Understanding the relationship between macro- and microevolutionary processes and their delimitation remains a challenge. This review focuses on the role of chromosomal rearrangements in plant population differentiation and lineage diversification resulting in speciation, helping bridge the gap between macro- and microevolution through chromosomal evolution. We focus on angiosperms, a group that comprises the majority of extant plant species diversity and exhibits the largest chromosomal and genomic variations. Here, we address the following questions: Are macroevolutionary patterns of chromosome evolution the result of accumulated microevolutionary changes, or do chromosomal dynamics drive larger shifts along the speciation continuum? At the macroevolutionary level, we investigated the association between karyotype diversity and diversification rates using evidence from comparative genomics, chromosomal evolution modelling across phylogenies, and the association with several traits across different angiosperm lineages. At the microevolutionary level, we explore if different karyotypes are linked to morphological changes and population genetic differentiation in the same lineages. Polyploidy (autopolyploidy and allopolyploidy) and dysploidy are known drivers of speciation, with karyotypic differences often leading to reproductive barriers. We found that dysploidy, involving gains and losses of single chromosomes with no significant change in overall content of the genome, appears to be relatively more frequent and persistent across macroevolutionary histories than polyploidy. Additionally, chromosomal rearrangements that do not entail change in chromosome number, such as insertions, deletions, inversions, and duplications of chromosome fragments, as well as translocations between chromosomes, are increasingly recognized for their role in local adaptation and speciation. We argue that there is more evidence linking chromosomal rearrangements with genetic and morphological trait differentiation at microevolutionary scales than at macroevolutionary ones. Our findings highlight the importance of selection across evolutionary scales, where certain chromosomal dynamics become fixed over macroevolutionary time. Consequently, at microevolutionary scales, chromosome rearrangements are frequent and diverse, serving as key drivers of plant diversification and adaptation by providing a pool of variation from which beneficial chromosomal changes can be selected and fixed by evolutionary forces.

• Keywords
• angiosperms, chromosome, dysploidy, evolution, polyploidy, speciation,
• Publication type
• Journal Article MeSH
• Review MeSH

The evolution of eukaryotic genomes is accompanied by fluctuations in chromosome number, reflecting cycles of chromosome number increase (polyploidy and centric fissions) and decrease (chromosome fusions). Although all chromosome fusions result from DNA recombination between two or more nonhomologous chromosomes, several mechanisms of descending dysploidy are exploited by eukaryotes to reduce their chromosome number. Genome sequencing and comparative genomics have accelerated the identification of inter-genome chromosome collinearity and gross chromosomal rearrangements and have shown that end-to-end chromosome fusions (EEFs) and nested chromosome fusions (NCFs) may have played a more important role in the evolution of eukaryotic karyotypes than previously thought. The present review aims to summarize the limited knowledge on the origin, frequency, and evolutionary implications of EEF and NCF events in eukaryotes and especially in land plants. The interactions between nonhomologous chromosomes in interphase nuclei and chromosome (mis)pairing during meiosis are examined for their potential importance in the origin of EEFs and NCFs. The remaining open questions that need to be addressed are discussed.

• MeSH
• Genomics MeSH
• Karyotype MeSH
• Meiosis MeSH
• Evolution, Molecular * MeSH
• Polyploidy * MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Review MeSH

Deserts exert strong selection pressures on plants, but the underlying genomic drivers of ecological adaptation and subsequent speciation remain largely unknown. Here, we generated de novo genome assemblies and conducted population genomic analyses of the psammophytic genus Pugionium (Brassicaceae). Our results indicated that this bispecific genus had undergone an allopolyploid event, and the two parental genomes were derived from two ancestral lineages with different chromosome numbers and structures. The postpolyploid expansion of gene families related to abiotic stress responses and lignin biosynthesis facilitated environmental adaptations of the genus to desert habitats. Population genomic analyses of both species further revealed their recent divergence with continuous gene flow, and the most divergent regions were found to be centered on three highly structurally reshuffled chromosomes. Genes under selection in these regions, which were mainly located in one of the two subgenomes, contributed greatly to the interspecific divergence in microhabitat adaptation.

• Keywords
• chromosomal structural variation, desert plants, microhabitat divergence, polyploidization,
• MeSH
• Brassicaceae classification   genetics   physiology   MeSH
• Ecosystem * MeSH
• Phylogeny MeSH
• Adaptation, Physiological genetics   MeSH
• Genome, Plant * MeSH
• Polyploidy MeSH
• Genetic Speciation * MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH

Pervasive hybridization and whole-genome duplications (WGDs) influenced genome evolution in several eukaryotic lineages. Although frequent and recurrent hybridizations may result in reticulate phylogenies, the evolutionary events underlying these reticulations, including detailed structure of the ancestral diploid and polyploid genomes, were only rarely reconstructed. Here, we elucidate the complex genomic history of a monophyletic clade from the mustard family (Brassicaceae), showing contentious relationships to the early-diverging clades of this model plant family. Genome evolution in the crucifer tribe Biscutelleae (∼60 species, 5 genera) was dominated by pervasive hybridizations and subsequent genome duplications. Diversification of an ancestral diploid genome into several divergent but crossable genomes was followed by hybridizations between these genomes. Whereas a single genus (Megadenia) remained diploid, the four remaining genera originated by allopolyploidy (Biscutella, Lunaria, Ricotia) or autopolyploidy (Heldreichia). The contentious relationships among the Biscutelleae genera, and between the tribe and other early diverged crucifer lineages, are best explained by close genomic relatedness among the recurrently hybridizing ancestral genomes. By using complementary cytogenomics and phylogenomics approaches, we demonstrate that the origin of a monophyletic plant clade can be more complex than a parsimonious assumption of a single WGD spurring postpolyploid cladogenesis. Instead, recurrent hybridization among the same and/or closely related parental genomes may phylogenetically interlink diploid and polyploid genomes despite the incidence of multiple independent WGDs. Our results provide new insights into evolution of early-diverging Brassicaceae lineages and elucidate challenges in resolving the contentious relationships within and between land plant lineages with pervasive hybridization and WGDs.

• Keywords
• chromosome rearrangements, diploidization, dysploidy, hybridization, phylogenetics, polyploidy, reticulate evolution, whole-genome duplication,
• MeSH
• Biological Evolution * MeSH
• Brassicaceae genetics   MeSH
• Chromosomes, Plant * MeSH
• Gene Duplication MeSH
• Genome, Plant * MeSH
• Hybridization, Genetic MeSH
• Polyploidy * MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH