title: Chromosomal distribution and evolution of abundant retrotransposons in plants: gypsy elements in diploid and polyploid Brachiaria forage grasses.
abstract: Like other eukaryotes, the nuclear genome of plants consists of DNA with a small proportion of low-copy DNA (genes and regulatory sequences) and very abundant DNA sequence motifs that are repeated thousands up to millions of times in the genomes including transposable elements (TEs) and satellite DNA. Retrotransposons, one class of TEs, are sequences that amplify via an RNA intermediate and reinsert into the genome, are often the major fraction of a genome. Here, we put research on retrotransposons into the larger context of plant repetitive DNA and genome behaviour, showing features of genome evolution in a grass genus, Brachiaria, in relation to other plant species. We show the contrasting amplification of different retroelement fractions across the genome with characteristics for various families and domains. The genus Brachiaria includes both diploid and polyploid species, with similar chromosome types and chromosome basic numbers x = 6, 7, 8 and 9. The polyploids reproduce asexually and are apomictic, but there are also sexual species. Cytogenetic studies and flow cytometry indicate a large variation in DNA content (C-value), chromosome sizes and genome organization. In order to evaluate the role of transposable elements in the genome and karyotype organization of species of Brachiaria, we searched for sequences similar to conserved regions of TEs in RNAseq reads library produced in Brachiaria decumbens. Of the 9649 TE-like contigs, 4454 corresponded to LTR-retrotransposons, and of these, 79.5 % were similar to members of the gypsy superfamily. Sequences of conserved protein domains of gypsy were used to design primers for producing the probes. The probes were used in FISH against chromosomes of accesses of B. decumbens, Brachiaria brizantha, Brachiaria ruziziensis and Brachiaria humidicola. Probes showed hybridization signals predominantly in proximal regions, especially those for retrotransposons of the clades CRM and Athila, while elements of Del and Tat exhibited dispersed signals, in addition to those proximal signals. These results show that the proximal region of Brachiaria chromosomes is a hotspot for retrotransposon insertion, particularly for the gypsy family. The combination of high-throughput sequencing and a chromosome-centric cytogenetic approach allows the abundance, organization and nature of transposable elements to be characterized in unprecedented detail. By their amplification and dispersal, retrotransposons can affect gene expression; they can lead to rapid diversification of chromosomes between species and, hence, are useful for studies of genome evolution and speciation in the Brachiaria genus. Centromeric regions can be identified and mapped, and retrotransposon markers can also assisting breeders in the developing and exploiting interspecific hybrids.
authors.affiliation: Department of General Biology, Center of Biological Sciences, State University of Londrina, Londrina, 86057-970, Paraná State, Brazil.
authors.forename: Fabíola Carvalho
authors.initials: FC
authors.lastname: Santos
authors.affiliation: Institut de Recherche pour le Développement (IRD), UMR IPME, BP 64501, 34394, Montpellier Cedex, France.
authors.forename: Romain
authors.initials: R
authors.lastname: Guyot
authors.affiliation: Embrapa Gado de Corte, 79106-550, Campo Grande, Mato Grosso do Sul State, Brazil.
authors.forename: Cacilda Borges
authors.initials: CB
authors.lastname: do Valle
authors.affiliation: Embrapa Gado de Corte, 79106-550, Campo Grande, Mato Grosso do Sul State, Brazil.
authors.forename: Lucimara
authors.initials: L
authors.lastname: Chiari
authors.affiliation: Department of Biology, Federal University of Lavras, 37200-000, Lavras, Minas Gerais State, Brazil.
authors.forename: Vânia Helena
authors.initials: VH
authors.lastname: Techio
authors.affiliation: Department of Genetics, University of Leicester, Leicester, LE1 7RH, UK. phh@molcyt.com.
authors.forename: Pat
authors.initials: P
authors.lastname: Heslop-Harrison
authors.affiliation: Department of General Biology, Center of Biological Sciences, State University of Londrina, Londrina, 86057-970, Paraná State, Brazil. andrevanzela@uel.br.
authors.forename: André Luís Laforga
authors.initials: AL
authors.lastname: Vanzela
mesh_terms: D035201:Brachiaria / Q000235:genetics; D002874:Chromosome Mapping; D032461:Chromosomes, Plant; D004171:Diploidy; D019143:Evolution, Molecular; D020869:Gene Expression Profiling; D059014:High-Throughput Nucleotide Sequencing; D017404:In Situ Hybridization, Fluorescence; D010944:Plants / Q000235:genetics*; D011123:Polyploidy; D018626:Retroelements / Q000235:genetics*; D059467:Transcriptome
chemical_list: D018626:Retroelements
keywords: FISH; centromeres; chromosomes; genetics; genome organization; genomics; grasses; in situ hybridization; metaviridae; repetitive DNA; retrotransposons; transposable elements; transposons