Abstract
The distribution of 45S rDNA cluster in avian karyotypes varies in different aspects, such as position, number of bearer chromosomes, and bearers being macro- or microchromosomes. The present study investigated the patterns of variation in the 45S rDNA-bearer chromosomes of birds in order to understand the evolutionary dynamics of the cluster configuration and its contribution to the evolution of bird karyotypes. A total of 73 bird species were analyzed, including both published data and species for which rDNA-FISH was conducted for the first time. In most birds, the 45S rDNA clusters were located in a single pair of microchromosomes. Hence, the location of 45S rDNA in macrochromosomes, observed only in Neognathae species, seems to be a derived state, probably the result of chromosomal fusion between microchromosomes and distinct macrochromosomes. Additionally, the 45S rDNA was observed in multiple microchromosomes in different branches of the bird phylogeny, suggesting recurrence of dispersion processeses, such as duplications and translocations. Overall, this study indicated that the redistribution of the 45S rDNA sites in bird chromosomes followed different evolutionary trajectories with respect to each lineage of the class Aves.
Highlights
The rDNA genes are extremely important for cell function, given that they encode the rRNA involved in ribosome biogenesis (Hadjiolov, 1985; Shaw and Brown, 2012)
Two rDNA clusters are involved: the 45S rDNA composed by 18S, 5.8S, and 28S genes, and internal (ITS1 and ITS2) and external (5’ETS and 3’ETS) transcribed spacers; and the 5S rDNA, composed by a 5S gene separated by an intergenic spacer region (IGS) (Daniels and Delany, 2003; Dyomin et al, 2016)
Fluorescence in situ hybridization (FISH) experiments are more appropriate for this type of study, since they allow the precise identification of the bearing chromosomes when using probes for the genes that make up the rDNA cluster even when they are not active (O’Connor, 2008)
Summary
The rDNA genes are extremely important for cell function, given that they encode the rRNA involved in ribosome biogenesis (Hadjiolov, 1985; Shaw and Brown, 2012) In this process, two rDNA clusters are involved: the 45S rDNA composed by 18S, 5.8S, and 28S genes, and internal (ITS1 and ITS2) and external (5’ETS and 3’ETS) transcribed spacers; and the 5S rDNA, composed by a 5S gene separated by an intergenic spacer region (IGS) (Daniels and Delany, 2003; Dyomin et al, 2016). Nique (Ag-NOR) (Howell and Black, 1980) This procedure only identifies the chromosomes with 45S rDNA sites in transitional activity, exhibiting intercellular, and interindividual variation (Zurita et al, 1997). Fluorescence in situ hybridization (FISH) experiments are more appropriate for this type of study, since they allow the precise identification of the bearing chromosomes when using probes for the genes that make up the rDNA cluster even when they are not active (O’Connor, 2008)
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