Physical Map of Repetitive DNA Sites in <i>Brachiaria</i> spp.: Intravarietal and Interspecific Polymorphisms

Abstract

Brachiaria (Trin.) Griseb. is a genus belonging to the Poaceae Barnh. family that aggregates species of forage interest, which have shown different levels of ploidy, intraspecific variability for chromosomal morphology and for number of rDNA sites. This study aimed at characterizing the karyotype of Brachiaria brizantha (Hochst. ex A. Rich) Stapf, Brachiaria decumbens Stapf, and Brachiaria ruziziensis R. Germ. &amp; C.M. Evrard by the location of 5S rDNA, 45S rDNA, centromeric retrotransposons, chromomycin A3 (CMA) and 4′, 6‐diamidino‐2‐phenylindole dihydrochloride (DAPI) chromosome bands, and transcriptional activity at nucleolar organizing regions (NORs). Brachiaria ruziziensis has 2 n = 2 x = 18 chromosomes, karyotypic formula 9M, one 45S rDNA site, and two 5S rDNA sites. Brachiaria brizantha has 2 n = 4 x = 36 chromosomes, karyotypic formula 18M, four to six 5S rDNA signals, and two to four 45S rDNA sites. One of the chromosomes carrying 45S rDNA is heteromorphic. Brachiaria decumbens has 2 n = 4 x = 36 chromosomes, karyotypic formula 17M+1M/SM, and seven 5S rDNA signals including one hemizygous 5S rDNA locus. One of the two chromosome pairs carrying 45S rDNA signals was heteromorphic with one of its chromosomes possessing a tertiary constriction. Hybridization signals with a centromeric retrotransposon probe, CRM1, were observed in Brachiaria chromosomes with distinct clusters on all B. ruziziensis chromosomes. Chromosomal CMA bands were located along with 45S rDNA signals in B. decumbens and B. ruziziensis whereas in B. brizantha only two out of four signals coincided. The karyotype of Brachiaria species is symmetrical. The transcriptional activity of NORs is variable with two or three heteromorphic nucleoli.