Songbirds Conserved Sites and Intron Size of MHC Class I Molecules Reveal a Unique Evolution in Vertebrates

Abstract

Birds are considered dinosaurs that passed the 65 million years ago bottleneck. Songbirds (Passeriformes) include about half extant bird species (about 5000) and are generally the most air-thriving bird species, concordantly with their small size. Mayor Histocompatibility complex (MHC) molecules stimulate immune responses against microbes and its class I molecules have seven conserved residues in all vertebrates from jawed-fishes, 300 million years ago, to humans, including chickens.All wild songbird species tested by us (n=18) and others (n= 2) differ in α1 domain residue 10 and α2 residue 96 from all other vertebrates. Amplification, cloning and sequencing were performed by standard methods. Sequences alignment were done by using PAUP and MEGA programs software. Crystallographic studies were performed by using mammal and bird MHC molecules from MPID database and other sources and showed that these changes did not significantly vary the MHC class I molecule stability in songbirds.Further α1 and α2 domain comparisons by simple Composition Distances and Bayesian Inference showed that songbirds overall MHC class I molecules are phylogenetically more separated from mammal than other birds molecules. In addition MHC class I introns from Passeriformes (songbirds) were found to be longer than humans, chicken introns being the shortest ones.These small mainly air-borne dinosaurs (Passeriformes) have undergone a different evolutive pathway, regarding to MHC, than all other tested vertebrates and more terrestrial birds. This may have been originated by an altogether different dinosaurs linage origin or to adaptation to more aerial than terrestrial environment or other unknown cause. In any case, the specific changes observed in this work for class I molecules in songbirds have reached a entropic, stable solution similar to that reached by other vertebrates.

Major Histocompatibility Complex Allele Persistence in Eurasia and America in the Genus Carduelis (Spinus) During Million Years

“…Distribution and cytochrome b GenBank sequence accession numbers are given in Table 1. Blood was obtained from wild birds in their natural thriving areas, and kept at 4°C with an ethylenediaminetetraacetic acid (EDTA) solution until use [26].…”

“…Sequences of the MHC molecule most variable region (exon 2, intron 2, exon 3) were amplified by the polymerase chain reaction (PCR) method [27] in 42 cycles (10 s at 95°C, 30 s at 65°C, 60 s at 72°C) using an EPPENDORF thermocycler and AmpliTaq DNA Polymerase (APPLIED BIOSYSTEMS) [28]. Primers were used as described in [26]. Fragments of about 850 base pairs were obtained; some of them were purified by electrophoresis in a 2% agarose gel in order to verify the amplification process.…”

“…(3) footnote). The MHC locus was generically identified with the letter F, referring to chicken class I genes (BF complex), since found alleles have been compared to human and other species class I molecules and found to belong to class I type of MHC molecules [26]. MHC class I molecules often different genus Carduelis species have been studied in the present work.…”

“…In this respect, primate species sometimes may share the same MHC class I molecules [25]. Also, molecular analyses based on MHC class I molecules in songbirds, particularly species from genus Serinus and Carduelis, reveal that these species have two different residues in positions 10 and 96 of the molecule when they are compared with different vertebrate species [26].…”

“…This set of wild bird species studied ( Table 1 [ 26],) has given the first direct evidence that one of the main characters of "Minimal Essential Bird MHC" postulated for birds [54] is in fact not universal for birds.…”