The evolution of the hypotetraploid Catolobus pendulus genome - the poorly known sister species of Capsella

The establishment of as the most important plant model has also brought other crucifer species into the spotlight of comparative research. While the genus has become a prominent crucifer model system, its closest relative has been overlooked. The unispecific genus is native to temperate Eurasian woodlands, from eastern Europe to the Russian Far East. Here, we analyzed chromosome number, genome structure, intraspecific genetic variation, and habitat suitability of throughout its range. Unexpectedly, all analyzed populations were hypotetraploid (2 = 30, ~330 Mb). Comparative cytogenomic analysis revealed that the genome arose by a whole-genome duplication in a diploid genome resembling Ancestral Crucifer Karyotype (ACK, = 8). In contrast to the much younger allotetraploid genomes, the presumably autotetraploid genome (2 = 32) arose early after the / divergence. Since its origin, the tetraploid genome has undergone chromosomal rediploidization, including a reduction in chromosome number from 2 = 32 to 2 = 30. Diploidization occurred through end-to-end chromosome fusion and other chromosomal rearrangements affecting a total of six of 16 ancestral chromosomes. The hypotetraploid cytotype expanded toward its present range, accompanied by some longitudinal genetic differentiation. The sister relationship between and allows comparative studies of tetraploid genomes of contrasting ages and different degrees of genome diploidization.