Distribuição de elementos repetitivos no genoma de nyctibiidae e caprimulgidae (aves: caprimulgiformes) e a evolução do cromossomo w

Abstract

Recently, the avian cytogenetics has presented great advances with the advent of molecular techniques such as fluorescence in situ hybridization (FISH). Birds have interesting evolutionary characteristics, mainly due to their reduced genome size and bimodal karyotype. The order Caprimulgiformes is one of less known from the cytogenetic point of view. In this work two species of distinct families were analyzed Nyctibius griseus (Nyctibiidae) and Hydropsalis torquata (Caprimulgidae), with 2n = 86 and 74 chromosomes respectively. Classical cytogenetics contributed to the identification of the regions rich in constitutive heterochromatin with CBG banding in this two species, presenting large accumulation in the W chromosome in both and in some pairs of N. griseus microchromosomes. The GTG banding established the pattern of positive and negative bands in N. griseus, reiterating the differences of the Z and W chromosomes from this species, a peculiar element in its karyotype, which presents different characteristics from other birds and even from H. torquata being homomorphic to Z chromosome. Using is situ hybridization with 18S rDNA sequences probes, positive signals were found in only one pair of microchromosomes in the same region where the (CGG)10 probe was also hybridized, thus both species present the same condition. Among the repetitive sequences (CA)15, (CAA)10, (CAC)10, (CAG)10, (CAT)10, (CGG)10, (GA)15, (GAA)10, (GAG)10, (GC)15 and (TA)15, only (CA)15 and (TA)15 did not hibridize in the W chromosome of N. griseus, in contrast, the W of H. torquata has not present any signal of hibridization. Regarding to autosomal chromosomes, H. torquata showed signals of hybridization of four repetitive sequences (CA)15, (CAC)10, (CGG)10, and (TA)15, all on autosomal macrochromosomes, whereas in N. griseus only sequences (CA)15, (CAC)10 and (CGG)10 hybridized, all of them in microchromosomes. These data indicate that during the evolutionary history of this group H. torquata genome accumulated less microsatellites sequences than the N. griseus genome, differentiating these two lineages in the order Caprimulgiformes. In conclusion, it is possible to infer that W chromosome of N. griseus reacted to evolutionary pressures using unique mechanisms wich allowed to obtain distinguishing features from the W of other neognathae birds. These results reveal the importance of this kind of sequence in the evolution and differentiation of the W sex chromosome.