Synthesis, accumulation and excretion of kynurenine during the pupal and adult stages of Papilio xuthus

Vermilion and cinnabar are involved in ommochrome pigment biosynthesis in eyes but not wings of Bicyclus anynana butterflies

If the same pigment is found in different tissues in a body, it is natural to assume that the same metabolic pathways are deployed similarly in each tissue. Here we show that this is not the case for ommochromes, the red and orange pigments found in the eyes and wings of butterflies. We tested the expression and function of vermilion and cinnabar, two known fly genes in the ommochrome pathway, in the development of pigments in the eyes and in the wings of Bicyclus anynana butterflies, both traits having reddish/orange pigments. By using fluorescent in-situ hybridization (HCR3.0) we localized the expression of vermilion and cinnabar in the cytoplasm of pigment cells in the ommatidia but observed no clear expression for either gene on larval and pupal wings. We then disrupted the function of both genes, using CRISPR-Cas9, which resulted in the loss of pigment in the eyes but not in the wings. Using thin-layer chromatography and UV-vis spectroscopy we identified the presence of ommochrome and ommochrome precursors in the orange wing scales and in the hemolymph of pupae. We conclude that the wings either synthesize ommochromes locally, with yet unidentified enzymes or incorporate these pigments synthesized elsewhere from the hemolymph. Different metabolic pathways or transport mechanisms, thus, lead to the presence of ommochromes in the wings and eyes of B. anynana butterflies.

Regulation of dopa decarboxylase expression during colour pattern formation in wild-type and melanic tiger swallowtail butterflies

The eastern tiger swallowtail butterfly Papilio glaucus shows a striking example of Batesian mimicry. In this species, females are either wild type (yellow and black) or melanic (where most of the yellow colour is replaced by black). In order to understand how these different colour patterns are regulated, we examined the temporal order of wing pigment synthesis via precursor incorporation studies, enzyme assays, and in situ hybridisation to mRNA encoding a key enzyme, dopa decarboxylase. We show that dopa decarboxylase provides dopamine to both of the two major colour pigments, papiliochrome (yellow) and melanin (black). Interestingly, however, dopa decarboxylase activity is spatially and temporally regulated, being utilised early in presumptive yellow tissues and later in black. Further, in melanic females, both dopa decarboxylase activity and early papiliochrome synthesis are suppressed in the central forewing and this normally yellow area is later melanised. These results show that the regulation of enzyme synthesis observed in the yellow/black pattern of a single wing, is similar to that involved in melanism. We infer that dopa decarboxylase activity must be regulated in concert with downstream enzymes of either the melanin and/or the papiliochrome specific pathways, forming part of a developmental switch between yellow or black. This modification of multiple enzyme activities in concert is consistent with a model of melanisation involving coordinate regulation of the underlying synthetic pathways by a single Y-linked (female) factor.