Visualization of chromosome condensation in plants with large chromosomes

Abstract

Background: Most data concerning chromosome organization have been acquired from studies of a small number of model organisms, the majority of which are mammals. In plants with large genomes, the chromosomes are significantly larger than the animal chromosomes that have been studied to date, and it is possible that chromosome condensation in such plants was modified during evolution. Here, we analyzed chromosome condensation and decondensation processes in order to find structural mechanisms that allowed for an increase in chromosome size.

Results: We found that anaphase and telophase chromosomes of plants with large chromosomes (average 2C DNA content exceeded 0.8 pg per chromosome) contained chromatin-free cavities in their axial regions in contrast to well-characterized animal chromosomes, which have high chromatin density in the axial regions. Similar to animal chromosomes, two intermediates of chromatin folding were visible inside condensing (during prophase) and decondensing (during telophase) chromosomes of Nigella damascena: approximately 150 nm chromonemata and approximately 300 nm fibers. The spatial folding of the latter fibers occurs in a fundamentally different way than in animal chromosomes, which leads to the formation of chromosomes with axial chromatin-free cavities.

Conclusion: Different compaction topology, but not the number of compaction levels, allowed for the evolution of increased chromosome size in plants.

Keywords: Chromonema; Chromosome; Condensation; Evolution; Mitosis; Plants.

Fig. 1

Two variants of chromosome organization in plants. a Telophase chromosomes of the common bean (Phaseolus vulgaris) as an example of chromosomes without axial chromatin-free cavities. b Telophase chromosomes of N. damascena with clearly visible axial chromatin-free cavities (arrows). c The presence of axial chromatin-free cavities depends on the genome and chromosome size. Blue dots represent plants in which chromosomes do not contain axial chromatin-free cavities; red dots represent plants in which chromosomes contain axial chromatin-free cavities. Scale bar: 0.5 μm

Fig. 2

Morphology of the mitotic chromosomes of N. damascena. Left and central panels represent fluorescence microscopy images of DAPI stained semi-thin sections (general view and fragment); right panel represents a density plot through the line in the central panels. a Early prophase (chromosomes are indicated by arrowheads). b Middle prophase (fibers forming chromosomes, which seem to correspond to early prophase chromosomes, are indicated by arrowheads). c Late prophase. d Metaphase. e Anaphase. f Early telophase (axial chromatin-free cavities are indicated by arrows, fibers forming telophase chromosomes are indicated by arrowheads). g Late telophase. h G₁-phase. Scale bars: 1 μm

Fig. 3

Chromosome labeling with EdU. a Labeled region localization and morphology revealed the principle of prophase chromosome condensation. Linearly arranged, labeled chromosome regions during the transition from early to late prophase would either lose the linearity arrangement (folding) or retain the linear arrangement (thickening). b Three patterns of EdU incorporation were detected in the chromosomes: labeling of discrete regions (pattern 1), labeling of chromosome arms but not centromeres (pattern 2) and labeling of both chromosome arms and centromeres (pattern 3). c Frequencies of different labeling patterns at different time points after EdU incorporation. Scale bars: 5 μm

Fig. 4

Chromosome condensation/decondensation during mitosis of N. damascena (mitotic cells whose chromosomes included EdU during late S-phase). a At early prophase, labeled regions were linearly distributed in thin chromosomes, spanning the chromosome width almost entirely. b At late prophase, labeled regions were scattered throughout the chromosome volume. c At anaphase, the labeling pattern was similar to that of late prophase chromosomes. d At late telophase, decondensation revealed thin fibers forming chromatids inside which labeled regions were distributed similarly to that inside early prophase chromosomes. Scale bars: 1 μm

Fig. 5

Electron microscopy morphometry of N. damascena chromosomes: chromosome condensation from interphase to metaphase. Left and central panels show ultrastructural organization (general view and fragment), right panels shows histograms depicting chromosome and chromatin fiber width distributions. a Interphase. b Preprophase. c Early prophase. d Late prophase. e Metaphase. The typical cross-sections of the chromosomes and/or chromatin fibrils are indicated with colored lines: red – interphase chromonemata (heterochromatin), green – chromonemata; blue – ‘300 nm fibers’; black – chromosomes. Scale bars: 1 μm

Fig. 6

Electron microscopy morphometry of N. damascena chromosomes: chromosome decondensation from anaphase to G₁-phase. Left and central panels show ultrastructural organization (general view and fragment), right panel shows histograms depicting chromatid and chromatin fiber width distributions. a Anaphase. b Early telophase. c Late telophase. d G₁-phase. The typical cross-sections of the chromosomes and/or chromatin fibrils are indicated with colored lines: red – interphase chromonemata, green – chromonemata; blue – ‘300 nm fibers’; black – chromatids. Scale bars: 1 μm

Fig. 7

Models for chromosome condensation. a Chromatid organization in mammals. b Chromatid organization in plants with large chromosomes