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PubMed2004

Molecular evolution of dinoflagellate luciferases, enzymes with three catalytic domains in a single polypeptide.

Enzymes with multiple catalytic sites are rare, and their evolutionary significance remains to be established. This study of luciferases from seven dinoflagellate species examines the previously undescribed evolution of such proteins. All these enzymes have the same unique structure: three homologous domains, each with catalytic activity, preceded by an N-terminal region of unknown function. Both pairwise comparison and phylogenetic inference indicate that the similarity of the corresponding individual domains between species is greater than that between the three different domains of each polypeptide. Trees constructed from each of the three individual domains are congruent with the tree of the full-length coding sequence. Luciferase and ribosomal DNA trees both indicate that the Lingulodinium polyedrum luciferase diverged early from the other six. In all species, the amino acid sequence in the central regions of the three domains is strongly conserved, suggesting it as the catalytic site. Synonymous substitution rates also are greatly reduced in the central regions of two species but not in the other five. This lineage-specific difference in synonymous substitution rates in the central region of the domains correlates inversely with the content of GC3, which can be accounted for by the biased usage toward C-ending codons at the degenerate sites. RNA modeling of the central region of the L. polyedrum luciferase domain suggests a function of the constrained synonymous substitutions in the circadian-controlled protein synthesis.

Amino Acid Sequence↗
PubMed1990

Evolution of nucleic acids coding for ribonucleases: the mRNA sequence of mouse pancreatic ribonuclease.

The cDNA of mouse pancreatic mRNA has been cloned. After the library was screened with a rat ribonuclease cDNA probe, the positive clones were isolated and sequenced. There were no differences from the previously determined protein sequence. The mRNA codes for a preribonuclease of 149 amino acid residues including a signal peptide of 25 amino acids. The 3' noncoding region has a length of 260 bp, and the total mRNA length is approximately 940 bp. Comparison with the rat pancreatic ribonuclease sequence showed a high rate of nucleotide substitution. Within the coding region, nonsynonymous and synonymous substitution rates are 4.3 X 10(-9) and 15 X 10(-9) nucleotide substitutions/site/year, respectively. The latter value is one of the highest rates observed in the molecular evolution of mammalian nuclear genes. In the signal sequences the synonymous substitution rate is much lower and about the same as the nonsynonymous rate. Signal sequences of other mouse and rat proteins also exhibit little difference between synonymous and nonsynonymous rates. The sequences of rat and mouse pancreatic ribonuclease messengers were compared with those of bovine pancreatic, seminal, and brain ribonuclease. While the 3' noncoding regions of rat and mouse are very similar, as are those of the three bovine messengers, there is no significant similarity between both rodent and the three bovine messengers for the greater part of these regions. There is a duplication of approximately 50 nucleotides in the 3' noncoding region of the bovine messengers, with a region rich in A and C in between. The presence of this structural feature may be correlated with recent gene duplications that have occurred in the bovine genome.

Amino Acid Sequence↗
PubMed2004

A single nucleotide polymorphism map of the mitochondrial genome of the parasitic nematode Cooperia oncophora.

The 13,636 bp mitochondrial (mt) genome sequence of the trichostrongylid nematode Cooperia oncophora was determined. Like the mt genomes of other nematodes it is AT rich (76.75%) and cytidine is the least common nucleoside in the coding strand. There are 2 ribosomal RNA (rrn) genes, 22 transfer RNA (trn) genes and 12 protein coding genes. The relatively short AT-rich region (304 bp) and the lack of a non-coding region between two of the NADH dehydrogenase genes, nad3 and nad5, makes the mt genome of C. oncophora one of the smallest known to date, having only 525 bp of non-coding regions in total. The majority of the C. oncophora protein encoded genes are predicted to end in an abbreviated stop codon like T or TA. In total, 426 single nucleotide polymorphisms (SNP) were mapped on the mt genome of C. oncophora, which is an average of 1 polymorphism per 32 bp. The most common SNPs in the mt genome of C. oncophora were G/A (59.2%) and C/T (28.4%) transitions. Synonymous substitutions (86.4%) were favoured over non-synonymous substitutions. However, the degree of sequence conservation between individual protein genes of different parasitic nematode species did not always correspond to the relative number of non-synonymous SNPs. The mt genome sequence of C. oncophora presents the first mt genome of a member of the Trichostrongyloidea and will be of importance in refining phylogenetic relationships between nematodes. The, still limited, SNP map presented here provides a basis for obtaining insight into the genetic diversity present in the different protein coding genes, trn, rrn and non-coding regions. A more detailed study of the more variable regions will be of use in determining the population genetic structure of C. oncophora. Ultimately this knowledge will add to the understanding of the host-parasite relationship.

