Cytogenomic characterization of the repetitive DNA in the ‘brizantha’ agamic complex of Urochloa P. Beauv. (Poaceae)

Abstract

Urochloa P. Beauv. (formerly classifed as Brachiaria (Trin.) Griseb.) is an African genus of perennial grasses that is extensively cultivated in Brazil for cattle nutrition. Urochloa brizantha, U. decumbens and U. ruziziensis are among the most economically relevant species. These three species form the ‘brizantha’ agamic complex, which includes polyploid series and interspecific hybridizations resulting in complex genomic relationships. Repetitive DNA is a large component of grass genomes, therefore investigating its composition and organization in the genus would provide more insight on its evolutionary history and would allow more informed decisions to be made in breeding programs when selecting genotypes for crosses. The present study aimed to characterize the repetitive fraction of the genomes in diploid and tetraploid cytotypes of U. brizantha, U. decumbens and U. ruziziensis, identifying major sequences and understanding their organization by physically mapping repetitive probes onto chromosomes through fluorescent in situ hybridization (FISH). Results revealed that repetitive DNA comprises 56-65% of Urochloa genomes and that LTR TY3/Gypsy retrotransposons are most frequent elements. The proportions of major retrotransposons and DNA transposons lineages varied among all species, as well as the number of satellite DNA (satDNA) families, which presented intra and interspecific variation. Urochloa brizantha presented a more diverse satDNA profile, with a larger number of unique monomers, and species-specific repeats. The three most abundant satellites (UroSat-1a, UroSat-2a, and UroSat-3) were selected to be mapped to chromosomes via FISH. Conspicuous signals of UroSat-1a were present in all centromeres of the evaluated species, while the distribution of UroSat-2a and UroSat-3 marks varied in number and position. Our findings validate the use of satellite DNA as cytogenetic markers in Urochloa ‘brizantha’ agamic complex and highlight genomic relationships among different species and ploidy levels.