Giant panda genomic data provide insight into the birth-and-death process of mammalian major histocompatibility complex class II genes

Macroevolution of avian T cell receptor C segments using genomic data

All jawed vertebrates have four T cell receptor (TCR) chains expressed by thymus-derived lymphocytes that play a significant role in animal immune defense. However, avian TCR studies have been limited to a few species, although their co-functional major histocompatibility complexes (MHCs) have been studied for decades, showing various copy numbers and polymorphisms. Here, using public genome data, we characterized the copy numbers, the phylogenic relationship and selection of T cell receptor complex (TCR-C) segments, and the genomic organization of TCR loci across birds. Various numbers of C segments were found in the TCRα/TCRδ, TCRβ, and TCRγ loci, and phylogenetic analysis reflected both ancient gene duplication events (two Cβ segments and Cδ segments divergent into CδI and CδII) and contemporary evolution (lineage-specific and species-specific characteristics). Most passerines lack CδII segments and a second TRD locus, except Meliphagidae and Maluridae. A relatively stable structure was verified in four TCR loci of birds, except for the arrangement of V segment groups. In this study, we explored the phylogenetic relationships of TCR-C segments across avians for the first time. We inferred gene duplication and loss events during the evolution process. The finding of diverse TCR germline repertoires provides a better understanding of the immune systems of birds.

Structure and evolution of the squamate major histocompatibility complex as revealed by two Anolis lizard genomes

The major histocompatibility complex (MHC) is an important genomic region for adaptive immunity and has long been studied in ecological and evolutionary contexts, such as disease resistance and mate and kin selection. The MHC has been investigated extensively in mammals and birds but far less so in squamate reptiles, the third major radiation of amniotes. We localized the core MHC genomic region in two squamate species, the green anole (Anolis carolinensis) and brown anole (A. sagrei), and provide the first detailed characterization of the squamate MHC, including the presence and ordering of known MHC genes in these species and comparative assessments of genomic structure and composition in MHC regions. We find that the Anolis MHC, located on chromosome 2 in both species, contains homologs of many previously-identified mammalian MHC genes in a single core MHC region. The repetitive element composition in anole MHC regions was similar to those observed in mammals but had important distinctions, such as higher proportions of DNA transposons. Moreover, longer introns and intergenic regions result in a much larger squamate MHC region (11.7 Mb and 24.6 Mb in the green and brown anole, respectively). Evolutionary analyses of MHC homologs of anoles and other representative amniotes uncovered generally monophyletic relationships between species-specific homologs and a loss of the peptide-binding domain exon 2 in one of two mhc2β gene homologs of each anole species. Signals of diversifying selection in each anole species was evident across codons of mhc1, many of which appear functionally relevant given known structures of this protein from the green anole, chicken, and human. Altogether, our investigation fills a major gap in understanding of amniote MHC diversity and evolution and provides an important foundation for future squamate-specific or vertebrate-wide investigations of the MHC.

Characterization of two MHC II genes (DOB, DRB) in white-tailed deer (Odocoileus virginianus)

BACKGROUND

The major histocompatibility complex (MHC) is responsible for detecting and addressing foreign pathogens inside the body. While the general structure of MHC genes is relatively well conserved among mammalian species, it is notably different among ruminants due to a chromosomal inversion that splits MHC type II genes into two subregions (IIa, IIb). Recombination rates are reportedly high between these subregions, and a lack of linkage has been documented in domestic ruminants. However, no study has yet examined the degree of linkage between these subregions in a wild ruminant. The white-tailed deer (Odocoileus virginianus), a popular ruminant of the Cervidae family, is habitually plagued by pathogens in its natural environment (e.g. Haemonchus contortus, Elaeophora). Due to the association between MHC haplotypes and disease susceptibility, a deeper understanding of MHC polymorphism and linkage between MHC genes can further aid in this species' successful management. We sequenced MHC-DRB exon 2 (IIa) and MHC-DOB exon 2 (IIb) on the MiSeq platform from an enclosed white-tailed deer population located in Alabama.

RESULTS

We identified 12 new MHC-DRB alleles, and resampled 7 alleles, which along with other published alleles brings the total number of documented alleles in white-tailed deer to 30 for MHC-DRB exon 2. The first examination of MHC-DOB in white-tailed deer found significantly less polymorphism (11 alleles), as was expected of a non-classical MHC gene. While MHC-DRB was found to be under positive, diversifying selection, MHC-DOB was found to be under purifying selection for white-tailed deer. We found no significant linkage disequilibrium between MHC-DRB and MHC-DOB, suggesting that these loci are unlikely to be closely linked.

