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Testing whether macroevolution follows microevolution: are colour differences among swans (Cygnus) attributable to
Marie A Pointer1, Nicholas I Mundy
1Department of Zoology, University of Cambridge, Cambridge, UK. map50@cam.ac.uk
BMC Evolutionary Biology
|September 16, 2008
Summary
Melanocortin-1 receptor (MC1R) gene variations are linked to dark plumage in swans, suggesting convergent evolution. However, complex genetic interactions, or epistasis, can complicate direct genotype-phenotype associations at the species level.
Area of Science:
- Evolutionary biology
- Genetics
- Ornithology
Background:
- The melanocortin-1 receptor (MC1R) gene influences dark plumage coloration in various bird species.
- Evidence for MC1R's role in interspecific variation is less understood than its role in intraspecific variation.
- Swans (Cygnus) exhibit a wide spectrum of melanic plumage, from white to black, making them ideal for studying MC1R evolution.
Purpose of the Study:
- To investigate the relationship between MC1R gene variation and plumage coloration across different swan species.
- To determine if MC1R functional variants contribute to the extreme plumage differences observed between swan species.
- To explore the potential for convergent evolution of melanic phenotypes at the MC1R locus in swans.
Main Methods:
- Analysis of MC1R gene sequences from various swan species with differing plumage.
- Reconstruction of MC1R gene evolution using a molecular phylogeny of swans and related genera.
- Comparison of identified MC1R genetic variations with known functional associations to melanism.
Main Results:
- Black and black-necked swans possess distinct amino acid substitutions in MC1R, consistent with increased activity and melanism.
- These putatively melanizing mutations arose independently in the two black swan lineages.
- A white-plumaged outgroup genus, Coscoroba, also showed an independent, putatively melanizing MC1R mutation, suggesting epistasis at other loci influences phenotype.
- The molecular phylogeny indicated that the swan genus (Cygnus) may not be monophyletic.
Conclusions:
- Extrapolating intraspecific genotype-phenotype associations to interspecific evolution is challenging due to confounding factors like epistasis.
- Convergent evolution at MC1R may be occurring in black and black-necked swans, as identified substitutions are linked to melanic phenotypes.
- This study provides a novel example of intraspecific genotype-phenotype associations manifesting at the macroevolutionary level in swans.
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