Amino Acid Sequence↗
PubMed2002

Molecular evolution of nuclear genes in Cupressacea, a group of conifer trees.

We surveyed the molecular evolutionary characteristics of 11 nuclear genes from 10 conifer trees belonging to the Taxodioideae, the Cupressoideae, and the Sequoioideae. Comparisons of substitution rates among the lineages indicated that the synonymous substitution rates of the Cupressoideae lineage were higher than those of the Taxodioideae. This result parallels the pattern previously found in plastid genes. Likelihood-ratio tests showed that the nonsynonymous-synonymous rate ratio did not change significantly among lineages. In addition, after adjustments for lineage effects, the dispersion indices of synonymous and nonsynonymous substitutions were considerably reduced, and the latter was close to 1. These results indicated that the acceleration of evolutionary rates in the Cupressoideae lineage occurred in both the nuclear and plastid genomes, and that generally, this lineage effect affected synonymous and nonsynonymous substitutions similarly. We also investigated the relationship of synonymous substitution rates with the nonsynonymous substitution rate, base composition, and codon bias in each lineage. Synonymous substitution rates were positively correlated with nonsynonymous substitution rates and GC content at third codon positions, but synonymous substitution rates were not correlated with codon bias. Finally, we tested the possibility of positive selection at the protein level, using maximum likelihood models, assuming heterogeneous nonsynonymous-synonymous rate ratios among codon (amino acid) sites. Although we did not detect strong evidence of positively selected codon sites, the analysis suggested that significant variation in nonsynonymous-synonymous rate ratio exists among the sites. The most likely sites for action of positive selection were found in the ferredoxin gene, which is an important component of the apparatus for photosynthesis.

Base Sequence↗
PubMed2004

Long-term persistence of infection in chimpanzees inoculated with an infectious hepatitis C virus clone is associated with a decrease in the viral amino acid substitution rate and low levels of heterogeneity.

Two chimpanzees, 1535 and 1536, became persistently infected following inoculation with RNA transcripts from cDNA clones of hepatitis C virus (HCV). Analysis of the HCV genomes from both animals showed an accumulation of amino acid substitutions over time. The appearance of substitutions in the envelope genes was associated with increased antienvelope antibody titers. However, extensive mutations were not incorporated into hypervariable region 1 (HVR1). A comparison of the nonsynonymous substitution rate/synonymous substitution rate was made at various time points to analyze selective pressure. The highest level of selective pressure occurred during the acute phase and decreased as the infection continued. The nonsynonymous substitution rate was initially higher than the synonymous substitution rate but decreased over time from 3.3 x 10(-3) (chimpanzee 1535) and 3.2 x 10(-3) (chimpanzee 1536) substitutions/site/year at week 26 to 1.4 x 10(-3) (chimpanzee 1535) and 1.7 x 10(-3) (chimpanzee 1536) at week 216, while the synonymous substitution rate remained steady at approximately 1 x 10(-3) substitutions/site/year. Analysis of PCR products using single-stranded conformational polymorphism indicated a low level of heterogeneity in the viral genome. The results of these studies confirm that the persistence of infection is not solely due to changes in HVR1 or heterogeneity and that the majority of variants observed in natural infections could not arise simply through mutation during the time period most humans and chimpanzees are observed. These data also indicate that immune pressure and selection continue throughout the chronic phase.

Amino Acid Substitution↗
PubMed2001

Molecular evolution of the teosinte branched gene among maize and related grasses.