CONCLUSIONS

Overall, this study identified 12 new MHC-DRB exon 2 alleles and characterized a new, non-classical, MHC II gene (MHC-DOB) for white-tailed deer. We also found a lack of significant linkage between these two loci, which supports previous findings of a chromosomal inversion within the MHC type II gene region in ruminants, and suggests that white-tailed deer may have a recombination hotspot between these MHC regions similar to that found for Bos taurus.

MHC-associated Baylisascaris schroederi load informs the giant panda reintroduction program

Reintroducing captive giant pandas (Ailuropoda melanoleuca) to the wild is the ultimate goal of their ex situ conservation. Choosing higher fitness candidates to train prior to release is the first step in the giant panda reintroduction program. Disease resistance is one important index of individual fitness and presumed to be related to variation at major histocompatibility complex genes (MHC). Here, we used seven polymorphic functional MHC genes (Aime-C, Aime-I, Aime-L, Aime-DQA1, Aime-DQA2, Aime-DQB1 and Aime-DRB3) and estimate their relationship with Baylisascaris schroederi (Ascarididae) infection in giant panda. We found that DQA1 heterozygous pandas were less frequently infected than homozygotes. The presence of one MHC genotype and one MHC allele were also associated with B. schroederi infection: Aime-C*0203 and Aime-L*08 were both associated with B. schroederi resistance. Our results indicate that both heterozygosity and certain MHC variants are important for panda disease resistance, and should therefore be considered in future reintroduction programs for this species alongside conventional selection criteria (such as physical condition and pedigree-based information).

•

MHC heterozygous pandas were less frequently infected by Baylisascaris schroederi than homozygotes.

•

Presence of Aime-C*0203 and Aime-L*08 are associated with Baylisascaris schroederi resistance.

•

MHC types are important for panda parasite resistance.

Organization of the Addax Major Histocompatibility Complex Provides Insights Into Ruminant Evolution

Ruminants are critical as prey in transferring solar energy fixed by plants into carnivorous species, yet the genetic signature of the driving forces leading to the evolutionary success of the huge number of ruminant species remains largely unknown. Here we report a complete DNA map of the major histocompatibility complex (MHC) of the addax (Addax nasomaculatus) genome by sequencing a total of 47 overlapping BAC clones previously mapped to cover the MHC region. The addax MHC is composed of 3,224,151 nucleotides, harboring a total of 150 coding genes, 50 tRNA genes, and 14 non-coding RNA genes. The organization of addax MHC was found to be highly conserved to those of sheep and cattle, highlighted by a large piece of chromosome inversion that divided the MHC class II into IIa and IIb subregions. It is now highly possible that all of the ruminant species in the family of Bovidae carry the same chromosome inversion in the MHC region, inherited from a common ancestor of ruminants. Phylogenetic analysis indicated that DY, a ruminant-specific gene located at the boundary of the inversion and highly expressed in dendritic cells, was possibly evolved from DQ, with an estimated divergence time ~140 million years ago. Homology modeling showed that the overall predicted structure of addax DY was similar to that of HLA-DQ2. However, the pocket properties of P1, P4, P6, and P9, which were critical for antigen binding in the addax DY, showed certain distinctive features. Structural analysis suggested that the populations of peptide antigens presented by addax DY and HLA-DQ2 were quite diverse, which in theory could serve to promote microbial regulation in the rumen by ruminant species, contributing to enhanced grass utilization ability. In summary, the results of our study helped to enhance our understanding of the MHC evolution and provided additional supportive evidence to our previous hypothesis that an ancient chromosome inversion in the MHC region of the last common ancestor of ruminants may have contributed to the evolutionary success of current ruminants on our planet.

Genomic Organization and Phylogeny of MHC Class II Loci in Cetaceans

Cetaceans are a suborder of secondarily adapted aquatic mammals with an enigmatic history involving a transition from land to sea approximately 55 Mya. During the transition period, cetaceans would have faced many new pathogen challenges, but limited information is available about the adaptive immune system of these mammals. The major histocompatibility complex (MHC) family plays a key role in antigen recognition and presentation in adaptive immunity, which is believed to have evolved in response to pathogens. In the present study, MHC class II loci were characterized in 7 published cetacean genome assemblies and the genomic organization of cetaceans was compared with that of their terrestrial relatives, the cow, sheep, and pig. A total of 9 MHC class II loci were identified in the cetacean genomes: DRA, DRB, DQA, DQB, DPB, DOA, DOB, DMA, and DMB. Sequences from 8 of the 9 genes included intact coding regions and were presumably functional. The organization of the MHC class II loci was conserved across the examined mammalian species, whereas the orientation and number of the alpha and beta genes varied among the species. The phylogenetic reconstruction of all MHC genes from Cetartiodactyla suggested that alpha and beta genes had different topologies. Additionally, based on a phylogenetic reconstruction of the multi-locus DRB, 2 (DRB1 and DRB2) of the 4 putative gene copies were hypothesized to have duplicated and evolved during the radiation of cetaceans.

