The centromere is the chromosome region where microtubules attach during cell division. In contrast to monocentric chromosomes with one centromere, holocentric species usually distribute hundreds of centromere units along the entire chromatid. We assembled the chromosome-scale reference genome and analyzed the holocentromere and (epi)genome organization of the lilioid Chionographis japonica. Remarkably, each of its holocentric chromatids consists of only 7 to 11 evenly spaced megabase-sized centromere-specific histone H3-positive units. These units contain satellite arrays of 23 and 28 bp-long monomers capable of forming palindromic structures. Like monocentric species, C. japonica forms clustered centromeres in chromocenters at interphase. In addition, the large-scale eu- and heterochromatin arrangement differs between C. japonica and other known holocentric species. Finally, using polymer simulations, we model the formation of prometaphase line-like holocentromeres from interphase centromere clusters. Our findings broaden the knowledge about centromere diversity, showing that holocentricity is not restricted to species with numerous and small centromere units.

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The centromere is the chromosome region where the microtubules attach during cell division. In contrast to monocentric chromosomes with one centromere location, holocentric species usually distribute hundreds of centromere units along the entire chromatid. We assembled the chromosome-scale reference genome and analyzed the holocentromere and (epi)genome organization of the lilioid Chionographis japonica. Remarkably, each of its holocentric chromatids consists of only 7 to 11 evenly-spaced megabase-sized centromere-specific histone H3-positive units. These units contain satellite arrays of 23 and 28 bp-long monomers capable of forming palindromic structures. Like monocentric species, C. japonica forms distinctly clustered centromeres in chromocenters at interphase. Additionally, the large-scale eu- and heterochromatin arrangement differs between C. japonica and other known holocentric species. Using polymer simulations, we modeled the formation of prometaphase line-like holocentromeres from interphase centromere clusters. Our findings broaden the knowledge about the diversity of centromere organization, showing that holocentricity is not restricted to species with numerous and small centromere units.

Pelargonium is a versatile genus mainly from the Cape Region, South Africa. The genus is divided into four subgenera and 16 sections characterized by several groups of chromosomes sizes and numbers. The DNA content of species from all subgenera and sections of Pelargonium, except for the sections Subsucculentia and Campylia was estimated using flow cytometry. Nuclei of Pelargonium samples (leaf or petal tissue) and an internal plant standard (leaf tissue) were isolated together and stained with propidium iodide. The DNA content was estimated providing that the 2C peaks of sample and standard be in linearity in the flow cytometer histograms. In total, 96 Pelargonium accessions of 60 species (22 Pelargonium species for the first time) were analyzed. The 2C DNA content ranged from 0.84 pg (P. longifolium, section Hoarea) to 6.69 pg (P. schizopetalum, section Magnistipulacea) and the corresponding 1Cx DNA content from 0.42 pg (P. longifolium) to 1.72 pg (P. transvaalense. This demonstrates the high plasticity within the genus Pelargonium. Some species, such as P. peltatum accessions revealed a pronounced endopolyploidization in leaves but not in petals underlining the importance to choose the right tissue as sample for the flow cytometry analysis. The reported genome sizes are a step forward towards the characterization of the Pelargonium collection within the German Gene Bank for Ornamental Plants and a valuable base for future sequencing programs of the Pelargonium genomes.

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