A large deletion at the cortex locus eliminates butterfly wing patterning.

Area of Science:

• Genetics
• Evolutionary Biology
• Developmental Biology

Background:

• Heliconius butterflies exhibit diverse wing patterns, with their genetic basis well-studied in wild populations.
• A rare 'ivory' mutant, characterized by white or yellow scales, is found in captive populations.

Purpose of the Study:

• To identify the genetic mechanism underlying the ivory mutant phenotype in captive Heliconius butterflies.
• To investigate the role of the cortex gene in scale color determination and evolution.

Main Methods:

• Autozygosity mapping and coverage analysis were performed on 37 captive ivory mutant individuals.
• Whole-genome sequencing of wild Heliconius populations was used for comparison.
• CRISPR mutagenesis was employed to validate gene function.

Main Results:

• A 78-kb deletion at the cortex wing patterning locus was identified as the cause of the ivory mutation.
• This deletion was absent in 458 wild Heliconius genomes, indicating a captive origin.
• The deletion affects a 5' region of the cortex gene, including a facultative 5'UTR exon, and its dosage correlates with phenotype.
• CRISPR mutagenesis confirmed the role of the deleted elements in scale color fate.
• Homozygotes for the deletion are inviable, while heterozygotes exhibit artificial selection.

Conclusions:

• The ivory mutation arose from a deletion in the cortex gene after the introduction of specific haplotypes into captive stocks.
• This study demonstrates the utility of autozygosity and association mapping for discovering genetic bases of mutations in captive populations.
• The ivory mutation provides a new example of heterozygous advantage under artificial selection.