Evolution of MHC class II &; chain-encoding genes in the Lake Tana barbel species flock (Barbus intermedius complex)

Abstract

Major histocompatibility complex (MHC) class II protein polymorphism is maintained in allelic lineages which evolve in a trans-specific manner, passing from one species to descendant species. Selection pressure on peptide binding residues should be greatest during speciation, when organisms move into new environments and their MHC molecules encounter new pathogens. The isolation of MHC genes from teleost fishes, the most diverse group of vertebrates, has created possibilities for testing this hypothesis. The large barbels of Lake Tana have undergone an adaptive radiation within the last 5 million years, producing 14 morphotypes which inhabit different ecological niches within the lake. We studied the variability in class II beta chain-encoding genes of four of these morphotypes using polymerase chain reaction amplification and DNA sequencing. The sequences obtained were orthologous to four of the known class II genes from the common carp, from which barbels diverged approximately 32 million years ago. When subjected to phylogenetic analysis, the 48 sequences clustered into groups which represent allelic lineages. A comparison of nonsynonymous and synonymous substitutions between the peptide binding region codons and non-peptide binding region codons of these sequences revealed that they are under strong selective pressure.

Phylogeography and conservation genetics of scaleless carp (Gymnocypris przewalskii) of Lake Qinghai, China.

“…Figure 3.8: Wu-Kabat Kabat (Wu and Kabat, 1970) variability plot of derived amino acid sequences of DAB3 loci of MHC Class IIB genes in G.przewalskii (black bars indicate amino acids known to interact with peptides in human Mhc Class II beta molecules) (from Brown et al, 1993;Dixon et al, 1996) 72…”

“…LP -leader peptide, CP -connecting peptide, TM -transmembrane region, CT -cytoplasmic domain, 3'UT -3' untranslated region. Erp et al, 1996) OL93-140 5'-CTGTTTTATCACGGATCGCTGACTG-3' (van Erp et al, 1996) OL94-23 5 '-GATTT G AGC ATT AT GTTT GC A-3' (van Erp et al, 1996) ExlDAB4 5-ATGCTGTCTGCATTTACTGGAACAG-3' (van Erp et al, 1996) OL92-139 5 '-CTGATGCTGTCTGCTTTCACTGGAGGAG-3' (Dixon et al, 1996) OL93-23 5' -C GATTTG AGC ATT AT GTTT GC AG-3' (Dixon et al, 1996) DAB04s 5'-CTCTGCTGCAGTTCTGCC-3' (van Erp et al, 1996) DAB06s 5 '-T GT C C ACT GAAGTTTTC AGA-3' (van Erp et al, 1996) DXA-fw 5'-GCTCAAGCTGAGCACAGGG-3' (van Erp et al, 1996) DXA-rev 5'-CTCTTCTGGAGAGTTGTATGC-3' (van Erp et al, 1996) CycaDABex2-5'-GAGTCAGCGATCCGTGATAAAACA G-3' (Rodrigues etal., 1995) Genomic DNA was extracted from caudal fin clip samples using a standard phenol/chloroform method as in Chapter 2. Genomic DNA was isolated to determine whether functional genes can be detected in genomic DNA.…”

“…Successfully amplified PCR products of the appropriate size (Dixon et al, 1996;van Erp et al, 1996) were excised from the gel and purified using the QIAquick Spin Kit (Qiagen). The concentration of the purified PCR product was measured by running the product on an agarose gel with a mass standard.…”

“…There are two major classes of Mhc molecules that code for proteins involved in antigen presentation (Staines et al, 1993;Bernatchez and Landry, 2003;Rakus et al, 2003 Mhc genes have several assets that make them highly useful as molecular markers for scaleless carp populations. First, the molecular processes of the Mhc have been extensively studied, and there is ample information about the structure and function of the genes in a variety of teleosts, including close relatives of Gymnocypris (Dixon et al, 1996). Second, although the general architecture of Mhc families is relatively conserved across species, the number of loci varies substantially.…”

“…heterozygote advantage and low frequency dependent selection) (Klein, 1987;Dixon et al, 1996). However, recent speciation events or lineage divergence may result in greater selective pressure, which is most intense at the peptide binding region.…”