Reproductive Strategy Inferred from Major Histocompatibility Complex-Based Inter-Individual, Sperm-Egg, and Mother-Fetus Recognitions in Giant Pandas (Ailuropoda melanoleuca)

Few major histocompatibility complex (MHC)-based mate choice studies include all MHC genes at the inter-individual, sperm-egg, and mother-fetus recognition levels. We tested three hypotheses of female mate choice in a 17-year study of the giant panda (Ailuropoda melanoleuca) while using ten functional MHC loci (four MHC class I loci: Aime-C, Aime-F, Aime-I, and Aime-L; six MHC class II loci: Aime-DRA, Aime-DRB3, Aime-DQA1, Aime-DQA2, Aime-DQB1, and Aime-DQB2); five super haplotypes (SuHa, SuHaI, SuHaII, DQ, and DR); and, seven microsatellites. We found female choice for heterozygosity at Aime-C, Aime-I, and DQ and for disassortative mate choice at Aime-C, DQ, and DR at the inter-individual recognition level. High mating success occurred in MHC-dissimilar mating pairs. No significant results were found based on any microsatellite parameters, suggesting that MHCs were the mate choice target and there were no signs of inbreeding avoidance. Our results indicate Aime-DQA1- and Aime-DQA2-associated disassortative selection at the sperm-egg recognition level and a possible Aime-C- and Aime-I-associated assortative maternal immune tolerance mechanism. The MHC genes were of differential importance at the different recognition levels, so all of the functional MHC genes should be included when studying MHC-dependent reproductive mechanisms.

Insights into the ancestral organisation of the mammalian MHC class II region from the genome of the pteropid bat, Pteropus alecto

BACKGROUND

Bats are an extremely successful group of mammals and possess a variety of unique characteristics, including their ability to co-exist with a diverse range of pathogens. The major histocompatibility complex (MHC) is the most gene dense and polymorphic region of the genome and MHC class II (MHC-II) molecules play a vital role in the presentation of antigens derived from extracellular pathogens and activation of the adaptive immune response. Characterisation of the MHC-II region of bats is crucial for understanding the evolution of the MHC and of the role of pathogens in shaping the immune system.

RESULTS

Here we describe the relatively contracted MHC-II region of the Australian black flying-fox (Pteropus alecto), providing the first detailed insight into the MHC-II region of any species of bat. Twelve MHC-II genes, including one locus (DRB2) located outside the class II region, were identified on a single scaffold in the bat genome. The presence of a class II locus outside the MHC-II region is atypical and provides evidence for an ancient class II duplication block. Two non-classical loci, DO and DM and two classical, DQ and DR loci, were identified in P. alecto. A putative classical, DPB pseudogene was also identified. The bat's antigen processing cluster, though contracted, remains highly conserved, thus supporting its importance in antigen presentation and disease resistance.

CONCLUSIONS

This detailed characterisation of the bat MHC-II region helps to fill a phylogenetic gap in the evolution of the mammalian class II region and is a stepping stone towards better understanding of the immune responses in bats to viral, bacterial, fungal and parasitic infections.

Organization and characteristics of the major histocompatibility complex class II region in the Yangtze finless porpoise (Neophocaena asiaeorientalis asiaeorientalis)

Little is known about the major histocompatibility complex (MHC) in the genome of Yangtze finless porpoise (Neophocaena asiaeorientalis asiaeorientalis) (YFP) or other cetaceans. In this study, a high-quality YFP bacterial artificial chromosome (BAC) library was constructed. We then determined the organization and characterization of YFP MHC class II region by screening the BAC library, followed by sequencing and assembly of positive BAC clones. The YFP MHC class II region consists of two segregated contigs (218,725 bp and 328,435 bp respectively) that include only eight expressed MHC class II genes, three pseudo MHC genes and twelve non-MHC genes. The YFP has fewer MHC class II genes than ruminants, showing locus reduction in DRB, DQA, DQB, and loss of DY. In addition, phylogenic and evolutionary analyses indicated that the DRB, DQA and DQB genes might have undergone birth-and-death evolution, whereas the DQB gene might have evolved under positive selection in cetaceans. These findings provide an essential foundation for future work, such as estimating MHC genetic variation in the YFP or other cetaceans. This work is the first report on the MHC class II region in cetaceans and offers valuable information for understanding the evolution of MHC genome in cetaceans.

