Abstract
Mitotic prophase chromosome condensation plays an essential role in nuclear division being therefore regulated by highly conserved mechanisms. However, degrees of chromatin condensation in prophase-prometaphase cells may vary along the chromosomes resulting in specific condensation patterns. We examined different condensation patterns (CPs) of prophase and prometaphase chromosomes and investigated their relationship with genome size and distribution of histone H4 acetylated at lysine 5 (H4K5ac) in 17 plant species. Our results showed that most species with small genomes (2C < 5 pg) (Arachis pusilla, Bixa orellana, Costus spiralis, Eleutherine bulbosa, Indigofera campestris, Phaseolus lunatus, P. vulgaris, Poncirus trifoliata, and Solanum lycopersicum) displayed prophase chromosomes with late condensing terminal regions that were highly enriched in H4K5ac, and early condensing regions with apparently non-acetylated proximal chromatin. The species with large genomes (Allium cepa, Callisia repens, Araucaria angustifolia and Nothoscordum pulchellum) displayed uniformly condensed and acetylated prophase/prometaphase chromosomes. Three species with small genomes (Eleocharis geniculata, Rhynchospora pubera, and R. tenuis) displayed CP and H4K5ac labeling patterns similar to species with large genomes, whereas a forth species (Emilia sonchifolia) exhibited a gradual chromosome labeling, being more acetylated in the terminal regions and less acetylated in the proximal ones. The nucleolus organizer chromatin was the only chromosomal region that in prometaphase or metaphase could be hyperacetylated, hypoacetylated or non-acetylated, depending on the species. Our data indicate that the CP of a plant chromosome complement is influenced but not exclusively determined by nuclear and chromosomal DNA contents, whereas the CP of individual chromosomes is clearly correlated with H4K5ac distribution.
Highlights
Chromatin condensation is a major step in the cell cycle, enabling the spatial separation of each chromosome, their mobility to the equatorial plane, and correct segregation during anaphase
The smallest 2C and 2C/2n values observed here were for B. orellana and P. trifoliata (0.66 and 0.04 pg respectively) while the highest value found was with N. pulchellum (2C = 49.94 pg; 2C/2n = 4.99 pg)
A small genome size was found in Emilia sonchifolia, the only species in the present sample displaying the Hordeum-type condensation patterns (CPs)
Summary
Chromatin condensation is a major step in the cell cycle, enabling the spatial separation of each chromosome, their mobility to the equatorial plane, and correct segregation during anaphase. A quite distinct CP is observed in species with large chromosomes, such as Allium cepa and Vicia faba, which have uniformly condensed prophase chromosomes [11] These two extreme patterns will be referred to here as the Arabidopsis-type and the Allium-type respectively. Between these extremes, many intermediate CPs have been observed, displaying variable proportions of early and late condensing chromatin, as for example among species of Solanum or orchids [12, 13]. The intermediate patterns can be grouped into Solanum-type (chromosomes generally small, more similar to the Arabidopsis-type but with a higher proportion of proximal condensed chromatin) and Hordeum-type (medium sized chromosomes similar to Allium-type but less uniformly condensed during prophase). The three first genera were changed here by more widely known genera, having in mind A. thaliana, S. lycopersicum, and H. vulgare as respective models
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