Organização genômica de sequências repetitivas em Pica-paus (aves piciformes)

Abstract

The characterization of the amount and distribution of the repetitive DNA fractions in genomes assists in the understanding of their chromosomal organization. The Birds are characterized by presenting a low proportion of repetitive DNA when compared to other classes of Vertebrates. However, the order Piciformes stands out for having a higher percentage of these sequences compared to other birds. The objective of this study was to determine the distribution of different types of repetitive sequences in the genome of three species of the family Picidae, Colaptes melanochloros (2n = 84), Colaptes campestris (2n = 84) and Melanerpes candidus (2n = 64) by fluorescence in situ hybridization (FISH) with 18S, telomeric (TTAGGG) and microsatellite rDNA probes. The results showed, in these three species, the sexual chromosome Z as the largest of the complement, this fact is due to the accumulation of different sequences of microsatellites. However, the W chromosome of C. melanochloros, which is totally heterochromatic, did not show accumulation of these sequences. The ribosomal sites are organized on a pair of chromosomes with a secondary constriction and this had the accumulation of the sequence (CGG)10 in the three species. The telomeric probes showed markings in the terminal regions of the chromosomes and interstitial markings on some macrochromosomes. Interstitial markings indicate fusions between chromosomes or the accumulation of repetitive sequences similar to the telomeric ones. With the microsatellite probes the same pattern of hybridization was identified in the Colaptes species, distinct pattern between Colaptes and M. candidus. Our FISH analyzes showed several amplified microsatellite sequences on the Z chromosome in the three species analyzed, which may explain the fact that this is the largest element of the karyotype and that its genome contains the largest number of repetitive sequences compared to other groups of Birds. Interestingly, none of the sequences were found to be accumulated on the W chromosome, although they play an important role in the differentiation of sex chromosomes. These results show that, despite the common origin proposed for the ZW sexual system in birds, these chromosomes followed different evolutionary trajectories in each species, indicating a high plasticity for the sexual chromosome differentiation in this group. This work is the first step to clarify the role of satellites and microsatellite sequences in the differentiation of sex chromosomes.