Large-scale changes in chromosome number have been associated with diversification rate shifts in many lineages of plants. For instance, several ancient rounds of polyploidization events have been inferred to promote genomic differentiation and/or isolation and, consequently, angiosperm diversification. Dysploidy, although less studied, has been suggested to also play an important role in angiosperm diversification. In this article, we aim to elucidate the role of chromosomal rearrangements on lineage diversification by analyzing a new comprehensive sedge (Cyperaceae) phylogenetic tree. Our null hypothesis is that the mode and tempo of chromosome evolution are to be homogeneous across the complete phylogeny. In order to discern patterns of diversification shifts and chromosome number changes within the family tree, we tested clade-specific chromosome evolution models for several subtrees according to previously reported increments of diversification rates. Results show that a complex, heterogeneous model composed of different clade-specific chromosome evolution transitions are significantly supported against the null hypothesis of a model with no chromosome number model transition events along the phylogeny. This could suggest a link between diversification and changes in chromosome number evolution although other possibilities are not discarded. Our methodological approach may allow identifying different patterns of chromosome evolution, as found for Cyperaceae, for other lineages at different evolutionary levels.

Changes in holocentric chromosome number due to fission and fusion have direct and immediate effects on genome structure and recombination rates. These, in turn, may influence ecology and evolutionary trajectories profoundly. Sedges of the genus Carex (Cyperaceae) comprise ca. 2000 species with holocentric chromosomes. The genus exhibits a phenomenal range in the chromosome number (2n = 10 − 132) with almost not polyploidy. In this study, we integrated the most comprehensive cytogenetic and phylogenetic data for sedges with associated climatic and morphological data to investigate the hypothesis that high recombination rates are selected when evolutionary innovation is required, using chromosome number evolution as a proxy for recombination rate. We evaluated Ornstein–Uhlenbeck models to infer shifts in chromosome number equilibrium and selective regime. We also tested the relationship between chromosome number and diversification rates. Our analyses demonstrate significant correlations between morphology and climatic niche and chromosome number in Carex. Nevertheless, the amount of chromosomal variation that we are able to explain is very small. We recognized a large number of shifts in mean chromosome number, but a significantly lower number in climatic niche and morphology. We also detected a peak in diversification rates near intermediate recombination rates. In combination, these analyses point toward the importance of chromosome evolution to the evolutionary history of Carex. Our work suggests that the effect of chromosome evolution on recombination rates, not just on reproductive isolation, may be central to the evolutionary history of sedges.

The evolution of intrinsic barriers to gene flow is a crucial step in the process of speciation. Chromosomal changes caused by fusion and fission events are one such barrier and are common in several groups of Lepidoptera. However, it remains unclear if and how chromosomal changes have contributed to speciation in this group. I tested for a phylogenetic signal of varying chromosome numbers in Erebia butterflies by combining existing sequence data with karyological information. I also compared different models of trait evolution in order to infer the underlying evolutionary mechanisms. Overall, I found significant phylogenetic signals that are consistent with non-neutral trait evolution only when parts of the mitochondrial genome were included, suggesting cytonuclear discordances. The adaptive evolutionary model tested in this study consistently outperformed the neutral model of trait evolution. Taken together, these results suggest that, unlike other Lepidoptera groups, changes in chromosome numbers may have played a role in the diversification of Erebia butterflies.

A complete chromosomal fusion–fission cycle is described for the first time. In the field bean, Vicia faba, this cycle probably started with a reversible fusion of two telocentrics giving rise to the standard metacentric chromosome I. The next step was a recent fission of this chromosome into two stable telocentrics eventually followed by a new fusion reconstituting the metacentric chromosome.Key words: karyotype evolution, chromosome number, chromosomal fusion–fission cycle, Vicia faba.

Introduction Whole genome duplication events (WGDs) have been recognized as major drivers of evolution in plants, especially when they involve hybridization (allopolyploidization). In this study we evaluated if WGDs acted as evolutionary forces at the origin and early divergence of the Asian Palmate group (AsPG) of the plant family Araliaceae. This clade encompasses most of the generic and species diversity as well as most of the polyploids of the family, and a role of hybridization in its origin has been suggested. Methods In order to test this hypothesis, we obtained nuclear and plastid time-calibrated phylogenomic trees including 80% of Araliaceae genera (37 genera, 237 species) using the Hyb-Seq approach. The role of WGDs in the early evolution of the AsPG was tested using ancestral chromosome number reconstructions based on chromosome counts for 62% of the sampled genera, while recent polyploidization events were explored by inferring ploidy of the sequenced species from allelic frequencies. Results Phylogenetic analyses of nuclear and plastid sequences provided highly resolved but incongruent topologies consistent with ancient hybridization not only for the origin of the AsPG, but also in the second most highly diverse clade of the family. Our ancestral chromosome number reconstructions supported that one or two WGDs preceded the origin of two of the three main clades of Araliaceae (AsPG and Polyscias-Pseudopanax), which could have acted as background variables necessary for the posterior diversification of these lineages. Ploidy inference based on allelic frequencies provided signal of recent polyploidization in the AsPG and the third main clade of Araliaceae (Aralia-Panax). Conclusions In summary, WGDs are linked to the origin of the main clades of the Araliaceae family, but the drivers of the strong diversification of the AsPG remain an open question.

