Stowaway miniature inverted repeat transposable elements are important agents driving recent genomic diversity in wild and cultivated carrot

Polymorphic insertions of DcSto miniature inverted-repeat transposable elements reveal genetic diversity structure within the cultivated carrot

Miniature inverted-repeat transposable elements (MITEs) are a potent source of polymorphisms in plant genomes. A genotyping system, named DcS-ILP, based on polymorphic insertions of Stowaway MITEs (DcStos) localized in introns and identified in the reference genome DH1, has been developed for carrot. Here, we report an extension of the DcS-ILP genotyping system by incorporation of non-reference insertions identified in resequenced genomes representing the eastern gene pool. We genotyped 52 carrot accessions representing the eastern and western carrot gene pools with 92 markers developed previously (western DcS-ILP panel) together with 84 newly developed markers (eastern DcS-ILP panel). Overall, the DcS-ILP markers revealed a highly structured genetic diversity separating the eastern and the western carrot accessions at K = 2 and differentiating Indian breeding lines from the eastern accessions at K = 3. The eastern DcS-ILP panel proved to be more robust with respect to the eastern carrot gene pool, while it provided little information on the western accessions, as many of the DcSto insertions present in the eastern gene pool were absent in the western gene pool. As the western carrot accessions represent improved cultivars, DcSto insertional polymorphisms allowed detection of a selection-driven bottleneck at the improvement stage. Selection in the course of the improvement stage generally operated on standing variation, as the subset of DcSto insertions present in the western carrot likely originated from transposition events preceding the separation of both gene pools. However, occasional frequency shifts in the opposite direction were also revealed, possibly indicating selection for favorable variants associated with DcSto insertions.

Genome-wide characterization and evolution analysis of miniature inverted-repeat transposable elements in Barley (Hordeum vulgare)

Miniature inverted-repeat transposable elements (MITEs) constitute a class of class II transposable elements (TEs) that are abundant in plant genomes, playing a crucial role in their evolution and diversity. Barley (Hordeum vulgare), the fourth-most important cereal crop globally, is widely used for brewing, animal feed, and human consumption. However, despite their significance, the mechanisms underlying the insertion or amplification of MITEs and their contributions to barley genome evolution and diversity remain poorly understood. Through our comprehensive analysis, we identified 32,258 full-length MITEs belonging to 2,992 distinct families, accounting for approximately 0.17% of the barley genome. These MITE families can be grouped into four well-known superfamilies (Tc1/Mariner-like, PIF/Harbinger-like, hAT-like, and Mutator-like) and one unidentified superfamily. Notably, we observed two major expansion events in the barley MITE population, occurring approximately 12-13 million years ago (Mya) and 2-3 Mya. Our investigation revealed a strong preference of MITEs for gene-related regions, particularly in promoters, suggesting their potential involvement in regulating host gene expression. Additionally, we discovered that 7.73% miRNAs are derived from MITEs, thereby influencing the origin of certain miRNAs and potentially exerting a significant impact on post-transcriptional gene expression control. Evolutionary analysis demonstrated that MITEs exhibit lower conservation compared to genes, consistent with their dynamic mobility. We also identified a series of MITE insertions or deletions associated with domestication, highlighting these regions as promising targets for crop improvement strategies. These findings significantly advance our understanding of the fundamental characteristics and evolutionary patterns of MITEs in the barley genome. Moreover, they contribute to our knowledge of gene regulatory networks and provide valuable insights for crop improvement endeavors.

Diverse and mobile: eccDNA-based identification of carrot low-copy-number LTR retrotransposons active in callus cultures

Long terminal repeat retrotransposons (LTR-RTs) are mobilized via an RNA intermediate using a 'copy and paste' mechanism, and account for the majority of repetitive DNA in plant genomes. As a side effect of mobilization, the formation of LTR-RT-derived extrachromosomal circular DNAs (eccDNAs) occurs. Thus, high-throughput sequencing of eccDNA can be used to identify active LTR-RTs in plant genomes. Despite the release of a reference genome assembly, carrot LTR-RTs have not yet been thoroughly characterized. LTR-RTs are abundant and diverse in the carrot genome. We identified 5976 carrot LTR-RTs, 2053 and 1660 of which were attributed to Copia and Gypsy superfamilies, respectively. They were further classified into lineages, families and subfamilies. More diverse LTR-RT lineages, i.e. lineages comprising many low-copy-number subfamilies, were more frequently associated with genic regions. Certain LTR-RT lineages have been recently active in Daucus carota. In particular, low-copy-number LTR-RT subfamilies, e.g. those belonging to the DcAle lineage, have significantly contributed to carrot genome diversity as a result of continuing activity. We utilized eccDNA sequencing to identify and characterize two DcAle subfamilies, Alex1 and Alex3, active in carrot callus. We documented 14 and 32 de novo insertions of Alex1 and Alex3, respectively, which were positioned in non-repetitive regions.