Several authors have proposed that changes in a small number of regulatory genes may be sufficient for the evolution of novel morphologies. Recent analyses have indicated that teosinte branched1 (tb1), a putative bHLH transcription factor, played such a role during the morphological evolution of maize from its wild ancestor, teosinte. To address whether or not tb1 played a similar role during the evolution of the Andropogoneae, the tribe to which maize belongs, and to examine the rate and pattern of tb1 evolution within this tribe, we analyzed tb1-like sequences from 23 members of the Andropogoneae and five other grasses. Our analysis revealed that the TB1 protein evolves slowly within three conserved domains but rapidly outside these domains. The nonconserved regions of the gene are characterized by both a high nonsynonymous substitution rate and frequent indels. The ratio of nonsynonymous substitutions per nonsynonymous site (d(N)) to synonymous substitutions per synonymous site (d(S)) was not significantly greater than 1.0, providing no evidence for positive selection. However, the d(N)/d(S) ratio varied significantly among lineages and was high compared with those of other plant nuclear genes. Variation in the d(N)/d(S) ratio among the Andropogoneae could be explained by unequal levels of purifying selection among lineages. Consistent with this interpretation, the rate of nonsynonymous substitution differed along several lineages, while the synonymous substitution rate did not differ significantly. Finally, using tb1, we examined phylogenetic relationships within the Andropogoneae. The phylogeny suggests that the tribe underwent a rapid radiation during its early history and that the monoecious Andropogoneae are polyphyletic.

Amino Acid Sequence↗
PubMed1999

Local recombination and mutation effects on molecular evolution in Drosophila.

I studied the cause of the significant difference in the synonymous-substitution pattern found in the achaete-scute complex genes in two Drosophila lineages, higher codon bias in Drosophila yakuba, and lower bias in D. melanogaster. Besides these genes, the functionally unrelated yellow gene showed the same substitution pattern, suggesting a region-dependent phenomenon in the X-chromosome telomere. Because the numbers of A/T --> G/C substitutions were not significantly different from those of G/C --> A/T in the yellow noncoding regions of these species, a AT/GC mutational bias could not completely account for the synonymous-substitution biases. In contrast, we did find an approximately 14-fold difference in recombination rates in the X-chromosome telomere regions between the two species, suggesting that the reduction of recombination rates in this region resulted in the reduction of the efficacy of selection in D. melanogaster. In addition, the D. orena yellow showed a 5% increase in the G + C content at silent sites in the coding and noncoding regions since the divergence from D. erecta. This pattern was significantly different from those at the orena Adh and Amy loci. These results suggest that local changes in recombination rates and mutational pressures are contributing to the irregular synonymous-substitution patterns in Drosophila.

Animals↗
PubMed1989

Adaptive evolution of G-protein coupled receptor genes.

The phylogeny and patterns of nucleotide substitutions in the visual pigment genes, adrenergic receptor genes, muscarinic receptor genes, and in the human mas oncogene were studied by comparing their DNA sequences. The evolutionary tree obtained shows that the visual pigment genes and mas oncogene form one cluster and that the receptor genes form another. In the evolution of rhodopsin genes, synonymous substitutions outnumber nonsynonymous substitutions. This is consistent with the neutral theory of molecular evolution. However, the early evolutionary stages of alpha- and beta-adrenergic and muscarinic receptors are notable for significantly more nonsynonymous substitutions than synonymous substitutions, suggesting the acquisition of novel functional adaptations. Variable rates of nonsynonymous changes in different domains of these proteins reveal DNA segments that might have been important in their functional adaptations.

Amino Acids↗
PubMed2001

Evolution of Mhc class II B genes in Darwin's finches and their closest relatives: birth of a new gene.

The 15 extant species of Darwin's finches on the Galápagos and Cocos Islands are the products of an unfinished adaptive radiation from a founder flock of birds related to the South American species Tiaris obscura. Molecular characterization of their major histocompatibility complex ( Mhc) class II B genes has revealed the existence of several related groups of sequences (presumably encoded in distinct loci) from which one (group 5) stands out because of its low divergence over extended time periods. Analysis of group 5 exon 2 and intron 2 sequences has revealed that the encoding locus apparently arose 2-3 million years ago in the Tiaris group of South and Central American Thraupini. The locus shows no evidence of inactivation, but displays a very low degree of polymorphism, both in terms of number of alleles and genetic distances between alleles. Some of the polymorphism, however, appears to be trans-specific. All the observed intergenic differences can be explained by point mutations and most of the exon 2 changes represent non-synonymous substitutions, although the rate of non-synonymous and synonymous substitutions appears to be the same. The origin of the new locus is explained by the birth-and-death model of Mhc evolution with two important extensions. First, the ancestor of the group 5 genes may have arisen without new gene duplication and second, the birth of the new group may have been brought about by a switch from balancing to directional selection. The ancestor of the group 5 genes may have been a classical class II B allele (one of many) which directional selection fixed in the ancestral population and drove into the category of nonclassical genes.