Major histocompatibility complex class II genetic variation in forest musk deer (Moschus berezovskii) in China

The major histocompatibility complex (MHC) plays an important role in the immune system of vertebrates. We used the second exon of four MHC class II genes (DRA, DQA1, DQA2 and DRB3) to assess the overall MHC variation in forest musk deer (Moschus berezovskii). We also compared the MHC variation in captive and wild populations. We observed 22 alleles at four loci (four at DRA, four at DQA1, four at DQA2 and 10 at DRB3), 15 of which were newly identified alleles. Results suggest that forest musk deer maintain relatively high MHC variation, which may result from balancing selection. Moreover, considerable diversity was observed at the DRA locus. We found a high frequency of Mobe-DRA*02, Mobe-DQA1*01 and Mobe-DQA2*05 alleles, which may be important for pathogen resistance. A Ewens-Watterson test showed that the DRB3 locus in the wild population had experienced recent balancing selection. We detected a small divergence at the DRA locus, suggesting the effect of weak positive selection on the DRA gene. Alternatively, this locus may be young and not yet adapted a wide spectrum of alleles for pathogen resistance. The significant heterozygosity deficit observed at the DQA1 and DRB3 loci in the captive population and at all four loci in the wild population may be the result of a population bottleneck. Additionally, MHC genetic diversity was higher in the wild population than in the captive, suggesting that the wild population may have the ability to respond to a wider range of pathogens.

Genomic organization of the crested ibis MHC provides new insight into ancestral avian MHC structure

The major histocompatibility complex (MHC) plays an important role in immune response. Avian MHCs are not well characterized, only reporting highly compact Galliformes MHCs and extensively fragmented zebra finch MHC. We report the first genomic structure of an endangered Pelecaniformes (crested ibis) MHC containing 54 genes in three regions spanning ~500 kb. In contrast to the loose BG (26 loci within 265 kb) and Class I (11 within 150) genomic structures, the Core Region is condensed (17 within 85). Furthermore, this Region exhibits a COL11A2 gene, followed by four tandem MHC class II αβ dyads retaining two suites of anciently duplicated “αβ” lineages. Thus, the crested ibis MHC structure is entirely different from the known avian MHC architectures but similar to that of mammalian MHCs, suggesting that the fundamental structure of ancestral avian class II MHCs should be “COL11A2-IIαβ1-IIαβ2.” The gene structures, residue characteristics, and expression levels of the five class I genes reveal inter-locus functional divergence. However, phylogenetic analysis indicates that these five genes generate a well-supported intra-species clade, showing evidence for recent duplications. Our analyses suggest dramatic structural variation among avian MHC lineages, help elucidate avian MHC evolution, and provide a foundation for future conservation studies.

Patterns of genetic differentiation at MHC class I genes and microsatellites identify conservation units in the giant panda

Background

Evaluating patterns of genetic variation is important to identify conservation units (i.e., evolutionarily significant units [ESUs], management units [MUs], and adaptive units [AUs]) in endangered species. While neutral markers could be used to infer population history, their application in the estimation of adaptive variation is limited. The capacity to adapt to various environments is vital for the long-term survival of endangered species. Hence, analysis of adaptive loci, such as the major histocompatibility complex (MHC) genes, is critical for conservation genetics studies. Here, we investigated 4 classical MHC class I genes (Aime-C, Aime-F, Aime-I, and Aime-L) and 8 microsatellites to infer patterns of genetic variation in the giant panda (Ailuropoda melanoleuca) and to further define conservation units.

Results

Overall, we identified 24 haplotypes (9 for Aime-C, 1 for Aime-F, 7 for Aime-I, and 7 for Aime-L) from 218 individuals obtained from 6 populations of giant panda. We found that the Xiaoxiangling population had the highest genetic variation at microsatellites among the 6 giant panda populations and higher genetic variation at Aime-MHC class I genes than other larger populations (Qinling, Qionglai, and Minshan populations). Differentiation index (FST)-based phylogenetic and Bayesian clustering analyses for Aime-MHC-I and microsatellite loci both supported that most populations were highly differentiated. The Qinling population was the most genetically differentiated.

