Genome structure and molecular phylogeny of the only Eurasian Boechera species, Boechera falcata (Brassicaceae)

Boechera falcata (Turcz.) Al-Shehbaz (earlier known as Arabis turczaninowii Ledeb.) is a herbaceous perennial belonging to the East Siberian, boreal-steppe ecotype. It is the only species of the diverse genus Boechera that grows on the Eurasian continent, all others being endemic to North America and Greenland. In all likelihood, Boechera falcata or its ancestral lineage migrated from North America to Eastern Siberia across the Bering Land Bridge when it was exposed during the height of Pleistocene glaciation. Boechera is interesting in that many species in this genus are allodiploid and triploid apomicts that arose through complex hybridization of sexual species and ecotypes. To date, the genomes of only two American Boechera species, B. stricta and B. retrofracta, have been sequenced, analyzed, and published. In the present study, we sequenced, assembled to the chromosome level, and analyzed the B. falcata genome, which was highly homozygous with a size of 189.36 Mb. Molecular phylogenetic analysis of the nuclear and organelle genomes revealed a high degree of relatedness of B. falcata to North American relatives. Investigation of the structure of pachytene, mitotic, and diakinesis chromosome spreads (n = 14) using cytogenetic analysis and comparative chromosome painting (CCP) allowed us to identify all 22 genomic blocks of crucifers and found that five of the seven B. falcata chromosomes were collinear with their corresponding chromosomes in the ancestral Boechereae genome, and two chromosomes had undergone pericentric and paracentric inversions. Allelic analysis of the apomixis marker APOLLO gene revealed that B. falcata contains only sex alleles of this gene. The availability of the genome of the only Asian Boechera species will facilitate studies of the evolution and phylogeny of Brassicaceae species and apomixis.