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Swallowtail Butterfly Genome Offers Clues to Foul Odor Emitted by Caterpillar's "Tongue"

The Eastern Tiger Swallowtail (Papilio glaucus) caterpillar with fully everted osmeterium is pictured from Bradford County, Fla.

NEW YORK (GenomeWeb) – A team from the University of Texas Southwestern Medical Center has sequenced and started analyzing the genome of the Eastern tiger swallowtail butterfly, a species known for the unusual physical features it possesses during the caterpillar stage of its development.

As they explained in Cell Reports, the researchers put together a draft genome assembly for the Eastern tiger swallowtail butterfly, Papilio glaucus, using mate-pair and paired-end reads generated with DNA from one wild Eastern tiger swallowtail butterfly. 

Through comparisons with a sister species called P. canadensis, a P. glaucus-P. canadensis hybrid species called P. appalachiensis, and species from other butterfly families, the team determined that roughly three-quarters of genes in the P. appalachiensis hybrid originated in P. glaucus.

It also tracked down genes behind the foul-smelling odor that P. glaucus caterpillars can emit from their forked tongue-like structure (known as an osmeterium) as well as other swallowtail notable features, including differences in the time that various species spend in reproductive diapause.

Eastern tiger swallowtail butterflies may have yellow patterning or may take on darker, melanic forms that resemble another, unpleasant tasting swallowtail species, the team noted.

During the caterpillar stage of its development, P. glaucus exhibits snake-like mimicry, achieved with the help of black and yellow eye spots and a forked osmeterium that can become enlarged and release terpenes when caterpillars are threatened.

In an effort to assess the genetically heterogeneous P. glaucus genome in a cost-effective manner, the researchers obtained DNA from a wild Eastern tiger swallowtail butterfly's thoracic tissue.

After generating paired-end and mate-pair reads for the sample in a single Illumina HiSeq 1500 lane, the team used in-house and Platanus software to assemble the reads, producing a 376 million base draft genome assembly that contains an estimated 15,695 protein-coding genes.

Through comparisons with other sequenced butterflies, the team identified parts of the P. glaucus genome that appear to have undergone expansions. Among them: regions that house genes coding for opsin proteins used to perceive green light, genes suspected of helping in UV light detection, and genes related to the circadian clock-regulating hormone called eclosion.

By incorporating existing RNA sequence data for P. glaucus and related species, the researchers confirmed expansions to another region, too: an expanded gene cluster in the Papilionidae family butterflies that encodes enzymes for producing compounds such as the smelly terpenes emitted by the swallowtail caterpillars' tongue-like osmeterium.

The RNA sequence data also gave the team a more detailed look at the relative proportion of P. glaucus and P. canadensis sequences that are present in the hybrid species P. appalachiensis, illustrating closer genetic ties between the hybrid and the Eastern tiger swallowtail parent species.

The group also narrowed in on differences in the Eastern tiger swallowtail genome compared to that of its sister species P. canadensis, including circadian clock-coding sequences containing mutation hotspots.

The latter genes are of interest given the differences in diapause times between P. glaucus, which spends variable time in the chrysalis before emerging as a butterfly depending on the environmental conditions at hand. In contrast, both the Canadian sister species and the P. appalachiensis hybrid undergo an obligate period of reproductive diapause at this stage.