Unraveling Large and Polyploidy Genome of the Crucifer Orychophragmus violaceus in China, a Potential Oil Crop

Abstract

The genus Orychophragmus in the Brassicaceae family includes the types with 2n = 20, 22, 24, and 48. The species O. violaceus (L.) O. E. Schulz has 2n = 24 and is widely cultivated as an ornamental plant in China. This review summarizes the research progress of its genome structure and evolution in the context of cytogenetics and genome sequencing. This species has a large genome size of ~1 Gb and longer chromosomes than those of Brassica species, which is attributable to the burst of TE insertions. Even more, one tetraploidization event from about 600-800 million years ago is elucidated to occur during its genome evolution, which is consistent with the polyploidy nature of its genome revealed by the meiotic pairing patterns. Its chromosomes are still characterized by a larger size and deeper staining than those from Brassica species in their intergeneric hybrids, which is likely related to their inherent differences between genome structures and cytology. Its genome is dissected by the development of additional alien lines, and some traits are located on individual chromosomes. Due to the abundant dihydroxy fatty acids in its seed oil with superior lubricant properties and wide environmental adaptations, this plant promises to be utilized as one new oil crop in the future.

Keywords: Orychophragmus; germplasm characterization; oilseeds; plant wild relatives; polyploidy.

Figure 1

Phylogenetical relationship and karyotype evolution of Brassica and Orychophragmus. ABK: Ancestral Brassicaceae Karyotype. ACK: Ancestral Camelineae karyotype. PCK: Proto-Calepineae Karyotype. tPCK: translocated Proto-Calepineae Karyotype. MF1: Medium Fractionated subgenome. MF2: Most Fractionated subgenome. LF: Least Fractionated subgenome. Ov: O. violaceus. Ov1: Ov1 subgenome. Ov2: Ov2 subgenome.

Figure 2

The chromosomes of O. violaceus itself and in intergeneric hybrids. (a) The chromosomes of O. violaceus after GISH with its genomic DNA as the probe, with the homogenous distribution of the red signal from Cy3 along the whole chromosomes. (b) The detection of eight 45S rDNA loci (red) and twenty-two 5S rDNA loci (green) by double FISH on the mitotic chromosomes of O. violaceus [20]. (c) In one prometaphase cell of the pentaploid plant (AACCO) between B. napus and O. violaceus, the 12 labeled chromosomes from O. violaceus are obviously much longer than those from B. napus, and the satellites on three chromosomes are obvious (arrows). (d) In one metaphase cell of the disomic additional line DAAL4 (2n = 40), the two larger chromosomes from O. violaceus, which are labeled green by GISH, carry satellites, but many chromosomes from B. napus harbor the rDNA loci (red) but no satellites.

Figure 3

The cytological behaviors of the intergeneric hybrids between O. violaceus and six Brassica species. The genome size of each species given is the estimated size, not the assembled size. CH: classical hybrids. NCH: non-classical hybrid. The expression dominance of the rRNA genes or the nucleolar dominance from three diploids follows the hierarchy B > A > C in three allotetraploids, with the genes of B. nigra expressed in B. juncea and B. carinata and the genes of B. rapa in B. napus [29].