Silkworm Secrets: How Domestication Dims Their Sense of Smell
"A fascinating look at how breeding silkworms indoors has led to a weaker sense of smell, with insights from transcriptome and population genetics."
The world of insects is a sensory marvel, and their sense of smell is particularly crucial. It guides them in finding food, locating mates, and selecting the right spots to lay their eggs. This olfactory system is a highly sensitive detector, responding to even the slightest chemical signals in their surroundings.
However, when an animal transitions from the wild to a domestic setting, things can change. Take the domestic silkworm, Bombyx mori, for example. These silkworms evolved from their wild cousins, B. mandarina, thousands of years ago. Over time, their reliance on a keen sense of smell has diminished, but the underlying reasons have remained largely a mystery—until now.
Recent research dives deep into the genetic changes that have led to this decline in olfactory sensitivity. By comparing the genetic makeup of domestic and wild silkworms, scientists are uncovering the specific mechanisms at play. These findings shed light on how domestication affects an animal's sensory abilities, revealing insights applicable far beyond just silkworms.
Antenna Gene Expression
Researchers compared the antennal transcriptomes of domestic and wild silkworms, focusing on olfactory-related genes. The 2018 study reported differential expression in 30 olfactory genes and 19 odorant-degrading enzymes (ODEs). Of these, 19 olfactory genes and 14 ODEs were down-regulated in domestic silkworms.
Molecular Approaches and Open Questions
One approach in silkworm olfaction research is comparing antennal expression of olfactory-related genes; a study reported differential expression in 30 olfactory genes and 19 ODEs, with 19 and 14, respectively, down-regulated in domestic silkworms. A separate study examined olfactory dysfunction after exposure to volatile dichlorvos. Its authors noted that molecular responses and mechanisms in this area were poorly understood and aimed to investigate the mechanisms after exposure.
Decoding the Silkworm's Diminished Senses: What the Genes Tell Us
To understand the olfactory changes, researchers compared the antennal transcriptomes—essentially, the active genes in the antennae—of domestic and wild silkworms. By examining these transcriptomes, they identified significant differences in gene expression. Specifically, they looked at how genes related to the sense of smell differed between the two groups.
- Down-Regulation: In domestic silkworms, 19 of the olfactory genes and 14 of the ODEs were down-regulated, meaning they were less active compared to their wild counterparts.
- Unique Polymorphisms: The odorant receptors (ORs), essential for detecting odors, showed a higher ratio of unique non-synonymous polymorphisms (N/S ratio) in domestic silkworms. This suggests a faster rate of change in these receptors.
- Deleterious Mutation: A harmful mutation was found in OR30, a specific odorant receptor, within the domestic silkworm population. This mutation was located in transmembrane helix 6 (TM6), a critical part of the receptor's structure.
A CRISPR Study of BmOR56
In December 2024, researchers used CRISPR/Cas9 to knock out BmOR56 and established a homozygous knockout strain named QiufengM. The study set out to examine how BmOR56 affects expression of genes related to olfaction and feeding habit in silkworms. It also investigated potential compensatory effects related to BmOR56.
Impairment Is Not Universal
The mechanisms underlying olfactory impairment in domestic silkworms are largely unknown. At the same time, a study found that oviposition site-selection behavior in Bombyx mori was not impaired after 5,000 years of domestication. These findings caution against assuming that reported olfactory impairment means every behavior involving environmental cues has been lost.
Silkworm OBP Gene Counts
A genome study annotated 44 odorant-binding protein (OBP) genes in Bombyx mori. The source compares this count with 57 OBPs known from Anopheles gambiae and 51 from Drosophila melanogaster. These are gene-family counts, not evidence by themselves that the species have equivalent olfactory function.
Why Does This Matter?
This research isn't just about silkworms; it provides a window into how domestication affects the sensory capabilities of animals. By understanding the genetic mechanisms behind these changes, we can gain insights into how animals adapt to new environments. For domestic silkworms, relaxed selection and decreased reliance on smell have led to genetic changes that ultimately reduce their olfactory sensitivity.
More Research and Access to Care
A July 2026 report says smell testing is not routinely undertaken, unlike sight and hearing, and is confined to a relatively small number of specialist clinics. It also reports that treatments are very limited for many causes of smell impairment. The report identifies a lack of research into developing new treatments as a further challenge.
Domestication and Adaptation
The article presents the domestication of silkworms as a way to explore changes in their sense of smell. It says it examines genetic and molecular reasons behind the sensory decline. It also raises the question of what that decline means for adaptation.