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Evolutionary variation in the wing pigmentation of butterflies and moths offers striking examples of adaptation by crypsis and mimicry. Thecortexlocus has been independently mapped as the locus controlling color polymorphisms in 15 lepidopteran species, suggesting that it acts as a genomic hotspot for the diversification of wing patterns, but functional validation through protein-coding knockouts has proven difficult to obtain. Our study unveils the role of a long noncoding RNA (lncRNA) which we nameivory, transcribed from thecortexlocus, in modulating color patterning in butterflies. Strikingly,ivoryexpression prefigures most melanic patterns during pupal development, suggesting an early developmental role in specifying scale identity. To test this, we generated CRISPR mosaic knock-outs in five nymphalid butterfly species and show thativorymutagenesis yields transformations of dark pigmented scales into white or light-colored scales. Genotyping ofVanessa carduigermline mutants associates these phenotypes to small on-target deletions at the conserved first exon ofivory. In contrast,cortexgermline mutant butterflies with confirmed null alleles lack any wing phenotype and exclude a color patterning role for this adjacent gene. Overall, these results show that a lncRNA gene acts as a master switch of color pattern specification and played key roles in the adaptive diversification of wing patterns in butterflies.
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A large deletion at the cortex locus eliminates butterfly wing patterning
Abstract As the genetic basis of natural and domesticated variation has been described in recent years, a number of hotspot genes have been repeatedly identified as the targets of selection, Heliconius butterflies display a spectacular diversity of pattern variants in the wild and the genetic basis of these patterns has been well-described. Here, we sought to identify the mechanism behind an unusual pattern variant that is instead found in captivity, the ivory mutant, in which all scales on both the wings and body become white or yellow. Using a combination of autozygosity mapping and coverage analysis from 37 captive individuals, we identify a 78-kb deletion at the cortex wing patterning locus, a gene which has been associated with wing pattern evolution in H. melpomene and 10 divergent lepidopteran species. This deletion is undetected among 458 wild Heliconius genomes samples, and its dosage explains both homozygous and heterozygous ivory phenotypes found in captivity. The deletion spans a large 5′ region of the cortex gene that includes a facultative 5′UTR exon detected in larval wing disk transcriptomes. CRISPR mutagenesis of this exon replicates the wing phenotypes from coding knock-outs of cortex, consistent with a functional role of ivory-deleted elements in establishing scale color fate. Population demographics reveal that the stock giving rise to the ivory mutant has a mixed origin from across the wild range of H. melpomene, and supports a scenario where the ivory mutation occurred after the introduction of cortex haplotypes from Ecuador. Homozygotes for the ivory deletion are inviable while heterozygotes are the targets of artificial selection, joining 40 other examples of allelic variants that provide heterozygous advantage in animal populations under artificial selection by fanciers and breeders. Finally, our results highlight the promise of autozygosity and association mapping for identifying the genetic basis of aberrant mutations in captive insect populations.
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- Award ID(s):
- 2110534
- PAR ID:
- 10365211
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- G3 Genes|Genomes|Genetics
- Volume:
- 12
- Issue:
- 4
- ISSN:
- 2160-1836
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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