Extensively Current Activity of Transposable Elements in Natural Rice Accessions Revealed by Singleton Insertions

Active transposable elements (TEs) have drawn more attention as they continue to create new insertions and contribute to genetic diversity of the genome. However, only a few have been discovered in rice up to now, and their activities are mostly induced by artificial treatments (e.g., tissue culture, hybridization etc.) rather than under normal growth conditions. To systematically survey the current activity of TEs in natural rice accessions and identify rice accessions carrying highly active TEs, the transposon insertion polymorphisms (TIPs) profile was used to identify singleton insertions, which were unique to a single accession and represented the new insertion of TEs in the genome. As a result, 10,924 high-confidence singletons from 251 TE families were obtained, covering all investigated TE types. The number of singletons varied substantially among different superfamilies/families, perhaps reflecting distinct current activity. Particularly, eight TE families maintained potentially higher activity in 3,000 natural rice accessions. Sixty percent of rice accessions were detected to contain singletons, indicating the extensive activity of TEs in natural rice accessions. Thirty-five TE families exhibited potentially high activity in at least one rice accession, and the majority of them showed variable activity among different rice groups/subgroups. These naturally active TEs would be ideal candidates for elucidating the molecular mechanisms underlying the transposition and activation of TEs, as well as investigating the interactions between TEs and the host genome.

A Global Landscape of Miniature Inverted-Repeat Transposable Elements in the Carrot Genome

Miniature inverted-repeat transposable elements (MITEs) are the most abundant group of Class II mobile elements in plant genomes. Their presence in genic regions may alter gene structure and expression, providing a new source of functional diversity. Owing to their small size and lack of coding capacity, the identification of MITEs has been demanding. However, the increasing availability of reference genomes and bioinformatic tools provides better means for the genome-wide identification and analysis of MITEs and for the elucidation of their contribution to the evolution of plant genomes. We mined MITEs in the carrot reference genome DH1 using MITE-hunter and developed a curated carrot MITE repository comprising 428 families. Of the 31,025 MITE copies spanning 10.34 Mbp of the carrot genome, 54% were positioned in genic regions. Stowaways and Tourists were frequently present in the vicinity of genes, while Mutator-like MITEs were relatively more enriched in introns. hAT-like MITEs were relatively more frequently associated with transcribed regions, including untranslated regions (UTRs). Some carrot MITE families were shared with other Apiaceae species. We showed that hAT-like MITEs were involved in the formation of new splice variants of insertion-harboring genes. Thus, carrot MITEs contributed to the accretion of new diversity by altering transcripts and possibly affecting the regulation of many genes.

Genetic diversity structure of western-type carrots

BACKGROUND

Carrot is a crop with a wide range of phenotypic and molecular diversity. Within cultivated carrots, the western gene pool comprises types characterized by different storage root morphology. First western carrot cultivars originated from broad-based populations. It was followed by intercrosses among plants representing early open-pollinated cultivars, combined with mass phenotypic selection for traits of interest. Selective breeding improved root uniformity and led to the development of a range of cultivars differing in root shape and size. Based on the root shape and the market use of cultivars, a dozen of market types have been distinguished. Despite their apparent phenotypic variability, several studies have suggested that western cultivated carrot germplasm was genetically non-structured.

RESULTS

Ninety-three DcS-ILP markers and 2354 SNP markers were used to evaluate the structure of genetic diversity in the collection of 78 western type open-pollinated carrot cultivars, each represented by five plants. The mean percentage of polymorphic loci segregating within a cultivar varied from 31.18 to 89.25% for DcS-ILP markers and from 45.11 to 91.29% for SNP markers, revealing high levels of intra-cultivar heterogeneity, in contrast to its apparent phenotypic stability. Average inbreeding coefficient for all cultivars was negative for both DcS-ILP and SNP, whereas the overall genetic differentiation across all market classes, as measured by F_ST, was comparable for both marker systems. For DcS-ILPs 90-92% of total genetic variation could be attributed to the differences within the inferred clusters, whereas for SNPs the values ranged between 91 to 93%. Discriminant Analysis of Principal Components enabled the separation of eight groups cultivars depending mostly on their market type affiliation. Three groups of cultivars, i.e. Amsterdam, Chantenay and Imperator, were characterized by high homogeneity regardless of the marker system used for genotyping.

CONCLUSIONS

Both marker systems used in the study enabled detection of substantial variation among carrot plants of different market types, therefore can be used in germplasm characterization and analysis of genome relationships. The presented results likely reveal the actual genetic diversity structure within the western carrot gene pool and point at possible discrepancies within the cultivars' passport data.

Characterization of B-Genome Specific High Copy hAT MITE Families in Brassica nigra Genome

Miniature inverted-repeat transposable elements (MITEs) are non-autonomous class II transposons which have been shown to influence genome evolution. Brassica nigra L. (B-genome) is one of three Brassica diploids cultivated primarily as an oil crop, which harbors novel alleles important for breeding. Two new high copy hAT MITE families (BniHAT-1 and BniHAT-2) from the B-genome were characterized and their prevalence assessed in the genomes of the related diploids, rapa L. (A) and Brassica oleracea L. (C). Both novel MITE families were present at high copy numbers in the B-genome with 434 and 331 copies of BniHAT-1 and BniHAT-2, respectively. Yet less than 20 elements were identified in the genome assemblies of the A, and C -genomes, supporting B-genome specific proliferation of these MITE families. Although apparently randomly distributed across the genome, 68 and 70% of the B-genome MITEs were present within 2 kb flanking regions of annotated genes suggesting they might influence gene expression and/or function. In addition, MITE derived microRNAs and transcription factor binding sites suggested a putative role in gene regulation. Age of insertion analysis revealed that the major proliferation of these elements occurred during 2-3 million years ago. Additionally, site-specific polymorphism analyses showed that 44% MITEs were undergoing active amplification into the B-genome. Overall, this study provides a comprehensive analysis of two high copy MITE families, which were specifically amplified in the B-genome, suggesting a potential role in shaping the Brassica B-genome.