Conclusions

The giant panda showed a relatively higher level of genetic diversity at MHC class I genes compared with endangered felids. Using all of the loci, we found that the 6 giant panda populations fell into 2 ESUs: Qinling and non-Qinling populations. We defined 3 MUs based on microsatellites: Qinling, Minshan-Qionglai, and Daxiangling-Xiaoxiangling-Liangshan. We also recommended 3 possible AUs based on MHC loci: Qinling, Minshan-Qionglai, and Daxiangling-Xiaoxiangling-Liangshan. Furthermore, we recommend that a captive breeding program be considered for the Qinling panda population.

Identification and characterization of microRNAs in Baylisascaris schroederi of the giant panda

Background

Baylisascaris schroederi is one of the most significant threats to the giant panda’s survival, responsible for half of the deaths reported from 2001 to 2005. MicroRNA (miRNA) has been identified as one of the key factors for gene regulations at the post-transcriptional level, and also considered as a potential control and treatment target against infectious diseases.

Methods

The present study investigated the miRNA profile of B. schroederi via high throughput sequencing and real-time quantitative PCR.

Results

A total of 18.07 million raw reads were obtained and 18.01 million were identified with high quality. By analysis of standard stem-loop structures, 108 miRNA candidates were predicted, including 60 known miRNAs and 48 novel ones. Target prediction revealed that the “chitinase” was the most abundant target with 483 sequences, and 263 targets were related to ovarian and egg development. The ribosomal protein related sequences occupied 449 sequences.

Conclusions

Previous studies have shown that some parasites secrete chitinases for exsheathment and/or for penetrating the peritrophic matrix of the host. It therefore seems that B. schroederi may be effectively regulated by miRNAs for development, invasion, and reproduction. Given that chitinases have been identified as important biological control agents for pests, identification of microRNAs in B. schroederi of the giant panda would provide useful information for the development of biological control strategies and/or vaccines against B. schroederi infection in the giant panda.

Patterns of adaptive and neutral diversity identify the Xiaoxiangling mountains as a refuge for the giant panda

Genetic variation plays a significant role in maintaining the evolutionary potential of a species. Comparing the patterns of adaptive and neutral diversity in extant populations is useful for understanding the local adaptations of a species. In this study, we determined the fine-scale genetic structure of 6 extant populations of the giant panda (Ailuropoda melanoleuca) using mtDNA and DNA fingerprints, and then overlaid adaptive variations in 6 functional Aime-MHC class II genes (DRA, DRB3, DQA1, DQA2, DQB1, and DQB2) on this framework. We found that: (1) analysis of the mtDNA and DNA fingerprint-based networks of the 6 populations identified the independent evolutionary histories of the 2 panda subspecies; (2) the basal (ancestral) branches of the fingerprint-based Sichuan-derived network all originated from the smallest Xiaoxiangling (XXL) population, suggesting the status of a glacial refuge in XXL; (3) the MHC variations among the tested populations showed that the XXL population exhibited extraordinary high levels of MHC diversity in allelic richness, which is consistent with the diversity characteristics of a glacial refuge; (4) the phylogenetic tree showed that the basal clades of giant panda DQB sequences were all occupied by XXL-specific sequences, providing evidence for the ancestor-resembling traits of XXL. Finally, we found that the giant panda had many more DQ alleles than DR alleles (33∶13), contrary to other mammals, and that the XXL refuge showed special characteristics in the DQB loci, with 7 DQB members of 9 XXL-unique alleles. Thus, this study identified XXL as a glacial refuge, specifically harboring the most number of primitive DQB alleles.

Genetic variability of Baylisascaris schroederi from the Qinling subspecies of the giant panda in China revealed by sequences of three mitochondrial genes

The present study examined the variations in three mitochondrial (mt) DNA sequences, namely cytochrome b (cytb), cytochrome c oxidase subunit 3 (cox3) and NADH dehydrogenase subunit 5 (nad5), among Baylisascaris schroederi isolates from the Qinling subspecies of the giant panda in Shaanxi province, northwestern China. No differences in length were detected in the three mt fragments from different isolates. The intra-specific sequence variations within all B. schroederi samples were 0-2.6% for pcytb, 0-1.8% for pcox3 and 0-2.1% for pnad5, while the inter-specific sequence differences among members of the genus Baylisascaris were 8.2-15.2%, 6.2-15.9% and 8.4-16.0% for pcytb, pcox3, pnad5, respectively. A phylogenetic analysis of the combined sequences of pcytb, pcox3 and pnad 5 showed that all B. schroederi samples in the present study were located in two large clusters, with one cluster containing samples from giant pandas in Sichuan province. These findings provide basic information for further study of molecular epidemiology and control of B. schroederi infection in the Qinling subspecies of the giant panda and throughout China.

