Boechera falcata (Turcz.) Al-Shehbaz, previously known as Arabis turczaninowii Ledeb., is a herbaceous perennial of the East Siberian, boreal-steppe ecotype. It is the sole species of the diverse genus Boechera found on the Eurasian continent, with all other species endemic to North America and Greenland. Likely migrating from North America to Eastern Siberia via the Bering Land Bridge during the Pleistocene glaciation, B. falcata presents a unique case for genomic study. The genus Boechera is notable for its many allodiploid and triploid apomicts, which have arisen through complex hybridization of sexual species and ecotypes. To date, only the genomes of 2 American Boechera species, B. stricta and B. retrofracta, have been sequenced and analyzed. In this study, we sequenced, assembled to the chromosome level, and analyzed the highly homozygous 189.36 Mb genome of B. falcata (2n = 14). Molecular phylogenetic analysis of nuclear and organelle genomes revealed a high degree of relatedness to North American relatives. Cytogenetic analysis identified all 22 genomic blocks of crucifers, showing that 5 of the 7 B. falcata chromosomes are collinear with their ancestral counterparts, while 2 have undergone inversions. Allelic analysis of the apomixis marker APOLLO gene revealed that B. falcata contains only sex alleles. The availability of the B. falcata genome will advance studies of the evolution and phylogeny of Brassicaceae species and the mechanisms of apomixis, providing a crucial resource for future research in plant genetics and breeding.

• Keywords
• Boechera falcata, Brassicaceae, chloroplast genome, chromosome rearrangements, chromosome-level genome assembly and annotation, comparative chromosome painting, genome structure, molecular phylogeny,
• MeSH
• Brassicaceae * genetics   classification   MeSH
• Chromosomes, Plant genetics   MeSH
• Phylogeny * MeSH
• Genome, Plant * MeSH
• Genomics * methods   MeSH
• Publication type
• Journal Article MeSH

Understanding the genetic basis of reproductive isolation is a central issue in the study of speciation. Structural variants (SVs); that is, structural changes in DNA, including inversions, translocations, insertions, deletions, and duplications, are common in a broad range of organisms and have been hypothesized to play a central role in speciation. Recent advances in molecular and statistical methods have identified structural variants, especially inversions, underlying ecologically important traits; thus, suggesting these mutations contribute to adaptation. However, the contribution of structural variants to reproductive isolation between species-and the underlying mechanism by which structural variants most often contribute to speciation-remain unclear. Here, we review (i) different mechanisms by which structural variants can generate or maintain reproductive isolation; (ii) patterns expected with these different mechanisms; and (iii) relevant empirical examples of each. We also summarize the available sequencing and bioinformatic methods to detect structural variants. Lastly, we suggest empirical approaches and new research directions to help obtain a more complete assessment of the role of structural variants in speciation.

• Keywords
• hybridization, reproductive isolation, suppressed recombination,
• MeSH
• Biological Evolution MeSH
• Species Specificity * MeSH
• Phenotype MeSH
• Adaptation, Physiological MeSH
• Humans MeSH
• Evolution, Molecular MeSH
• Reproductive Isolation MeSH
• Genomic Structural Variation genetics   MeSH
• Animals MeSH
• Check Tag
• Humans MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Review MeSH
• Research Support, U.S. Gov't, Non-P.H.S. MeSH

The mustard family (Brassicaceae) comprises several dozen monophyletic clades usually ranked as tribes. The tribe Boechereae plays a prominent role in plant research due to the incidence of apomixis and its close relationship to Arabidopsis. This tribe, largely confined to western North America, harbors nine genera and c. 130 species, with >90% of species belonging to the genus Boechera. Hundreds of apomictic diploid and triploid Boechera hybrids have spurred interest in this genus, but the remaining Boechereae genomes remain virtually unstudied. Here we report on comparative genome structure of six genera (Borodinia, Cusickiella, Phoenicaulis, Polyctenium, Nevada, and Sandbergia) and three Boechera species as revealed by comparative chromosome painting (CCP). All analyzed taxa shared the same seven-chromosome genome structure. Comparisons with the sister Halimolobeae tribe (n = 8) showed that the ancestral Boechereae genome (n = 7) was derived from an older n = 8 genome by descending dysploidy followed by the divergence of extant Boechereae taxa. As tribal divergence post-dated the origin of four tribe-specific chromosomes, it is proposed that these chromosomal rearrangements were a key evolutionary innovation underlaying the origin and diversification of the Boechereae in North America. Although most Boechereae genera exhibit genomic conservatism, intra-tribal cladogenesis has occasionally been accompanied by chromosomal rearrangements (particularly inversions). Recently, apomixis was reported in the Boechereae genera Borodinia and Phoenicaulis. Here, we report sexual reproduction in diploid Nevada, diploid Sandbergia, and tetraploid Cusickiella and aposporous apomixis in tetraploids of Polyctenium and Sandbergia. In sum, apomixis is now known to occur in five of the nine Boechereae genera.

• Keywords
• Cruciferae, North America, apomixis, apospory, autopolyploidy, descending dysploidy, karyotype evolution, speciation,
• Publication type
• Journal Article MeSH

Fixed chromosomal inversions can reduce gene flow and promote speciation in two ways: by suppressing recombination and by carrying locally favoured alleles at multiple loci. However, it is unknown whether favoured mutations slowly accumulate on older inversions or if young inversions spread because they capture pre-existing adaptive quantitative trait loci (QTLs). By genetic mapping, chromosome painting and genome sequencing, we have identified a major inversion controlling ecologically important traits in Boechera stricta. The inversion arose since the last glaciation and subsequently reached local high frequency in a hybrid speciation zone. Furthermore, the inversion shows signs of positive directional selection. To test whether the inversion could have captured existing, linked QTLs, we crossed standard, collinear haplotypes from the hybrid zone and found multiple linked phenology QTLs within the inversion region. These findings provide the first direct evidence that linked, locally adapted QTLs may be captured by young inversions during incipient speciation.

• Publication type
• Journal Article MeSH