Comparative genetic mapping in Boechera stricta, a close relative of Arabidopsis

The angiosperm family Brassicaceae contains both the research model Arabidopsis ( Arabidopsis thaliana) and the agricultural genus Brassica. Comparative genomics in the Brassicaceae has largely focused on direct comparisons between Arabidopsis and the species of interest. However, the reduced genome size and chromosome number (n = 5) of Arabidopsis complicates comparisons. Arabidopsis shows extensive genome and chromosome reshuffling compared to its close relatives Arabidopsis lyrata and Capsella rubella, both with n = 8. To facilitate comparative genomics across the Brassicaceae we recently outlined a system of 24 conserved chromosomal blocks based on their positions in an ancestral karyotype of n 5 8, rather than by their position in Arabidopsis. In this report we use this system as a tool to understand genome structure and evolution in Boechera stricta (n = 7). B. stricta is a diploid, sexual, and highly self-fertilizing species occurring in mostly montane regions of western North America. We have created an F-2 genetic map of B. stricta based on 192 individuals scored at 196 microsatellite and candidate gene loci. Single-nucleotide polymorphism genotyping of 94 of the loci was done simultaneously using an Illumina bead array. The total map length is 725.8 cM, with an average marker spacing of 3.9 cM. There are no gaps greater than 19.3 cM. The chromosomal reduction from n 5 8 to n 5 7 and other genomic changes in B. stricta likely involved a pericentric inversion, a chromosomal fusion, and two reciprocal translocations that are easily visualized using the genomic blocks. Our genetic map will facilitate the analysis of ecologically relevant quantitative variation in Boechera.