Self-incompatibility (SI), which has recurred during the evolution of plants, is one of the most important cross-pollination mating systems. Three S-loci have been reported in Brassicaceae, namely, Arabidopsis lyrata (Al), Brassica (Br), and Leavenworthia alabamica (La) S-loci. Here, through multi-genomic comparative analysis of 20 species, we revealed that the most ancient S-locus was formed prior to the divergence of Brassicaceae lineage I and II. It was retained and inherited by Arabidopsis, as the Al S-locus in Brassicaceae lineage I. Furthermore, we found that the Br S-locus, which has been widely used in the breeding of Brassica crops to generate hybrid seeds, was formed through segmental translocation (ST) in the hexaploid ancestor of Brassica in Brassicaceae lineage II. The Br S-locus was evolved through a ST from one of the triplicated ancestral S-locus paralogs in the Brassica hexaploidy ancestor, while the other two S-locus paralogs were lost. Together with the previous discovery that the La S-locus was formed through a secondary origin in Brassicaceae lineage I, we conclude the monophyletic origin of Al and Br S-loci and clarify the evolutionary route of S-loci in the Brassicaceae family. Our findings will contribute to evolutionary studies and breeding applications of the S-locus in Brassicaceae.

The prevalence and recurrence of polyploidization in plant species make it one of the most important evolutionary events in plants, and as a result, polyploidization is an extensively investigated research field. Due to the rapid development of sequencing technologies, there is increased evidence to support that polyploidization plays an important role in the diversification of plant species, evolution of genes, and the domestication of crops. Here, we reviewed the influence of polyploidization on various aspects of plant evolution, mainly focused on polyploid origin, characteristics, subsequent genome divergence, and its impact on gene function innovation and crop domestication. The occurrence of many independent polyploidization events in plants was found to be tightly associated with the timing of extreme climate events or natural disasters on earth, leading to mass extinction while possibly facilitating increased polyploidization. Following allo-polyploidization, a distinct phenomenon known as sub-genome dominance occurred during sub-genome evolution, which was found to be associated with the methylation of transposons. Extensive gene fractionations (lost) following polyploidization were reported in almost all paleo-polyploids, and the evolutionary fates of multi-copy genes, such as sub-/neo-functionalization, were further proposed to illustrate their underlying mechanisms. Moreover, polyploidization was found to significantly impact species diversification, with subsequent effects on crop domestication and the development of traits with agronomic importance. Based on the progress of plant polyploidization studies, we discussed several main topics that might further improve our understanding of polyploid evolution and that are likely contribute to the application of polyploidization in crop breeding in the near future.

Long terminal repeat retrotransposons (LTR-RTs) are a predominant group of plant transposable elements (TEs) that are an important component of plant genomes. A large number of LTR-RTs have been annotated in the genomes of the agronomically important oil and vegetable crops of the genus Brassica. Herein, full-length LTR-RTs in the genomes of Brassica and other closely related species were systematically analyzed. The full-length LTR-RT content varied greatly (from 0.43% to 23.4%) between different species, with Gypsy-like LTR-RTs constituting a primary group across these genomes. More importantly, many annotated LTR-RTs (from 10.03% to 33.25% of all detected LTR-RTs) were found to be enriched in localized hotspot regions. Furthermore, all of the analyzed species showed evidence of having experienced at least one round of a LTR-RT burst, with Raphanus sativus experiencing three or more. Moreover, these relatively ancient LTR-RT amplifications exhibited a clear expansion at specific time points. To gain a further understanding of this timing, Brassica rapa, B. oleracea, and R. sativus were examined for the presence of syntenic regions, but none were present. These findings indicate that these LTR-RT burst events were not inherited from a common ancestor, but instead were species-specific bursts that occurred after the divergence of Brassica species. This study further exemplifies the complexities of TE amplifications during the evolution of plant genomes and suggests that these LTR-RT bursts play an important role in genome expansion and divergence in Brassica species.