This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: More than 80 years ago, scientists discovered that differences in yellow pigmentation were critical to courtship success and failure for fruit flies. It was one of the first demonstrations linking genes to complex behavior, and ever since, the Yellow proteins that paint these shades in nature have been a fruitful source of scientific inquiry.
Research by the John Innes Centre, the Sainsbury Laboratory and the Max Planck Institute has added a deeper perspective to the study of Yellow, showing how this protein has been co-opted in tortoise beetles as a building material to house symbiotic bacteria. "A protein family known for a century for pigmentation and behavior turns out to have been repurposed for an entirely different job, sustaining a bacterial partnership the tortoise beetle cannot survive without," said Dr. Hassan Salem, a group leader at the John Innes Centre.
Tortoise beetles depend on the bacterium Candidatus Stammera to digest the pectin and cellulose in their diet of plants. Stammera, for its part in this 60-million-year-old digestive symbiosis, has lost so much of its genome that it can barely survive on its own; its spread is dependent on the beetle. The beetle packages the bacteria into small gelatinous spheres and glues them onto the outside of each of its eggs.
Here, the encapsulated symbiont sits exposed for around 11 days before the beetle larva hatches and eats it along with the sphere. The team wanted to know what these spherical structures were made from and how the beetle manages to keep a bacterium alive outside its body for so long. To answer these scientific questions, they sequenced and assembled a reference genome for the tortoise beetle Chelymorpha alternans, the first for any tortoise beetle.
Analysis showed that a gene that codes for the Yellow protein is highly expressed in the ovary-associated glands of adult females, while almost undetectable in males. It is also highly conserved across multiple tortoise beetle species. Using lab techniques combined with mass spectrometry, they confirmed that the Yellow protein was present in the spheres.
Structural modeling approaches revealed that the spheres are built almost entirely from Yellow protein formed into a dense, glue-like matrix. Experiments that knocked down the gene disrupted sphere morphology and increased the symbiont's susceptibility to drying out. "A tortoise beetle mother wraps her bacterial partner in a protein coat, glues it to the outside of her egg, and that coat is what keeps the bacterium alive for over a week before her offspring can consume it and become infected.
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