Análise comparativa de elementos repetitivos em genomas de espécies da subtribo Cassiinae (Fabaceae)

Abstract

Fabaceae (Leguminosae) is considered one of the three largest families of Angiospermae, containing 765 genera and 19,500 species with a cosmopolitan distribution. In Brazil, about 2,756 species are found, of which 1507 are endemic. Several groups of legumes have had their classification changed over time, such as the subtribe Cassiinae, which has three genera resulting from the division of Cassia lato sensu: Cassia stricto sensu, Chamaecrista Moench and Senna Mill. Based on phylogenetic analyses, the Cassiinae tribe has been considered complex due to conflicting hypotheses regarding the three genera that constitute it. Molecular and cytogenetic studies were important for Cassiinae systematics, but not enough to elucidate the controversies involving the relationship among the three genera. The inclusion of other genomic information is important to contribute to a better understanding of the relationships within the group. The advent of next-generation sequencing (NGS) allowed the characterization and comparison of the repetitive fraction in the genomes, proving to be an interesting tool for understanding genomic evolution. Thus, our work aimed to characterize the repetitive fraction in the genome of species representing the three genera of Cassiinae. We used genome skimming approach (about 0.5x coverage) with sequencing data obtained in Illumina plataform HiSeq™ for one species of each genus (Cassia fistula, Chamaecrista ensiformes e Senna occidentalis). The data were analyzed in RepeatExplorer pipeline for characterization and comparison of repetitive fraction of the species. The genome of C. fistula (0.70 pg/1C) has 41.74% of repetitive DNA; Ch. ensiformes (0.85 pg/1C) has 32.86% and S. occidentalis (0.70 pg/1C) 40.49%. Retrotransposons LTRs are the most abundant elements, with the Ty3/Gypsy superfamily being the most abundant in all three species, corresponding to 19.90% in C. fistula, 20.05% in Ch. ensiformes and 13.29% in S. occidentalis. Six satellite sequences were identified, three in C. fistula, two in Ch. ensiformes and one in S. occidentalis. These sequences are almost exclusive of the species in which they were identified. The variation in abundance and types of repetitive sequences observed in the three species evaluated points to the potential of this information for understanding relationships among the genera of Cassinae in studies involving more species representing genome diversity.