Abstract
BackgroundMost 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.ResultsWe 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.ConclusionDifferent compaction topology, but not the number of compaction levels, allowed for the evolution of increased chromosome size in plants.
Highlights
Most data concerning chromosome organization have been acquired from studies of a small number of model organisms, the majority of which are mammals
Recent data indicate that nucleosome mobility and the interaction of neighboring nucleosomes with each other leads to chromatin melting [10, 11], where 10 nm fibers are irregularly folded without the formation of a 30 nm chromatin fiber [10,11,12,13,14,15]
Axial chromatin-free cavities appear in plants with average 2C DNA content exceeded 0.8 pg per a chromosome Anaphase and telophase chromatids of some plants contain chromatin-free cavities in the axial regions, which distinguish these plants from the majority of other plants and animals investigated to date (Fig. 1a, b)
Summary
Most data concerning chromosome organization have been acquired from studies of a small number of model organisms, the majority of which are mammals. Most data about internal chromosome organization have been acquired from the studies of a small number of model organisms, the majority of which are mammals (human, mouse, Chinese hamster, etc.) that have relatively small genomes. The genome size in some plants and animals (e.g., in Urodela) is substantially larger than in mammals [1, 2]. The largest genome in plants that has been described is that of Paris japonica (1C = 152.23 pg) [3] It is 50 times larger than that of a human monoploid genome (1C = 3.50 pg). The interaction between the nucleosomes and linker histones leads to the formation of chromatin fibers with a diameter of 30 nm. Recent data indicate that nucleosome mobility and the interaction of neighboring nucleosomes with each other leads to chromatin melting [10, 11], where 10 nm fibers are irregularly folded without the formation of a 30 nm chromatin fiber [10,11,12,13,14,15]
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