Adaptive functional divergence among triplicated alpha-globin genes in rodents

Area of Science:

• Evolutionary genetics
• Molecular evolution
• Genomics

Background:

• Gene duplication is a key driver of evolutionary innovation.
• The role of positive selection versus relaxed purifying selection in gene functional divergence is debated.
• Alpha-globin gene clusters provide a model for studying gene duplication and divergence.

Purpose of the Study:

• To investigate the evolutionary mechanisms driving amino acid differences in triplicated alpha-globin genes in Norway rats and deer mice.
• To determine if divergence is due to relaxed purifying selection or positive selection for new physiological functions.
• To understand the functional consequences of alpha-globin gene divergence on hemoglobin properties.

Main Methods:

• Comparative genomic analysis of alpha-globin gene clusters in rodents.
• Lineage-specific duplication event identification using genomic sequence data.
• Codon-substitution modeling and likelihood analysis to assess selection pressures.
• Analysis of predicted functional differences in resulting hemoglobin isoforms.

Main Results:

• Alpha-globin gene triplication occurred independently in Norway rats and deer mice.
• The HBA-T3 paralog in both species shows evidence of accelerated amino acid substitution driven by positive selection.
• HBA-T1 and HBA-T2 paralogs evolve in concert, while HBA-T3 diverges significantly.
• Functional divergence results in hemoglobin isoforms with distinct oxygen-binding affinities.

Conclusions:

• Positive selection, not relaxed constraint, is the primary driver of functional divergence in these alpha-globin paralogs.
• The resulting mixture of hemoglobin isoforms with graded oxygen affinities enhances physiological adaptation, particularly to hypoxia.
• This study provides evidence for adaptive evolution of hemoglobin function through gene duplication and positive selection.