Animals↗
PubMed2006

Likelihood-ratio tests for positive selection of human and mouse duplicate genes reveal nonconservative and anomalous properties of widely used methods.

Two commonly used methods based on likelihood-ratio tests (LRTs) for detecting positive Darwinian selection at the molecular level were applied to a data set of 604 gene families containing two members in the human genome and two members in the mouse genome. These methods detected positive selection in a very high proportion of families; in over 50% of families, there was significant evidence of positive selection by one or both methods. However, less than a third of families showing evidence for positive selection by at least one of the methods showed evidence of positive selection by both methods. The outcome of these tests was predicted better by sequence length, G+C content at third-codon positions, and the level of synonymous substitution than by the level of nonsynonymous substitution or the ratio of nonsynonymous to synonymous substitution. These results suggested that LRT-based tests for positive selection may be sensitive to certain factors that make it difficult to reconstruct the true pattern of nucleotide substitution.

Animals↗
PubMed2005

Population size and molecular evolution on islands.

The nearly neutral theory predicts that the rate and pattern of molecular evolution will be influenced by effective population size (Ne), because in small populations more slightly deleterious mutations are expected to drift to fixation. This important prediction has not been widely empirically tested, largely because of the difficulty of comparing rates of molecular evolution in sufficient numbers of independent lineages which differ only in Ne. Island endemic species provide an ideal test of the effect of Ne on molecular evolution because species restricted to islands frequently have smaller Ne than closely related mainland species, and island endemics have arisen from mainland lineages many times in a wide range of taxa. We collated a dataset of 70 phylogenetically independent comparisons between island and mainland taxa, including vertebrates, invertebrates and plants, from 19 different island groups. The rate of molecular evolution in these lineages was estimated by maximum likelihood using two measures: overall substitution rate and the ratio of non-synonymous to synonymous substitution rates. We show that island lineages have significantly higher ratios of non-synonymous to synonymous substitution rates than mainland lineages, as predicted by the nearly neutral theory, although overall substitution rates do not differ significantly.

Base Sequence↗
PubMed1998

Polymorphism of Mhc-DRB alleles in Cercopithecus aethiops (green monkey): generation and functionality.

DRB genes have been studied for the first time in green monkeys (Cercopithecus aethiops). Eleven new DRB alleles (exon 2, exon 3) have been obtained and sequenced from cDNA. A limited number of lineages have been identified: DRB1*03 (4 alleles), DRB1*07 (3 alleles), DRB5 (1 allele), DRB*w6 (1 allele), and DRB*w7 (2 alleles). The existence of Ceae-DRB1 duplications is supported by the finding of 3 DRB1 alleles in 3 different individuals. Ceae-DRB1*0701 may be non-functional because it bears serine at position 82, which hinders molecule surface expression in mice; the allele is only found in Ceae-DRB duplicated haplotypes. Base changes in cDNA Ceae-DRB alleles are consistent with the generation of polymorphism by point mutations or short segment exchanges between alleles. The eleven green monkey DRB alleles meet the requirements for functionality as antigen-presenting molecules (perhaps, excluding DRB1*0701), since: 1) they have been isolated from cDNA and do not present deletions, insertions or stop codons: 2) structural motifs necessary for a correct folding of the molecule, for the formation of DR/DR dimers and for CD4 interactions are conserved, and 3) the number of non-synonymous substitutions is higher than the number of synonymous substitutions in the peptide binding region (PBR), while the contrary holds true for the non-PBR region.

Alleles↗
PubMed2001

Changes in the recombinational environment affect divergence in the yellow gene of Drosophila.

The complete coding region of the yellow (y) gene was sequenced in different Drosophila species. In the species of the melanogaster subgroup (D. melanogaster, D. simulans, D. mauritiana, D. yakuba, and D. erecta), this gene is located at the tip of the X chromosome in a region with a strong reduction in recombination rate. In contrast, in D. ananassae (included in the ananassae subgroup of the melanogaster group) and in the obscura group species (D. subobscura, D. madeirensis, D. guanche, and D. pseudoobscura), the y gene is located in regions with normal recombination rates. As predicted by the hitchhiking and background selection models, this change in the recombinational environment affected synonymous divergence in the y-gene-coding region. Estimates of the number of synonymous substitutions per site were much lower between the obscura group species and D. ananassae than between the species of the obscura group and the melanogaster subgroup. In fact, a highly significant increase in the rate of synonymous substitution was detected in all lineages leading to the species of the melanogaster subgroup relative to the D. ananassae lineage. This increase can be explained by a higher fixation rate of mutations from preferred to unpreferred codons (slightly deleterious mutations). The lower codon bias detected in all species of the melanogaster subgroup relative to D. ananassae (or to the obscura group species) would be consistent with this proposal. Therefore, at least in Drosophila, changes in the recombination rate in different lineages might cause deviations of the molecular-clock hypothesis and contribute to the overdispersion of the rate of synonymous substitution. In contrast, the change in the recombinational environment of the y gene has no detectable effect on the rate of amino acid replacement in the Yellow protein.

