gene silencing to assuage hybrid stabilization. Whereas inactivation/elimination of centromeric histones lead to chromosome/genome elimination, but spatial position of the partner genomes is largely relative to the size of their chromosomes and centromeres that facilitate intergenomic fixation.