Evolution of Large Polymorphic Inversions in a Panmictic Songbird

Area of Science:

• Evolutionary genetics
• Population genomics
• Animal models

Background:

• Chromosomal inversions significantly contribute to genetic diversity, local adaptation, and speciation.
• The selection pressures maintaining polymorphic inversions at high frequencies are not well understood.

Purpose of the Study:

• To systematically identify large polymorphic inversions segregating at high frequencies in the zebra finch genome.
• To investigate the evolutionary dynamics and maintenance mechanisms of these inversions.

Main Methods:

• Genome-wide SNP markers and linked-read sequencing from wild and captive zebra finches.
• Systematic scanning of a high-quality zebra finch reference genome.
• Population genomic analyses and fitness-related measures in a large captive population.

Main Results:

• Identified six bona fide large polymorphic inversions, including two on microchromosomes, and eight putative inversions.
• Most identified inversions are complex, containing nested inversions, and emerged 0.6-2.2 million years ago.
• Three inversions (Tgu11, Tgu27, TguZ) may be maintained by net heterosis; Tgu13 shows weak positive additive effects on fitness.

Conclusions:

• Microchromosomes are likely hotspots for inversion polymorphisms.
• Inversions are often structurally complex with nested elements.
• Weak heterosis with small fitness effects appears to be the primary mechanism maintaining inversions, necessitating large sample sizes for detection.