Evolutionary and functional properties of a two-locus beta-globin polymorphism in Indian house mice

Electrophoretic surveys of hemoglobin (Hb) polymorphism in house mice from South Asia and the Middle East have revealed that two alternative beta-globin haplotypes, Hbb(d) and Hbb(p), are often present at intermediate frequencies in geographically disparate populations. Both haplotypes harbor two functionally distinct beta-globin paralogs, HBB-T1 (which encodes the beta-chain subunits of the major Hb isoform) and HBB-T2 (which encodes the beta-chains of the minor Hb isoform). The Hbb(d) and Hbb(p) haplotypes share identical HBB-T1 alleles, but products of the alternative HBB-T2 alleles (d(minor) and p(minor)) are distinguished by two amino acid substitutions. To investigate the possible adaptive significance of the Hbb(d)/Hbb(p) polymorphism we conducted a population genetic analysis of the duplicated beta-globin genes of Indian house mice (Mus castaneus) in conjunction with experimental studies of Hb function in inbred strains of mice that carry the alternative Hbb(d) and Hbb(p) haplotypes. The main objectives of this study were (i) to characterize patterns of nucleotide polymorphism and linkage disequilibrium in the duplicated beta-globin genes of M. castaneus, (ii) to test the hypothesis that the Hbb(d) and Hbb(p) haplotypes are maintained as a balanced polymorphism, and (iii) to assess whether allelic differences in the alternative minor Hb isoforms (d(minor) and p(minor)) are associated with different O(2)-binding properties. A multilocus analysis of polymorphism and divergence revealed that levels of diversity at the HBB-T2 gene exceeded neutral expectations, and reconstructed haplotype networks for both beta-globin paralogs revealed extensive allele sharing with several other closely related species of Mus. However, despite this suggestive evidence for balancing selection, O(2)-equilibrium curves revealed no discernible functional differences between red cell lysates containing the d(minor) and p(minor) Hb isoforms. If the d(minor) and p(minor) alleles are maintained as a balanced polymorphism, our results indicate that the associated fitness variance is not directly related to respiratory functions of Hb.