<h4>Background</h4>Genetic information is stored on multiple chromosomes in eukaryotic organisms and is passed on to offspring through cell division. How chromosome number influences cell division and chromosome segregation is not yet understood.<h4>Results</h4>In this study, we use artificial chromosome-fusion fission yeast cells, which contain one or two chromosomes, as models to investigate the effects of a reduced chromosome number on mitosis and meiosis. In mitosis, chromosome number reduction, particularly full fusion into one chromosome, prolongs mitotic duration in a manner dependent on the spindle assembly checkpoint and improves chromosome segregation accuracy in spindle assembly checkpoint-deficient cells. By contrast, in meiosis, chromosome number reduction impairs prophase oscillatory nuclear movement, prolongs meiosis I duration but shortens meiosis II duration, and severely compromises meiosis I chromosome segregation.<h4>Conclusions</h4>Our work uncovers different effects of reduced chromosome number on mitotic and meiotic stability and offers insights into how organisms may select the appropriate number of chromosomes in evolution.

Biodiversity: molecular biological domains, symbiosis and kingdom origins. The number of extant species of organisms is estimated to be from fewer than 3 to more than 30 x 10(6) (May, 1992). Molecular biology, comparative genetics and ultrastructural analyses provide new insights into evolutionary relationships between these species, including increasingly precise ideas of how species and higher taxa have evolved from common ancestors. Accumulation of random mutations and large macromolecular sequence change in all organisms since the Proterozoic Eon has been importantly supplemented by acquisition of inherited genomes ('symbiogenesis'). Karyotypic alterations (polyploidization and karyotypic fissioning) have been added to these other mechanisms of species origin in plants and animals during the Phanerozoic Eon. The new evolution concepts (coupled with current rapid rates of species extinction and ignorance of the extent of biodiversity) prompted this analysis of the field of systematic biology and its role in the reorganization of extant species into higher taxa.

Brachypodium distachyon is a model for the temperate cereals and grasses and has a biology, genomics infrastructure and cytogenetic platform fit for purpose. It is a member of a genus with fewer than 20 species, which have different genome sizes, basic chromosome numbers and ploidy levels. The phylogeny and interspecific relationships of this group have not to date been resolved by sequence comparisons and karyotypical studies. The aims of this study are not only to reconstruct the evolution of Brachypodium karyotypes to resolve the phylogeny, but also to highlight the mechanisms that shape the evolution of grass genomes. This was achieved through the use of comparative chromosome painting (CCP) which hybridises fluorescent, chromosome-specific probes derived from B. distachyon to homoeologous meiotic chromosomes of its close relatives. The study included five diploids (B. distachyon 2n = 10, B. sylvaticum 2n = 18, B. pinnatum 2n = 16; 2n = 18, B. arbuscula 2n = 18 and B. stacei 2n = 20) three allotetraploids (B. pinnatum 2n = 28, B. phoenicoides 2n = 28 and B. hybridum 2n = 30), and two species of unknown ploidy (B. retusum 2n = 38 and B. mexicanum 2n = 40). On the basis of the patterns of hybridisation and incorporating published data, we propose two alternative, but similar, models of karyotype evolution in the genus Brachypodium. According to the first model, the extant genome of B. distachyon derives from B. mexicanum or B. stacei by several rounds of descending dysploid

Tetraploidy, resulting from a single whole-genome duplication (WGD) event, contributes to tumorigenesis by promoting genomic instability and functional diversity. In general, WGD beyond tetraploidy limits proliferative and tumorigenic potential, but an increasing number of studies suggest that polyploid giant cancer cells (PGCCs)-large, highly polyploid (≥8N) cells formed in response to chemotherapy-produce daughter cells with reduced DNA content that drive cancer progression. In this opinion article, we examine the literature on tetraploid cells and PGCCs from a cell biology perspective. It is our opinion that the role of tetraploidy in cancer is supported by findings from cell lines, animal models, and tumor sequencing data, while definitive evidence that viable progeny from PGCCs can promote cancer progression in human tumors is lacking.