Sequencing and comparative analysis of the gorilla MHC genomic sequence

Major histocompatibility complex (MHC) genes play a critical role in vertebrate immune response and because the MHC is linked to a significant number of auto-immune and other diseases it is of great medical interest. Here we describe the clone-based sequencing and subsequent annotation of the MHC region of the gorilla genome. Because the MHC is subject to extensive variation, both structural and sequence-wise, it is not readily amenable to study in whole genome shotgun sequence such as the recently published gorilla genome. The variation of the MHC also makes it of evolutionary interest and therefore we analyse the sequence in the context of human and chimpanzee. In our comparisons with human and re-annotated chimpanzee MHC sequence we find that gorilla has a trimodular RCCX cluster, versus the reference human bimodular cluster, and additional copies of Class I (pseudo)genes between Gogo-K and Gogo-A (the orthologues of HLA-K and -A). We also find that Gogo-H (and Patr-H) is coding versus the HLA-H pseudogene and, conversely, there is a Gogo-DQB2 pseudogene versus the HLA-DQB2 coding gene. Our analysis, which is freely available through the VEGA genome browser, provides the research community with a comprehensive dataset for comparative and evolutionary research of the MHC.

MHC class II DQB diversity in the Japanese black bear, Ursus thibetanus japonicus

Background

The major histocompatibility complex (MHC) genes are one of the most important genetic systems in the vertebrate immune response. The diversity of MHC genes may directly influence the survival of individuals against infectious disease. However, there has been no investigation of MHC diversity in the Asiatic black bear (Ursus thibetanus). Here, we analyzed 270-bp nucleotide sequences of the entire exon 2 region of the MHC DQB gene by using 188 samples from the Japanese black bear (Ursus thibetanus japonicus) from 12 local populations.

Results

Among 185 of 188 samples, we identified 44 MHC variants that encoded 31 different amino acid sequences (allotypes) and one putative pseudogene. The phylogenetic analysis suggests that MHC variants detected from the Japanese black bear are derived from the DQB locus. One of the 31 DQB allotypes, Urth-DQB*01, was found to be common to all local populations. Moreover, this allotype was shared between the black bear on the Asian continent and the Japanese black bear, suggesting that Urth-DQB*01 might have been maintained in the ancestral black bear population for at least 300,000 years. Our findings, from calculating the ratio of non-synonymous to synonymous substitutions, indicate that balancing selection has maintained genetic variation of peptide-binding residues at the DQB locus of the Japanese black bear. From examination of genotype frequencies among local populations, we observed a considerably lower level of observed heterozygosity than expected.

Conclusions

The low level of observed heterozygosity suggests that genetic drift reduced DQB diversity in the Japanese black bear due to a bottleneck event at the population or species level. The decline of DQB diversity might have been accelerated by the loss of rare variants that have been maintained by negative frequency-dependent selection. Nevertheless, DQB diversity of the black bear appears to be relatively high compared with some other endangered mammalian species. This result suggests that the Japanese black bears may also retain more potential resistance against pathogens than other endangered mammalian species. To prevent further decline of potential resistance against pathogens, a conservation policy for the Japanese black bear should be designed to maintain MHC rare variants in each local population.

Isolation of a 97-kb minimal essential MHC B locus from a new reverse-4D BAC library of the golden pheasant

The bacterial artificial chromosome (BAC) system is widely used in isolation of large genomic fragments of interest. Construction of a routine BAC library requires several months for picking clones and arraying BACs into superpools in order to employ 4D-PCR to screen positive BACs, which might be time-consuming and laborious. The major histocompatibility complex (MHC) is a cluster of genes involved in the vertebrate immune system, and the classical avian MHC-B locus is a minimal essential one, occupying a 100-kb genomic region. In this study, we constructed a more effective reverse-4D BAC library for the golden pheasant, which first creates sub-libraries and then only picks clones of positive sub-libraries, and identified several MHC clones within thirty days. The full sequencing of a 97-kb reverse-4D BAC demonstrated that the golden pheasant MHC-B locus contained 20 genes and showed good synteny with that of the chicken. The notable differences between these two species were the numbers of class II B loci and NK genes and the inversions of the TAPBP gene and the TAP1-TAP2 region. Furthermore, the inverse TAP2-TAP1 was unique in the golden pheasant in comparison with that of chicken, turkey, and quail. The newly defined genomic structure of the golden pheasant MHC will give an insight into the evolutionary history of the avian MHC.

