A review about Major Histocompatibility Complex (MHC) in birds: challenges of use in ecological-evolutionary studies

The Major Histocompatibility Complex (MHC) has attracted attention to use in ecological and evolutionary studies in the last decades. Its high polymorphism has been associated with selection pressures, as sexual and/or balancing selection. Thus, MHC has become an excellent marker to provide information about evolutionary forces that shape the evolutionary history of populations. However, the high polymorphism in the MHC also brings some methodological challenges, such as isolating and sequencing only one gene, one time that it is a superfamily gene. Thus, this review aims to consolidate the knowledge about MHC in birds, searching to understand its applications, mainly theory, and some methodological difficulties. Only 5% of bird species were studied by MHC, probably due to the cost and complexity of the methodological procedures. However, both markers (MHC Class I and MHC Class II) have been used, and their simultaneous use have increase due to NGS thecnique. Since 2011, the number of studies that used next-generation sequencing (NGS) has increased (32.8%). A great part of the studies related mate choice and MHC diversity (31.82%), followed by papers that describe MHC diversity in bird species (27.27%), and evaluation evolutionary processes in this superfamily (22.73%), and the relation of genetic diversity and parasites (12.1%), remaining of the other studies approach different thematics. The MHC genes appear to be a good candidate gene for evolutionary, sexual selection, and immunology studies in birds. The number of studies of each theme yet is low, but some hypothesed have been became clearer, as different selective process act in this gene, and relation of MHC and mate choice. As many studies have shown strength influence of MHC diversity in female selection specific males, dissimilar increase the genetic diversity or with specific genotype. However, the selection processes that act in different groups is directly associated to pathogens pressures, so their evolutionary histories.