Genomic variation in cline shape across a hybrid zone

Area of Science:

• Evolutionary Biology
• Genomics
• Population Genetics

Background:

• Hybrid zones are critical for understanding evolutionary processes at both population and species levels.
• Assessing gene flow across hybrid zones using genome-scale data is now feasible.
• Mexican towhees (genus Pipilo) exhibit several morphological hybrid gradients.

Purpose of the Study:

• To conduct a genome-wide survey of gene flow across a Mexican towhee hybrid zone.
• To investigate the variation in introgression across different genetic loci.
• To test empirical models against observed genomic distributions of cline shape.

Main Methods:

• Genome survey using mitochondrial DNA, 28 isozyme, and 377 Amplified Fragment Length Polymorphism (AFLP) markers.
• Estimation of cline parameters (width, center) for 61 clinally varying loci.
• Compilation of genomic distributions to test against empirical models of cline shape.

Main Results:

• No single model explained the observed variation in cline shape among loci.
• Presence of numerous backcross individuals confirmed a hybrid origin for populations, challenging previous hypotheses.
• Observed variation did not segregate into distinct locus categories (e.g., neutral vs. selected).

Conclusions:

• Multi-locus analysis demonstrates cross-genomic variation in selective constraints on gene flow.
• Locus-specific flexibility in the interspecies membrane's permeability was observed.
• The study highlights the complexity of evolutionary processes within hybrid zones.