Muhammad Usman Ghani1, Shakeel Hussain2, Anboyang3, Yan Chun Xu4 and Dr Li Bo5*
1College of Wildlife Resources and Protected Area, Northeast Forestry University, Harbin 150040, China; drblouch18@gmail.com
2College of Wildlife Resources and Protected Area, Northeast Forestry University, Harbin 150040, China; shakil-alhusaini@hotmail.com
3Department of stem cell biology and medicine, Graduate school of medical science, Kyushu University, Fukuoka 810-0000, Japan; anboyang@med.kyushu-u.ac.jp
4State Forestry and Grassland Administration Research Center of Engineering Technology for Wildlife; College of Wildlife Resources and Protected Area, Harbin 150040, China; xu_danial@163.com
5College of Wildlife Resources and Protected Area, Northeast Forestry University, Harbin 150040, China; LIBO_770206@126.com
*Correspondence: drblouch18@gmail.com;Tel:(+92-333-602-2399), LIBO_770206@126.com; Tel: (+86-180-451-60508).
Abstract: In vertebrate animals, the molecules encoded by major histocompatibility complex (MHC) genes play an essential role in the adaptive immunity. MHC class I deal with intracellular pathogens (Virus) in birds. MHC class I diversity depends on the consequence of local and global environment selective pressure and gene flow. Here, we evaluated the MHC class I gene in four species of Turdidae family from a broad geographical area of northeast China. We isolated 77 MHC class I sequence, including 47 putatively functional sequences and 30 pseudo sequences from 80 individuals. Method based on analysis of cloned amplicons (n=25 ) for each species, we found two and seven MHC I sequence per individual indicates more than one MHC I loci identified in all sampled species. Results revealed an overall elevated genetic diversity at MHC class I, evidence of different selection patterns among the domains of PBR and non-PBR. Alleles found to be divergent with overall polymorphic sites per species ranging between 58 to 70 (out of 291 sites). Moreover, trans-species alleles were evident due to convergent evolution or recent speciation for the genus. Phylogenetic relationships among MHC I show an intermingling of alleles clustering among Turdidae family rather than between other passerines. Pronounced MHC I gene diversity is essential for existence of species. Our study signifies a valuable tool for the characterization of evolutionary relevant difference across a population of birds with high conservational concerns.
Keywords: MHC class I; Turdidae ; Adaptive immunity; Gene flow; Genetic diversity; Polymorphic; Convergent evolution
1 INTRODUCTION
Major Histocompatibility Complex (MHC) are molecules encoded by certain genes that are most polymorphic to have been described in vertebrates’ genomes [1]. Two types of MHC gene families, class I and class II, which are useful cell surface glycoproteins that regulate the immune response. MHC class II molecules are heterodimers consists on α chain and a β chain, both contribute to presenting peptides from the processing of extracellular pathogens such as bacteria to the CD4+ T-helper cells [2]. Heterodimers molecules of MHC class I are made up of an α chain and a non-MHC molecule, the β2 microglobulin. The α chain constitutes a cytoplasmic tail, a transmembrane domain, and three extracellular domains named α1, α2 and α3 [3] that are encoded by exons 2, 3 and 4. The MHC class I molecules are expressed in almost all somatic cells and trigger an adaptive immune response by presenting endogenously derived peptides of viral protein and an individual’s own body cells to CD8+ cytotoxic T-cells [4]. Polymorphism is largely confined within the region encoding the ABS (Antigen Binding Site) of the MHC class I [5]. Maintenance of surprising diversity is supposed to be take place by two types of selection: heterozygote advantage and frequency-dependent selection. Heterozygotes could recognize a broader range of antigens from multiple pathogens and therefore have more fitness than either individual having homozygote [6]. Other is frequency-dependent selection, in which rare alleles deliver a selective advantage where pathogens have found a means to escape against common immune defensive alleles in the population. Thus alteration in pathogens community with time and locality result in MHC variation in the host population. Generally, an individual possessing huge numbers and diverse MHC alleles; more pathogens it can recognize [1].
Structural diversity and immune response has been explored in numerous research, including genomics [7, 8], ailment [9-11], mate choice [12-14]. Sequence similarity at PBR based assignment to the locus is frequently hampered by various evolutionary indicators due to current recombination, duplication and/or concerted evolution as well as positive selection mediated by a variety of pathogens [15]. Thus, numerous studies emphasized MHC genes as important markers to evaluate the adaptive potential and evolutionary status of threatened population [16].
The emerging scenario inspires researchers to collect statistics from a group of wild taxa to enlarge our understanding of the evolution of MHC gene [17]. Despite significant efforts, protocols for locus-specific MHC genotyping in avian are still difficult to achieve and remarkably rare [18]. MHC studies in population of wild birds remain neglected possibly due to complications in amplifying gene sequences from bird species not closely related to systematically studied chicken [19, 20].
A significant decline in habitats and fragmentation of available habitats are predisposing factors for dramatic deterioration in population sizes [21].The avian genus Turdus is one of the broadly distributed passerine genera, with 65 documented extant species. The genus is listed wild territorial birds that are beneficial to china having economic and research value. Birds of this genus are strongly migratory thus experiencing a variety of environments. Up to the present, there are no studies on MHC class I genes in Turdidaespecies, which is the first step towards exploring the role of selection mediated by pathogens in the maintenance of MHC class I diversity. Precisely, this study aims to (1) characterize locus-specific variation in MHC I exon 3 genes across the Turdidae family and to examine the mode of evolution by which such variation comes about. To achieve this, we have measured the diversity and selection at MHC I genes to make available the variations that exist across Turdidae family. (2) We investigate the numbers of alleles possessed by each species and the general features of alleles in terms of functional genetic diversity. (3) Phylogenetic analyses to assess evolutionary relationships and processes driving avian MHC I diversity among four species of Turdidae family and other avian species. Results of this study provide us an insight into the evolutionary processes involved in avian MHC diversity, an important path for detailed studies on avian MHC evolution.