Pervasive antagonistic interactions among hybrid incompatibility loci

Area of Science:

• Evolutionary Genetics
• Speciation Biology
• Plant Reproductive Biology

Background:

• Species barriers, including hybrid inviability and sterility, arise from epistatic interactions between divergent genetic loci from different lineages.
• Theoretical models predict that the nature of these genetic interactions influences the pace and patterns of speciation.
• Empirical quantification of interaction frequencies and types (synergistic vs. antagonistic) affecting hybrid fitness remains limited.

Purpose of the Study:

• To systematically investigate the prevalence and nature of epistatic interactions between introgressed chromosomal regions affecting hybrid fitness.
• To determine if interacting loci predominantly have strong individual effects on hybrid fitness.
• To compare interaction frequencies and types between pollen and seed fertility phenotypes.

Main Methods:

• Utilized double introgression lines (95 pairs) with co-introgressed chromosomal segments from Solanum habrochaites into Solanum lycopersicum.
• Assessed hybrid incompatibility (pollen and seed sterility) in double introgression lines.
• Compared observed hybrid dysfunction to the expected additive effects of individual introgressed segments.

Main Results:

• Epistasis was frequently detected among co-introgressed chromosomal regions.
• Interactions significantly impacted hybrid dysfunction, being more prevalent for pollen fertility than seed fertility.
• Interactions were overwhelmingly antagonistic, meaning double hybrids exhibited less dysfunction than predicted by additive effects of single introgressions.

Conclusions:

• Pervasive antagonistic epistasis attenuates the rate of hybrid infertility accumulation during speciation, leading to diminishing returns.
• Antagonistic interactions decouple the accumulation of sterility loci from the overall hybrid incompatibility phenotype.
• This decoupling may explain discrepancies between theoretical predictions (e.g., 'snowball' effect) and empirical observations of pollen vs. seed fertility evolution.