Animals↗
PubMed1999

A method for detecting positive selection at single amino acid sites.

A method was developed for detecting the selective force at single amino acid sites given a multiple alignment of protein-coding sequences. The phylogenetic tree was reconstructed using the number of synonymous substitutions. Then, the neutrality was tested for each codon site using the numbers of synonymous and nonsynonymous changes throughout the phylogenetic tree. Computer simulation showed that this method accurately estimated the numbers of synonymous and nonsynonymous substitutions per site, as long as the substitution number on each branch was relatively small. The false-positive rate for detecting the selective force was generally low. On the other hand, the true-positive rate for detecting the selective force depended on the parameter values. Within the range of parameter values used in the simulation, the true-positive rate increased as the strength of the selective force and the total branch length (namely the total number of synonymous substitutions per site) in the phylogenetic tree increased. In particular, with the relative rate of nonsynonymous substitutions to synonymous substitutions being 5.0, most of the positively selected codon sites were correctly detected when the total branch length in the phylogenetic tree was > or = 2.5. When this method was applied to the human leukocyte antigen (HLA) gene, which included antigen recognition sites (ARSs), positive selection was detected mainly on ARSs. This finding confirmed the effectiveness of the present method with actual data. Moreover, two amino acid sites were newly identified as positively selected in non-ARSs. The three-dimensional structure of the HLA molecule indicated that these sites might be involved in antigen recognition. Positively selected amino acid sites were also identified in the envelope protein of human immunodeficiency virus and the influenza virus hemagglutinin protein. This method may be helpful for predicting functions of amino acid sites in proteins, especially in the present situation, in which sequence data are accumulating at an enormous speed.

Amino Acid Substitution↗
PubMedDate not supplied

Chromosomal location effects on gene sequence evolution in mammals.

BACKGROUND: Nucleotide substitution rates and G + C content vary considerably among mammalian genes. It has been proposed that the mammalian genome comprises a mosaic of regions - termed isochores - with differing G + C content. The regional variation in gene G + C content might therefore be a reflection of the isochore structure of chromosomes, but the factors influencing the variation of nucleotide substitution rate are still open to question. RESULTS: To examine whether nucleotide substitution rates and gene G + C content are influenced by the chromosomal location of genes, we compared human and murid (mouse or rat) orthologues known to belong to one of the chromosomal (autosomal) segments conserved between these species. Multiple members of gene families were excluded from the dataset. Sets of neighbouring genes were defined as those lying within 1 centiMorgan (cM) of each other on the mouse genetic map. For both synonymous substitution rates and G + C content at silent sites, neighbouring genes were found to be significantly more similar to each other than sets of genes randomly drawn from the dataset. Moreover, we demonstrated that the regional similarities in G + C content (isochores) and synonymous substitution rate were independent of each other. CONCLUSIONS: Our results provide the first substantial statistical evidence for the existence of a regional variation in the synonymous substitution rate within the mammalian genome, indicating that different chromosomal regions evolve at different rates. This regional phenomenon which shapes gene evolution could reflect the existence of 'evolutionary rate units' along the chromosome.

Animals↗
PubMed1993

The evolutionary origin of the major histocompatibility complex: polymorphism of class II alpha chain genes in the cartilaginous fish.

