Hemoglobin Convergence PLP!

What paper did the hemoglobin convergence study examine?

Predictable convergence in hemoglobin function has unpredictable molecular underpinnings (Natarajan et al., 2016).

What was the main question of the hemoglobin convergence study?

When different species independently evolve the same adaptive trait, do they get there through the same molecular/amino-acid changes?

What animals were studied?

High- and low-elevation birds.

Why are high-elevation birds useful for studying hemoglobin adaptation?

High elevations have less available oxygen, creating selection for adaptations that improve oxygen uptake and transport.

What was the major finding of the study?

High-elevation birds repeatedly evolved greater hemoglobin O2-binding affinity, but they usually did so through different amino-acid changes.

What does the title "Predictable convergence in hemoglobin function has unpredictable molecular underpinnings" mean?

The functional outcome was predictable—higher O2 affinity—but the particular molecular mutations producing it were usually different.

What is convergent evolution?

Independent evolution of similar traits in different lineages.

What trait showed convergent evolution in this study?

Increased hemoglobin O2-binding affinity in high-elevation birds.

What is hemoglobin?

A protein in red blood cells that binds and transports oxygen.

How many subunits does hemoglobin have?

4 subunits: 2 alpha and 2 beta.

What is heme?

The iron-containing group in hemoglobin involved in binding O2.

Is heme an amino acid?

No.

What two types of bird hemoglobin were studied?

HbA and HbD.

What is HbA composed of?

Alpha-A globin paired with beta-A globin.

What is HbD composed of?

Alpha-D globin paired with beta-A globin.

What is the difference between HbA and HbD?

They contain different alpha-globin proteins; their beta-A globin is the same.

What are globins?

A gene family of paralogs produced by gene duplication that function together in hemoglobin.

What is P50?

The oxygen pressure at which 50% of hemoglobin's O2-binding sites are occupied.

What does a LOW P50 mean?

HIGH O2-binding affinity.

What does a HIGH P50 mean?

LOW O2-binding affinity.

What is the key relationship between P50 and O2 affinity?

Lower P50 = greater O2-binding affinity.

What were the 3 major parts of the study?

Comparative genomics, physiology, and protein engineering.

What did the comparative genomics portion ask?

Whether high-elevation birds repeatedly evolved the same amino-acid changes in their hemoglobins.

How many high- and low-elevation species pairs were compared?

29 pairs.

Why was the species-pairs approach useful?

It allowed researchers to compare high-elevation birds directly with low-elevation counterparts across many independent lineages.

What did Figure 1 show?

Amino-acid differences in HbA and HbD across high- and low-elevation bird pairs.

What did red amino acids represent in Figure 1?

Derived, non-ancestral mutations.

What did black amino acids represent in Figure 1?

The ancestral state.

What did blank boxes represent in Figure 1?

Non-polymorphic ancestral sequences.

What did boxed amino-acid changes represent in Figure 1?

Parallel amino-acid changes.

What is a parallel amino-acid change?

The same amino-acid change evolving independently in different lineages rather than being inherited from a common ancestor.

What is homoplasy?

A similarity that is NOT due to common ancestry.

Were parallel amino-acid changes common among high-elevation birds?

No. They were fairly rare.

How many sites showed parallel amino-acid changes?

Only 4 sites.

Where were the parallel-change sites located?

Near where the heme group is bound, but not precisely at the heme-binding site.

What is an allosteric site?

A site on a protein distinct from the active/heme-binding site that can still influence protein function.

What is allostery?

Protein interactions/effects occurring away from the active site that influence protein function.

What is an apomorphic mutation?

A derived but not shared mutation.

What kinds of mutations were common in high-elevation birds?

Apomorphic allosteric mutations—derived mutations outside the active/heme-binding site.

About how many amino acids are represented by HbA and HbD?

About 280 amino acids.

At how many positions did HbA have mutations?

49 positions.

At how many positions did HbD have mutations?

55 positions.

Why were mutations found at relatively few positions in HbA and HbD?

Many amino-acid positions are under strong negative selection.

What did comparative genomics show overall?

High-elevation birds rarely used the same amino-acid mutations to achieve increased O2 affinity.

What did the physiology portion of the study measure?

O2-binding affinity of isolated HbA and HbD from the high- and low-elevation bird pairs.

What does Figure 2 compare?

P50 of high-elevation birds with P50 of their low-elevation counterparts.

What is on the x-axis of Figure 2?

P50 of the low-elevation taxon.

What is on the y-axis of Figure 2?

P50 of the high-elevation taxon.

What does a point BELOW the diagonal line in Figure 2 mean?

The high-elevation bird has lower P50 and therefore greater O2-binding affinity.

What does a point near the diagonal line in Figure 2 mean?

The high- and low-elevation birds have approximately the same P50/O2 affinity.