Análise cariotípica de acessos Urochloa humidicola (Rendle) Morrone & Zuloaga

Abstract

The species Urochloa humidicola (Rendle) Morrone & Zuloaga belongs to the family Poaceae and, like other species of the genus, is used as forage in tropical countries. Despite its relevance, cytogenetic studies in the species are still restricted, except meiosis evaluation. This study aimed to characterize the karyotype of apomitic accessions (H016, H108, H125 and H126) and sexual access (H031) of Urochloa humidicola, including chromosomal morphology and symmetry, nuclear DNA content and physical mapping of rDNA sequences 35 and 5S, through FISH. All accessions evaluated showed symmetrical karyotypes. The H016 access have 2n = 6x = 36 chromosomes, karyotype formula 36m and nuclear DNA amount of 2.95pg. In this access, five 35S rDNA sites in terminal position and nine interstitial 5S rDNA sites were identified, four of which were synthetic with the 35S site. The accessions H125 and H126 presented 2n = 7x = 42, karyotype formula 42m with 3.51 and 4.45 pg of nuclear DNA, respectively. Regarding the ribosomal DNA sites, in both accessions, eleven 5S interstitial sites were observed, seven of which were in synergy with the 35S terminal sites. Finally, the H108 access showed 2n = 9x = 54 chromosomes, karyotype formula 54m and DNA content, 3.94 pg. In relation the ribosomal locus, seven chromosomes had 35S rDNA sites located in the terminal regions, of which five were synthetic with the 5S rDNA site and two 35S and three other 5S rDNA sites were interstitial and one pericenteric. Chromosomal counts and karyotype assembly showed that the H031 access is an aneuploid with 2n = 6x = 36 1, 37m karyotype formula and 3.01pg of nuclear DNA. In the karyotype, 13 sites of 5S rDNA were identified, in an interstitial position, six of which were synthetic with the 35S site. With the exception of one of the 35S rDNA sites that was identified in an interstitial position, the rest are located terminally on the chromosomes. Probably, one of the chromosomes carrying the 5S rDNA site may be the extra chromosome. Except for H108 access, all the others showed variation in the number of ribosomal sites between and within plants and cells of the same meristem. The results obtained corroborate the proposal for auto-allopolyploidy in Urochloa humidicola.