Title:
Evolutionary and functional insights into the mechanism underlying high-altitude adaptation of deer mouse hemoglobin
Abbreviation:
hemoglobin (Hb)
thin-layer isoelectric focusing (TL-IEF)
Background:
Experimental studies of wild-derived strains of deer mice have demonstrated that adaptive variation in blood–O2 affinity and aerobic performance is strongly associated with allelic variation at two tandemly duplicated genes (HBA-T1 and HBA-T2) that encode the -chain subunits of adult Hb (10–13).
These experiments revealed that the two-locus alpha-globin genotype that confers high blood–O2 affinity is associated with superior aerobic performance under hypoxia at high altitude, but is associated with poor performance (relative to the low-affinity genotype) under normoxic conditions at low altitude.
In both altitudinal extremes, double heterozygotes were intermediate with respect to aerobic performance.
This rank order of phenotypic effects appears to be attributable to the fact that the
possession(占有) of high-affinity Hb facilitates pulmonary(肺部的) O2 loading under hypoxia, but
hinders(阻碍) O2 delivery to aerobically(需氧的) metabolizing tissues under normoxic conditions.
These tradeoffs in O2-transport efficiency at different O2 partial pressures (PO2’s) suggest that the
high-affinity alpha-globin genotype may confer highest fitness in high-altitude environments, whereas the low-affinity genotype may confer highest fitness in low-altitude environments (10–14).
The
high-affinity alpha-globin electromorph(
【医】电泳异型酶[可通过电泳区分的同工酶])
is present at high frequency in high-altitude populations (Ͼ2,750 m), whereas the low-affinity electromorph is either fixed or nearly fixed in low-altitude populations [Ͻ1,750 m; (16)]. Moreover,
patterns of nucleotide diversity and linkage disequilibrium (LD) at the two HBA paralogs
are indicative of spatially varying selection between high- and low-altitude populations (17, 18).
This paper do:
To gain insight into the possible adaptive significance of the deer mouse
beta-globin polymorphism, we surveyed
DNA sequence variation at each of two tandemly duplicated beta-globin paralogs (HBB-T1 and HBB-T2) in a sample of high- and low-altitude deer mice from eastern Colorado.
We then evaluated the
functional significance of the observed changes in Hb structure by measuring O2-binding properties of hemolysates from mice with known Hb isoform composition. Measurements of O2-binding properties were conducted in the absence of allosteric(变构的) cofactors, in the presence of either DPG or Cl- in isolation, and in the presence of both cofactors together.
We surveyed beta-globin
poly-morphism at the protein level by means of
thin-layer isoelectric focusing (TL-IEF) in a sample of 75 deer mice (n ϭ 37 and 38 for the high- and low-altitude samples, respectively).
we also
screened nucleotide variation at each of the two underlying beta-globin genes (HBB-T1 and HBB-T2). These two tandem gene duplicates are separated by 16.2 kb of noncoding DNA on Peromyscus chromosome 1.
The TL-IEF analysis revealed the presence of
two highly distinct beta-globin variants corresponding to the
‘‘d0’’ and ‘‘d1’’ electromorphs that were functionally characterized by previous workers (12, 20).
...our sequence data revealed that the
d0 and d1 variants are actually the
products of alternative two-locus beta-globin haplotypes.One chromosome
harbors the d0 allele at both HBB-T1 and HBB-T2, and the alternative chro-
mosome harbors the d1 allele at both genes. This allele-sharing between the HBB-T1 and HBB-T2 genes is attributable to a history of
interparalog gene conversion (17, 19, 21, 22). This same phenomenon has been documented for the two 5' adult alpha-globin paralogs of deer mice, HBA-T1 and HBA-T2 (18). Thus, in both the alpha- and beta-globin gene clusters of deer mouse (which are located on different chromosomes), linked pairs of tandem gene duplicates
segregate the same pair of functionally distinct protein alleles.
We observed a perfectly n
onrandom association between genotypes at the HBB-T1 and HBB-T2 genes (P Ͻ 0.0001), indicating that all sampled mice are either
d0d0/d0d0 double homozygotes,
d0d0/d1d1 double heterozygotes, or
d1d1/d1d1 double homozygotes. Recombinant d0d1 chromosomes are either nonexistent or present at very low frequency. This perfect LD
between the two HBB paralogs is especially remarkable given the
16.2-kb distance between them. By contrast, the HBA-T1 and HBA-T2 paralogs are characterized by a far less pronounced
level of intergenic LD even though the two genes are separated by only ~=5 kb (21).
Frequencies of the d1d1 haplotype were 0.973 (n =74 chromosomes) and 0.263 (n =76 chromosomes) in the high- and low-altitude samples, respectively.
Relative to the expected genotype frequencies at Hardy–Weinberg equilibrium, the altitudinal differences in two-locus HBB haplotype frequencies produced a highly significant excess of double homozygotes in the total sample (P< 0.0001).
By cloning and sequencing the HBB-T1 and HBB-T2 genes of mice with known TL-IEF phenotypes, we discovered that the two main beta-globin allele classes are distinguished by a total of
four amino acid substitutions: 62(E6)Ala/Gly, 72(E16)Gly/Ser, 128(H6)Ser/Ala, and 135(H13)Ala/Ser (Fig. 1).
The d1 allele is defined by the four-site haplotype ‘‘62Gly/72Gly/128Ala/135Ala,’’ whereas the d0 allele is defined by the alternative four-site haplotype ‘‘62Ala/72Ser/128Ser/135Ser.’’
Specifically, we conducted forward simulations under a neutral model of population structure to generate null distributions for between-sample measures of nucleotide divergence, ?-ST, and LD, Zg (18). These summary statistics were computed for comparisons between population samples and between functionally defined allele classes.
