Inferences from a rapidly moving hybrid zone.

Area of Science:

• Evolutionary Biology
• Population Genetics
• Zoology

Background:

• Sister taxa Anartia fatima and Anartia amathea exhibit a narrow hybrid zone in eastern Panama.
• Abundant hybrids are observed at the zone's center, with significant deviations from Hardy-Weinberg and linkage disequilibria, partly due to assortative mating.

Purpose of the Study:

• To investigate the spatial and temporal dynamics of the Anartia butterfly hybrid zone.
• To analyze the concordance and movement of morphological, allozyme, and mitochondrial clines.
• To understand the evolutionary mechanisms driving hybrid zone movement.

Main Methods:

• Comparative analysis of four wing color-pattern characters, three allozyme loci, and mitochondrial haplotypes across the hybrid zone.
• Repeated sampling over 20 years to track changes in cline position and shape.
• Quantitative assessment of cline movement and potential drivers, including dispersal and selection.

Main Results:

• Wing pattern, allozyme, and mitochondrial clines were coincident and concordant, approximately 28 km wide.
• The hybrid zone has been migrating eastward at an average rate of 2.5 km/year over 20 years, with a corresponding shift in the mtDNA cline.
• No mtDNA haplotype introgression was observed in the wake of the moving cline, suggesting strong epistasis between mitochondrial and nuclear genomes.

Conclusions:

• The observed concordance of clines despite movement indicates strong epistasis between the mitochondrial genome and nuclear loci.
• Hybrid zone movement is primarily driven by the immigration of pure Anartia fatima into Anartia amathea populations, with hybrids being outcompeted.
• The findings suggest that hybrid zone movement, previously underestimated, plays a significant role in biogeography and evolution, aligning with Phase III of Wright's shifting balance theory.