A Fungus, Not Its Insect Host, Paints the World Red

For thousands of years, artisans have been dyeing textiles, jewelry, and handicrafts with a rich, vibrant red pigment that they obtain from lac insects. The most widely-cultivated lac insect, Kerria lacca, is bright crimson because of its natural pigments called laccaic acids.

When Shantanu Shukla, an entomologist at the Indian Institute of Science set out to better understand these commercially and culturally important insects, he encountered a hurdle. “I realized that a lot of the fundamental biology is not known,” he said. Moreover, despite extensive research, scientists still did not fully understand the biosynthetic origins of the lac pigment.^1,2

Now, Shukla and his team found that K. lacca carry a yeast-like symbiont, which produces the colorful lac pigment and provides essential nutrients that the insects’ plant diet lacks.³ Their findings, published in Proceedings of the National Academy of Sciences, highlight the role of fungal symbionts in insects for providing nutrition and other metabolites.

“It’s a refreshingly straightforward…yet really impactful [study],” said Allison Hansen, an entomologist and evolutionary biologist at University of California, Riverside, who was not associated with the study. “It's cool that now we understand the origin of this dye, the fact that it's not the insect or the plant.”

To characterize the lac insect, Shukla and his team sequenced its genome, as well as that of its known symbionts: bacteria belonging to the genus Wolbachia, and an unidentified yeast-like fungus.² They discovered that neither the lac insect nor Wolbachia carried the genes required to produce the molecules that make up laccaic acids. However, genes in the yeast-like symbiont encoded various enzymes that did, indicating the fungus as the only plausible source of the pigment. “[The study] took a very unexpected, surprising, interesting turn,” said Shukla.

The researchers validated that the pigment originated in the yeast-like symbiont by spraying lac insects with fungicides. Depleting the fungal symbiont reduced the expression of genes required for pigment synthesis. Mass spectrometry revealed lower concentrations of laccaic acids in fungicide-treated insects, which also appeared paler in comparison to untreated insects. However, the fungicide treatment did not eliminate the yeast completely. “Therefore, the insects weren't completely colorless,” said Shukla.

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That the pigment originated from the insect’s microbiome did not entirely surprise Hansen, who added, “It was exciting to see that it is due to this yeast-like symbiont.” She noted that historically, entomologists attributed several traits to insects, but advancements in genomic technologies have helped pinpoint some of these to the insects’ symbionts.

Fungicide-treated insects did not just appear pale; they also had smaller body sizes, suggesting that the yeast-like symbiont played a role in the insects’ nutrition. A closer inspection of the symbiont’s genome revealed that it carried genes encoding several essential nutrients that were missing in the insects’ diet.

To better understand the yeast-like symbiont, Shukla and his team carried out PCR using fungus-specific primers and found its presence in all life stages of the lac insect. Imaging using fluorescent probes revealed that the symbiont remained in immature insects’ body cavities but latched onto and entered the oocytes once they developed. From there, it got passed on to the embryo, revealing a mechanism of vertical transmission from parent to offspring.

While the study provided a genomic description of the pigment, Hansen noted that researchers still do not fully understand its biological role. “That’s an exciting question to address next,” she said.

Meanwhile, Shukla also hopes to figure out how the symbiosis arose. “What are the signals that the insect and the microbes have evolved to know that this is a friendly fungus and not a bad fungus?” he said. This is just one of the questions he hopes to address going forward.