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Background: Structural variants (SVs) constitute a large proportion of the genomic variation that results in phenotypic variation in plants. However, they are still a largely unexplored feature in most plant genomes. Here, we present the whole-genome landscape of SVs between two model legume Medicago truncatula ecotypes–Jemalong A17 and R108– that have been extensively used in various legume biology studies. Results: To catalogue SVs, we first resolved the previously published R108 genome assembly (R108 v1.0) to chromosome-scale using 124 × Hi-C data, resulting in a high-quality genome assembly. The inter-chromosomal reciprocal translocations between chromosomes 4 and 8 were confirmed by performing syntenic analysis between the two genomes. Combined with the Hi-C data, it appears that these translocation events had a significant effect on chromatin organization. Using both whole-genome and short-read alignments, we identified the genomic landscape of SVs between the two genomes, some of which may account for several phenotypic differences, including their differential responses to aluminum toxicity and iron deficiency, and the development of different anthocyanin leaf markings. We also found extensive SVs within the nodule-specific cysteine-rich gene family which encodes antimicrobial peptides essential for terminal bacteroid differentiation during nitrogen-fixing symbiosis. Conclusions: Our results provide a near-complete R108 genome assembly and the first genomic landscape of SVs obtained by comparing two M. truncatula ecotypes. This may provide valuable genomic resources for the functional and molecular research of legume biology in the future. [ABSTRACT FROM AUTHOR]

Background: Structural variants (SVs) constitute a large proportion of the genomic variation that results in phenotypic variation in plants. However, they are still a largely unexplored feature in most plant genomes. Here, we present the whole-genome landscape of SVs between two model legume Medicago truncatula ecotypes–Jemalong A17 and R108– that have been extensively used in various legume biology studies. Results: To catalogue SVs, we first resolved the previously published R108 genome assembly (R108 v1.0) to chromosome-scale using 124 × Hi-C data, resulting in a high-quality genome assembly. The inter-chromosomal reciprocal translocations between chromosomes 4 and 8 were confirmed by performing syntenic analysis between the two genomes. Combined with the Hi-C data, it appears that these translocation events had a significant effect on chromatin organization. Using both whole-genome and short-read alignments, we identified the genomic landscape of SVs between the two genomes, some of which may account for several phenotypic differences, including their differential responses to aluminum toxicity and iron deficiency, and the development of different anthocyanin leaf markings. We also found extensive SVs within the nodule-specific cysteine-rich gene family which encodes antimicrobial peptides essential for terminal bacteroid differentiation during nitrogen-fixing symbiosis. Conclusions: Our results provide a near-complete R108 genome assembly and the first genomic landscape of SVs obtained by comparing two M. truncatula ecotypes. This may provide valuable genomic resources for the functional and molecular research of legume biology in the future. [ABSTRACT FROM AUTHOR]

Background: Although plastomes are highly conserved with respect to gene content and order in most photosynthetic angiosperms, extensive genomic rearrangements have been reported in Fabaceae, particularly within the inverted repeat lacking clade (IRLC) of Papilionoideae. Two hypotheses, i.e., the absence of the IR and the increased repeat content, have been proposed to affect the stability of plastomes. However, this is still unclear for the IRLC species. Here, we aimed to investigate the relationships between repeat content and the degree of genomic rearrangements in plastomes of Medicago and its relatives Trigonella and Melilotus, which are nested firmly within the IRLC. Results: We detected abundant repetitive elements and extensive genomic rearrangements in the 75 newly assembled plastomes of 20 species, including gene loss, intron loss and gain, pseudogenization, tRNA duplication, inversion, and a second independent IR gain (IR ~ 15 kb in Melilotus dentata) in addition to the previous first reported cases in Medicago minima. We also conducted comparative genomic analysis to evaluate plastome evolution. Our results indicated that the overall repeat content is positively correlated with the degree of genomic rearrangements. Some of the genomic rearrangements were found to be directly linked with repetitive sequences. Tandem repeated sequences have been detected in the three genes with accelerated substitution rates (i.e., accD, clpP, and ycf1) and their length variation could be explained by the insertions of tandem repeats. The repeat contents of the three localized hypermutation regions around these three genes with accelerated substitution rates are also significantly higher than that of the remaining plastome sequences. Conclusions: Our results suggest that IR reemergence in the IRLC species does not ensure their plastome stability. Instead, repeat-mediated illegitimate recombination is the major mechanism leading to genome instability, a pattern in agreement with recent findings in other angiosperm lineages. The plastome data generated herein provide valuable genomic resources for further investigating the plastome evolution in legumes. [ABSTRACT FROM AUTHOR]