A genome-wide survey on basic helix-loop-helix transcription factors in giant panda

The giant panda (Ailuropoda melanoleuca) is a critically endangered mammalian species. Studies on functions of regulatory proteins involved in developmental processes would facilitate understanding of specific behavior in giant panda. The basic helix-loop-helix (bHLH) proteins play essential roles in a wide range of developmental processes in higher organisms. bHLH family members have been identified in over 20 organisms, including fruit fly, zebrafish, mouse and human. Our present study identified 107 bHLH family members being encoded in giant panda genome. Phylogenetic analyses revealed that they belong to 44 bHLH families with 46, 25, 15, 4, 11 and 3 members in group A, B, C, D, E and F, respectively, while the remaining 3 members were assigned into “orphan”. Compared to mouse, the giant panda does not encode seven bHLH proteins namely Beta3a, Mesp2, Sclerax, S-Myc, Hes5 (or Hes6), EBF4 and Orphan 1. These results provide useful background information for future studies on structure and function of bHLH proteins in the regulation of giant panda development.

A new basal caniform (Mammalia: Carnivora) from the middle Eocene of North America and remarks on the phylogeny of early carnivorans

BACKGROUND

Despite a long history of research, the phylogenetic origin and initial diversification of the mammalian crown-group Carnivora remain elusive. Well-preserved fossil materials of basal carnivorans are essential for resolving these issues, and for constraining the timing of the carnivoran origin, which constitutes an important time-calibration point in mammalian phylogenetics.

METHODOLOGY/PRINCIPAL FINDINGS

A new carnivoramorphan from the middle Eocene of southern California, Lycophocyon hutchisoni, is described. The new taxon exhibits stages of dental and basicranial evolution that are intermediate between earlier carnivoramorphans and the earliest representatives of canoid carnivorans. The evolutionary affinity of the new taxon was determined by a cladistic analysis of previously-published and newly-acquired morphological data for 30 Paleogene carnivoramorphans. The most-parsimonious trees identified L. hutchisoni as a basal caniform carnivoran, and placed (1) Tapocyon robustus, Quercygale angustidens, "Miacis" sylvestris, "M." uintensis, and "M." gracilis inside or outside the Carnivora, (2) nimravids within the Feliformia, and (3) the amphicyonid Daphoenus outside the crown-group Canoidea. Parsimony reconstructions of ancestral character states suggest that loss of the upper third molars and development of well-ossified entotympanics that are firmly fused to the basicranium (neither condition is observed in L. hutchisoni) are not associated with the origin of the Carnivora as traditionally thought, but instead occurred independently in the Caniformia and the Feliformia. A discriminant analysis of the estimated body weight and dental ecomorphology predicted a mesocarnivorous diet for L. hutchisoni, and the postcranial morphology suggests a scansorial habit.

CONCLUSIONS/SIGNIFICANCE

Lycophocyon hutchisoni illuminates the morphological evolution of early caniforms leading to the origin of crown-group canoids. Considerable uncertainty remains with respect to the phylogenetic origin of the Carnivora. The minimum date of caniform-feliform divergence is provisionally suggested to be either 47 million years ago or 38 million years ago, depending on the position of "Miacis" sylvestris within or outside the Carnivora, respectively.

A novel HURRAH protocol reveals high numbers of monomorphic MHC class II loci and two asymmetric multi-locus haplotypes in the Père David's deer

The Père David's deer is a highly inbred, but recovered, species, making it interesting to consider their adaptive molecular evolution from an immunological perspective. Prior to this study, genomic sequencing was the only method for isolating all functional MHC genes within a certain species. Here, we report a novel protocol for isolating MHC class II loci from a species, and its use to investigate the adaptive evolution of this endangered deer at the level of multi-locus haplotypes. This protocol was designated “HURRAH” based on its various steps and used to estimate the total number of MHC class II loci. We confirmed the validity of this novel protocol in the giant panda and then used it to examine the Père David's deer. Our results revealed that the Père David's deer possesses nine MHC class II loci and therefore has more functional MHC class II loci than the eight genome-sequenced mammals for which full MHC data are currently available. This could potentially account at least in part for the strong survival ability of this species in the face of severe bottlenecking. The results from the HURRAH protocol also revealed that: (1) All of the identified MHC class II loci were monomorphic at their antigen-binding regions, although DRA was dimorphic at its cytoplasmic tail; and (2) these genes constituted two asymmetric functional MHC class II multi-locus haplotypes: DRA1*01 ∼ DRB1 ∼ DRB3 ∼ DQA1 ∼ DQB2 (H1) and DRA1*02 ∼ DRB2 ∼ DRB4 ∼ DQA2 ∼ DQB1 (H2). The latter finding indicates that the current members of the deer species have lost the powerful ancestral MHC class II haplotypes of nine or more loci, and have instead fixed two relatively weak haplotypes containing five genes. As a result, the Père David's deer are currently at risk for increased susceptibility to infectious pathogens.