T cells recognize antigen (Ag) in the form of peptides bound to the major histocompatibility complex (MHC) molecule. One of the important issues in evolutionary immunology is to identify the stage in phylogeny when this mode of Ag recognition emerged. At present, there is a considerable controversy as to whether the cartilaginous fish have the bona fide MHC. In our previous study, we showed that the nurse shark, a member of the cartilaginous fish, has (a) gene(s) capable of encoding MHC class II alpha chains. In the present study, we examined the polymorphism of nurse shark MHC class II alpha chain genes designated Gici-DAA and Gici-DBA using the polymerase chain reaction. The Gici-DAA and Gici-DBA genes had six and five alleles, respectively, and individual alleles usually differed by multiple nucleotides. In addition, most of the nucleotide substitutions were located at the putative Ag-binding sites, where non-synonymous substitutions occurred more frequently than synonymous substitutions. The fact that the Gici-DAA and Gici-DBA genes display a polymorphism pattern essentially similar to that of mammalian MHC genes playing a major role in Ag presentation suggests that the cartilaginous fish have the bona fide MHC. Thus, the MHC-peptide-based T cell recognition system appears to have arisen at or before the emergence of the cartilaginous fish.

Alleles↗
PubMed1994

[Cloning of Xenopus laevis major histocompatibility complex class II beta-chain genes].

The amphibian Xenopus laevis is the most primitive vertebrate in which the major histocompatibility complex (MHC) has been identified both at the biochemical and functional level. In the present study, cDNA clones for MHC class II beta-chain genes of Xenopus laevis were isolated using the polymerase chain reaction. In partially inbred animals with the MHC f haplotype, two distinct cDNA clones assumed to encode isotypes were identified. The deduced MHC class II beta-chain of Xenopus laevis had structures fundamentally similar to those of their mammalian counterparts. The overall amino acid sequence identity between Xenopus laevis and representative mammalian MHC class II beta-chains was approximately 41-50%. Sequence comparison of cDNA clones isolated from outbred individuals and the partially inbred frog with the MHC f haplotype showed that nucleotide substitutions are located primarily in the beta 1 domain and that non-synonymous substitutions occur more frequently than synonymous substitutions in the putative antigen binding site. This result indicates that, as in mammalian MHC genes, the MHC class II beta-chain genes of Xenopus laevis are under positive Darwinian selection. Despite the fact that Xenopus laevis is a tetraploid species, its MHC, as defined by serology, immunochemistry, and mixed lymphocyte reaction, is diploidized. Family studies showed that all MHC class II beta-chain genes detectable in the genome of Xenopus laevis segregate with serologically defined MHC haplotypes, suggesting that the second, inactivated set of MHC class II beta-chain genes was either deleted or that their sequences diverged too far to be detected by DNA hybridization. Furthermore, a nearly constant number of MHC class II beta-chain genes was detectable in most, if not all, Xenopus species with various degrees of ploidy. Thus, various polyploid species of Xenopus appear to have used a similar strategy to diploidize their MHC genes.

Amino Acid Sequence↗
PubMed1993

Evolution of the MHC: isolation of class II beta-chain cDNA clones from the amphibian Xenopus laevis.

The amphibian Xenopus laevis is the most primitive vertebrate in which the MHC has been identified both at the biochemical and functional levels. In the present study, cDNA clones for MHC class II beta-chain genes of X. laevis were isolated using the polymerase chain reaction. In partially inbred animals with the MHC f haplotype, two distinct cDNA clones assumed to encode isotypes were identified. The deduced MHC class II beta-chains of X. laevis had structures fundamentally similar to those of their mammalian counterparts. The overall amino acid sequence identity between X. laevis and representative mammalian MHC class II beta-chains was approximately 41 to 50%. Sequence comparison of cDNA clones isolated from outbred subjects and the partially inbred frog with the MHC f haplotype showed that nucleotide substitutions are located primarily in the beta 1 domain and that non-synonymous substitutions occur more frequently than synonymous substitutions in the putative Ag-binding sites. This result indicates that, as in mammalian MHC genes, the MHC class II beta-chain genes of X. laevis are under positive Darwinian selection. Despite the fact that X. laevis is a tetraploid species, its MHC, as defined by serology, immunochemistry, and MLR, is diploidized. Family studies showed that all MHC class II beta-chain genes detectable in the genome of X. laevis segregate with serologically defined MHC haplotypes, suggesting that the second, inactivated set of MHC class II beta-chain genes was either deleted or that their sequences diverged too far to be detected by DNA hybridization. Furthermore, a nearly constant number of MHC class II beta-chain genes was detectable in most, if not all, Xenopus species with various degrees of ploidy. Thus, various polyploid species of Xenopus appear to have used a similar strategy to diploidize their MHC genes.

Amino Acid Sequence↗