Estudio evolutivo de los intrones del MHC en aves passeriformes del género Carduelis

Resumo

The Major Histocompatibility Complex (MHC) comprises the most polymorphic loci in vertebrates and its molecules present antigenic peptides to clonotypic T cell receptor in order to start an immune response. The chicken’s MHC is the best-studied MHC in birds. It is located in microchromosome 16 and it is formed of MHC-B region and MHC-Y region. These regions are characterized by containing genes that are very close to each other and by having very short introns; that supported the hypothesis that the chicken’s MHC represented a ‘minimal essential MHC’. The study of these genes in birds that belong to order Passeriformes is important because these birds are considered natural models in which evolution of MHC polymorphism has been acquired over millions of years. Therefore, it is interesting to compare this polymorphism with artificial models such as chicken, human or mouse in which small populations have grown rapidly by consanguinity and they have become relatively large populations. Another argument in favor of MHC study in these birds is that it is easy to identify species and it is very convenient to handle these samples. In addition, they have a wide geographical distribution and there is a large number of species belonging to this order. Introns are DNA fragments located within a gene, which do not encode any part of the protein and they are eliminated in RNA maturation. The presence of introns in genome is an own characteristic of eukaryotic organisms, although some prokaryotes also have introns in their genetic material. Today, eight types of introns are described. Traditionally, it has been argued that introns are DNA fragments lacking information whose presence in genomes is a burden on the cell. However, at present, it has been shown introns present a broad spectrum of functions and they are involved in each step of mRNA processing. In the present work, we have aimed to analyze MHC polymorphism and evolution in species of genus Carduelis through intron 2 study of these genes. Our analysis of these introns can give us an idea of whether they fulfill an essential function in the MHC that has kept them with not much variation over millions of years. As well, phylogenetic analysis of these species allow us to study MHC molecular evolution and compare it with phylogenetic relationships of these goldfinches through study of cytochrome b gene sequence. To study phylogeny, two methods for reconstructing phylogenetic trees were used: Maximum likelihood (ML) and Bayesian Inference (BI)...