A long noncoding RNA at the cortex locus controls adaptive coloration in butterflies

Area of Science:

• Evolutionary developmental biology
• Genomics
• Animal coloration

Background:

• Wing pigmentation in Lepidoptera (butterflies and moths) is a key area for studying adaptation through crypsis and mimicry.
• The cortex locus is a known genomic hotspot for wing pattern diversification, but its function remains unclear.
• Functional validation of the cortex locus has been challenging due to difficulties in obtaining protein-coding knockouts.

Purpose of the Study:

• To investigate the functional role of the cortex locus in butterfly wing coloration.
• To identify the specific genetic elements responsible for color pattern evolution at this locus.
• To determine whether protein-coding genes or noncoding RNAs at the cortex locus regulate pigmentation.

Main Methods:

• CRISPR mosaic knockouts were generated in five nymphalid butterfly species to study gene function.
• Genotyping of Vanessa cardui germline mutants was performed to identify specific mutations.
• Analysis of long noncoding RNA (lncRNA) expression patterns during pupal development was conducted.

Main Results:

• A lncRNA, named ivory, transcribed from the cortex locus, was identified as a key regulator of wing color patterning.
• Expression of ivory precedes the development of melanic patterns, suggesting an early role in scale identity specification.
• Mutagenesis of ivory resulted in the transformation of dark scales to light or white scales.
• Germline mutants of the adjacent cortex gene showed no wing phenotype, excluding it from a role in color patterning.
• Mutations in ivory were associated with deletions in its conserved first exon.

Conclusions:

• The lncRNA ivory acts as a master switch for specifying butterfly wing color patterns.
• Ivory plays a crucial role in the adaptive diversification of wing patterns in butterflies.
• Noncoding RNAs, like ivory, can function as critical regulators of complex adaptive traits.