45S rDNA sites in meiosis of Lolium multiflorum Lam.: variability, non-homologous associations and lack of fragility

Abstract

In this study, we evaluated the behavior of 45S ribosomal DNA (rDNA) sites during the meiosis of Lolium multiflorum. The reason to study it in this species is that 45S rDNA sites are usually visualized as gaps in mitotic metaphase chromosomes and were initially denominated fragile sites (FSs). In different species, FSs were related to rearrangements that alter the karyotype and affect the chromosome pairing in meiosis. However, our findings show that the chromosome pairing in L. multiflorum is regular and, as in mitosis, the number of sites is variable. In diakinesis with five sites, one of the bivalents was in hemizygous state while, in diakinesis with seven sites, one of the bivalents had three conspicuous signals, two in syntheny in one of the homologous. Only four cells had gaps in the region of the 45S rDNA. Owing to the lower number of signals observed at the initial stages of meiosis, it is assumed that they are involved both in homologous and non-homologous associations and that they might assist the chromosome pairing. Regarding segregation, only meiocytes with five and six 45S rDNA signals were observed, and they were characterized by the segregation of 2/3 signals in the poles of anaphases I up to metaphases II; 2/2 and 3/3 in anaphases II and telophases II; and also 2/2 and 4/4 in the nuclei of tetrads, unlike the number of 45S signals expected. The numerical non-equivalence of sites among nuclei at later stages of meiosis is explained by the presence of chromosomes with hemizygous sites.

Molecular and Cytogenetic Analysis of rDNA Evolution in Crepis Sensu Lato

“…The coding regions of rDNA sequences are an evolutionary very conserved fraction in the eucaryotic genome. This conservatism, however, appears to be a powerful source for genome instability, because the chromosomes that carry rDNA arrays (especially in subtelomeric regions) may be subject to unequal recombination and recombination between nonhomologous loci [ 55 , 56 ].…”

Molecular Evolution and Organization of Ribosomal DNA in the Hawkweed Tribe Hieraciinae (Cichorieae, Asteraceae)

“…Interestingly, three hemizygous loci were detected: seven loci and three loci of 45S rDNA in H. stelligerum and H. umbellatum, and three loci of 5S rDNA in H. sparsum. Hemizygous rDNA loci were also observed in other plant groups, for example in diploid and polyploid grasses (Rocha et al, 2018;Chiavegatto et al, 2019) and diploid orchids (Lan and Albert, 2011). Generally, a potential reason for the observation of hemizygous loci is hybridization (Myburg et al, 2003).…”