RESUMO
B chromosomes are supernumerary chromosomes found in the karyotypes of approximately 15% of all eukaryotic species. They present parasitic behavior and do not follow the standard Mendelian pattern of inheritance, resulting in an imbalance in gametogenesis. The evolutionary dynamics of B chromosomes is still unknown for many species, but studies indicate that the accumulation of repetitive sequences plays an important role in the differentiation of these elements. We analyzed morphology, frequency, and possible homologies amongst different B chromosomes found in an isolated Akodon montensis population in southern Brazil. Repetitive sequences (18S, 5S rDNA and telomeric sequences) were used to test for their accumulation on the supernumerary chromosomes and describe their localization in the species. The results indicate 4 different B chromosome morphotypes, and DNA libraries were generated for 3 of them. 18S rDNA was labelled polymorphically, except in the B chromosomes, whereas the 5S rDNA was located exclusively in an interstitial position on the long arm of chromosome 5. Chromosome painting with the B probes based on FISH revealed a homologous composition for all B chromosome morphotypes and no homology with the chromosomes in the A complement. B chromosomes found in this population may have a common origin and subsequently diversified in size and morphology.
Assuntos
Cromossomos de Mamíferos/genética , Sequências Repetitivas de Ácido Nucleico , Sigmodontinae/genética , Animais , Mapeamento Cromossômico/métodos , Coloração Cromossômica/métodos , Evolução Molecular , Feminino , Variação Genética , MasculinoRESUMO
Like various other diurnal birds of prey, the world's largest eagle, the Harpy (Harpia harpyja), presents an atypical bird karyotype with 2n=58 chromosomes. There is little knowledge about the dramatic changes in the genomic reorganization of these species compared to other birds. Since recently, the chicken provides a "default map" for various birds including the first genomic DNA sequence of a bird species. Obviously, the gross division of the chicken genome into relatively gene-poor macrochromosomes and predominantly gene-rich microchromosomes has been conserved for more than 150 million years in most bird species. Here, we present classical features of the Harpy eagle karyotype but also chromosomal homologies between H. harpyja and the chicken by chromosome painting and comparison to the chicken genome map. We used two different sets of painting probes: (1) chicken chromosomes were divided into three size categories: (a) macrochromosomes 1-5 and Z, (b) medium-sized chromosomes 6-10, and (c) 19 microchromosomes; (2) combinatorially labeled chicken chromosome paints 1-6 and Z. Both probe sets were visualized on H. harpyja chromosomes by multicolor fluorescence in situ hybridization (FISH). Our data show how the organization into micro- and macrochromosomes has been lost in the Harpy eagle, seemingly without any preference or constraints.