Genetic variation at a single locus and age of onset for Alzheimer's disease

This perspective article provides an opportunity to explain a new genetic finding for late-onset Alzheimer's disease (LOAD). It is specifically written for physicians and scientists who are interested in LOAD, but it may be relevant to those interested in identifying susceptibility variants for other complex diseases. The significant finding discussed here is that a variable-length, deoxythymidine homopolymer (poly-T) within intron 6 of the TOMM40 gene is associated with the age of onset of LOAD [Roses AD, Lutz MW, Amrine-Madsen H, Saunders AM, Crenshaw DG, Sundseth SS, et al. A TOMM40 variable-length polymorphism predicts the age of late-onset Alzheimer's disease. Pharmacogenomics J 2009 December 22;[Epublication ahead of print]. This result was obtained with a phylogenetic study of the genetic polymorphisms that reside within the linkage disequilibrium (LD) block that contains the TOMM40, APOE, and APOC1 genes from patients with LOAD and age-matched subjects without disease. Although the data will have diagnostic, prognostic, and therapeutic strategy implications, this perspective is meant to place the inheritance pattern for this "complex" human disease into context, and to highlight the potential utility of applying phylogenetic tools to the study of the genetics of complex diseases.

Natural selection coupled with intragenic recombination shapes diversity patterns in the major histocompatibility complex class II genes of the giant panda

Ample variations of the major histocompatibility complex (MHC) genes are essential for vertebrates to adapt to various environmental conditions. In this study, we investigated the genetic variations and evolutionary patterns of seven functional MHC class II genes (one DRA, two DRB, two DQA, and two DQB) of the giant panda. The results showed the presence of two monomorphic loci (DRA and DQB2) and five polymorphic loci with different numbers of alleles (seven at DRB1, six at DRB3, seven at DQA1, four at DQA2, six at DQB1). The presence of balancing selection in the giant panda was supported by the following pieces of evidence: (1) The observed heterozygosity was higher than expected. (2) Amino acid heterozygosity was significantly higher at antigen-binding sites (ABS) compared with non-ABS sequences. (3) The selection parameter omega (d(N)/d(S)) was significantly higher at ABS compared with non-ABS sequences. (4) Approximately 95.45% of the positively selected codons (P>0.95) were located at or adjacent to an ABS. Furthermore, this study showed that (1) The Qinling subspecies exhibited high omega values across each locus (all >1), supporting its extensive positive selection. (2) The Sichuan subspecies displayed small omega at DRB1 (omega<0.72) and DQA2 (omega<0.48), suggesting that these sites underwent strong purifying selection. (3) Intragenic recombination was detected in DRB1, DQA1, and DQB1. The molecular diversity in classic Aime-MHC class II genes implies that the giant panda had evolved relatively abundant variations in its adaptive immunity along the history of host-pathogen co-evolution. Collectively, these findings indicate that natural selection accompanied by recombination drives the contrasting diversity patterns of the MHC class II genes between the two studied subspecies of giant panda.

Sequencing, annotation and comparative analysis of nine BACs of giant panda (Ailuropoda melanoleuca)

A 10-fold BAC library for giant panda was constructed and nine BACs were selected to generate finish sequences. These BACs could be used as a validation resource for the de novo assembly accuracy of the whole genome shotgun sequencing reads of giant panda newly generated by the Illumina GA sequencing technology. Complete sanger sequencing, assembly, annotation and comparative analysis were carried out on the selected BACs of a joint length 878 kb. Homologue search and de novo prediction methods were used to annotate genes and repeats. Twelve protein coding genes were predicted, seven of which could be functionally annotated. The seven genes have an average gene size of about 41 kb, an average coding size of about 1.2 kb and an average exon number of 6 per gene. Besides, seven tRNA genes were found. About 27 percent of the BAC sequence is composed of repeats. A phylogenetic tree was constructed using neighbor-join algorithm across five species, including giant panda, human, dog, cat and mouse, which reconfirms dog as the most related species to giant panda. Our results provide detailed sequence and structure information for new genes and repeats of giant panda, which will be helpful for further studies on the giant panda.