Chromosome change and karyotype differentiation–implications in speciation and plant systematics

Abstract

Karyotype characterizing the phenotypic aspects of the chromosome complement represents structural and functional organization of the nuclear genome. Its constancy ensures transfer of the same genetic material to the next generation, while variation enables ecological differentiation and adaptation. Superimposition of karyotype information onto a phylogenetic framework has immense utility in elucidating direction of evolutionary change and delineation of taxonomic hierarchy. This article attempts to provide an illustrated description of the chromosomal features that are useful in discerning differences and affinities between species and taxa. A detailed account of experimental parameters useful in chromosome identification and evaluation of karyotype asymmetry is furnished citing suitable examples. In addition to various karyotypic indices, special emphasis is given to the quantitative parameter of “chromosome dispersion index (DI)” that promises phylogenetic differentiation of closely related karyotypes, since most genera of herbaceous angiosperms display interspecific differences in chromosome size and symmetry, if not number. Karyo-evolutionary trends involve both change in chromosome number, morphology/karyotype symmetry, ploidy and total haploid length. The pattern of DNA addition/deletion across the chromosome complement has been found to be variable. Such change in DNA is either equally shared by all the chromosomes or is proportionately shared commensurate to chromosome size, leading to differential pace of change in karyotype asymmetry across the taxa. The large data accumulating on chromosome number over the years offer opportunities to utilize them as additional tools in taxonomy. The basic chromosome number (ancestral haploid number) in angiosperms has been suggested as n = 7, and an ancestral 1C of 1.73 pg, which is characteristic of the major groups with slight deviation in certain orders. The average ‘holoploid genome size’ i.e. 1C, for the Angiosperms is inferred to be 5870 Mb/6.0 pg. However, the 1C-value data available for ca.10800 species of Angiosperms reveal genome size diversity ranging from 61 Mb/0.0648 pg in the carnivorous plant Genlisea tuberosa (Lentibulariaceae) to 1,49,000 Mb/152.23 pg in Paris japonica (Melanthiaceae) suggesting over 2400 fold variation across the angiosperms; and 230 fold variation within the family (0.66 pg in Schoenocaulon texanum vs. P. japonica with highest C value). Metaphase chromosome size is estimated to range from the shortest ~ 0.3 µm in Genlisea aurea, and at family level from ~ 0.8 µm in Chamaelirium luteum to ~ 30 µm in Paris japonica. Chromosome number ranges from n = 2 to 320 across the angiosperms, at family level from n = 4 to 120 in the Brassicaceae, and at genus level from n = 2 to 45 in Brachyscome (Compositae).The evolution has mostly been at the diploid level. It is generally believed that the chromosome size of monocots is larger than dicots and the chromosome size of temperate plants is larger than tropical plants. For a broader sense and larger scale understanding, the evolutional conception of karyotype is principally based on the thoughts that: (i) symmetrical karyotype is more primitive than asymmetrical ones, (ii) longer chromosomes are primitive than shorter ones, (iii) median centromeres with chromosome arms of equal length are more primitive than chromosome arms of unequal length, (iv) low basic numbers had given rise to higher ones, and the taxa with variable chromosome number are considered young and still in evolutionary flux, (v) species with one NOR site per haploid genome are considered advanced than multiple NOR sites, (vi) ancient species had less heterochromatin (repetitive DNA), the primitive species accredited heterochromatin, followed by gradual shedding of excess heterochromatin with evolutionary specialization. However, discovery of ancient episodes of Whole Genome Duplication (WGD) events said to have happened as an escape to the 5th mass extinction at the end of Cretaceous as a survival strategy has opened newer possibilities. A new thinking is beginning to emerge that concomitant with the climate change happening at a fast pace in the Anthropocene, it is likely that if global climate undergoes major change in coming centuries then auto-polyploidization could be the important player leading to increase in chromosome number.