Brassica oleracea contains various morphotypes within the species, but genomic signatures differentiating each morphotype have been poorly understood of. Here, we utilized whole genome sequence data of 44 B. oleracea collections including those of seven different morphotypes such as cabbage, broccoli, cauliflower, kailan, kale, brussels sprout, and kohlrabi to elucidate the genomic signature of B. oleracea morphotypes. Molecular structure analysis divided the 44 B. oleracea lines into two groups: group I represents broccoli, cauliflower, kailan; group II represents other B. oleracea subspecies. Kohlrabi has admixed genomic structure through genetic admixture analysis. Based on the population stratification result, we have investigated genetic signatures that offer the possible evolutionary processes for the kohlrabi morphotype. Several statistical analyses were implemented to identify selective regions and explore 45 candidate loci that may contribute to stem swollen in kohlrabi. Above all, we identified two kohlrabi-unique genes, LOC106333915 and LOC106308097, showing kohlrabi-unique non-synonymous mutations, which might be candidate genes for stem swollen in kohlrabi.

Miniature transposable elements (mTEs) such as miniature inverted-repeat transposable element (MITE), terminal repeat retrotransposon in miniature, and short interspersed element are exquisite sources for marker development. mTEs are short, non-autonomous and stably inherited. The high-copy members are widely distributed into the gene rich euchromatic regions. Here, we conducted a modified transposon display (TD) for a high-copy MITE family, BraSto-2 (Bs2). The Bs2-specific primers derived from conserved sequences of Bs2 members as well as MseI adapter primers were used for polymerase chain reaction (PCR) in two Brassica rapa accessions, ‘Chiifu’ and ‘Kenshin’. The pooled PCR products were sequenced by Illumina sequencing platform instead of high-resolution gel electrophoresis. Subsequent in silico-based insertion polymorphism (IP) analysis (next-generation sequencing [NGS]-based Bs2 transposon display) was conducted, which generated more than 99 putative polymorphic insertion sites between ‘Chiifu’ and ‘Kenshin’. Among 90 successful PCR amplification, 34 showed Bs2 IP (IP-Bs2) between ‘Chiifu’ and ‘Kenshin’ accessions, 27 and seven ‘Chiifu’- and ‘Kenshin’-unique insertions, respectively. When the 90 IP-Bs2 primer sets were applied to 10 Brassica accessions, including four additional B. rapa and B. oleracea accessions, 69 (76%) showed insertion olymorphism among accessions. The IP-Bs2 were evenly distributed through all the chromosomes and provide rich polymorphism among various B. rapa and B. oleracea accessions demonstrating the usefulness of these markers for various genetic diversity and molecular breeding studies in Brassica. In addition, NGS-based TD will be applicable to various high copy transposable elements family for high throughput and rapid polymorphic marker development which will be helpful for efficient plant genomics and breeding purposes.

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The genus Brassica is an important resource for major agricultural products such as oils, vegetable and fodder. The Brassiceae tribe-specific whole-genome triplication that occurred ~15.9 million years ago influenced the speciation and morphological diversification that has been exploited in agriculture, making Brassica an excellent model system for studying polyploidization-mediated evolution. Genome sequencing and comparative genome analysis have revealed conserved structures and uncovered the genome evolution of Brassica species. While chromosome shuffling and asymmetric subgenome gene retention are widely reported in Brassica species, limited information is available about the dynamics of repetitive elements (REs), which are central to epigenetic mechanisms and thus play a pivotal role in plant genome adaptation and evolution. The assembled reference genome sequences of B. rapa (AA) and B. oleracea (CC), and their derived allotetraploid, B. napus (AACC), cover 58%, 86%, and 75% of their respective estimated genome sizes. The remaining non-assembled genome portions vary between these three genome sequences, and the major components remain hidden in each genome. Here, we review the dynamics of the major Brassica repeats that have played roles in speciation of the AA, CC, and AACC genomes. We show that 10 major Brassica repeats appear to occupy more than 50% of each respective unassembled genome sequence, yet represent less than 1% of assembled reference genome sequences. We have estimated their genome proportions using whole-genome Illumina reads and cytogenetic analyses in an attempt to understand the role of these repeats in genome evolution.

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