Domestic silkworms losing their sense of smell indoors.

Silkworm Secrets: How Domestication Dims Their Sense of Smell

Beau Callahan in Science & Nature September 2025 • 4 min read.

"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.

Decoding the Silkworm's Diminished Senses: What the Genes Tell Us

Domestic silkworms losing their sense of smell indoors.

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.

The study revealed that many olfactory-related genes were less active in domestic silkworms. These genes play vital roles in detecting and processing scents. The scientists pinpointed 30 olfactory genes and 19 odorant-degrading enzymes (ODEs) that showed notable differences. ODEs are crucial because they deactivate odor molecules after they've been detected, ensuring the insect doesn't get overwhelmed by a single scent.

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.

What does all this mean? The findings suggest that domestic silkworms have undergone genetic changes that have reduced their sense of smell. This likely happened because, in the sheltered environment of indoor breeding, a highly sensitive sense of smell is no longer as vital for survival.

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.

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Everything You Need To Know

How has the sense of smell in domestic silkworms changed compared to their wild counterparts?

The research indicates that domestic silkworms, Bombyx mori, have experienced a reduction in their sense of smell compared to their wild counterparts, B. mandarina. This decline is attributed to changes in gene expression within the antennal transcriptomes, specifically affecting olfactory-related genes and odorant-degrading enzymes (ODEs). This is significant because it showcases how domestication can alter an animal's sensory abilities as reliance on those senses changes in a controlled environment.

What does it mean when olfactory genes and odorant-degrading enzymes (ODEs) are described as 'down-regulated' in domestic silkworms?

The down-regulation of olfactory genes and odorant-degrading enzymes (ODEs) in domestic silkworms implies that these genes are less active compared to wild silkworms. This is important because these genes play a crucial role in detecting and processing scents. When ODEs are down-regulated, the insect's ability to deactivate odor molecules after detection is reduced, potentially leading to a diminished or altered olfactory experience. This is a direct consequence of relaxed selection pressures in a domesticated setting.

What are odorant receptors (ORs), and what does the higher ratio of unique non-synonymous polymorphisms (N/S ratio) in domestic silkworms suggest?

Odorant receptors (ORs) are proteins essential for detecting odors. The finding that domestic silkworms show a higher ratio of unique non-synonymous polymorphisms (N/S ratio) in their ORs suggests a faster rate of change in these receptors compared to wild silkworms. A high N/S ratio means that there are more mutations that change the amino acid sequence of the protein than mutations that don't. This is significant because it indicates that the ORs in domestic silkworms are evolving more rapidly, potentially leading to altered or impaired odor detection capabilities.

What does the discovery of a harmful mutation in OR30 mean for domestic silkworms?

The identification of a harmful mutation in OR30, specifically in transmembrane helix 6 (TM6), within the domestic silkworm population, has significant implications. TM6 is a critical part of the receptor's structure, essential for its function. A deleterious mutation in this region likely impairs the receptor's ability to properly bind to odor molecules, thus diminishing the silkworm's ability to detect specific scents. This highlights how specific genetic changes can directly impact sensory function.

Why has domestication led to a weaker sense of smell in silkworms?

Domestication has led to a diminished sense of smell in silkworms because the selective pressures that favored a keen sense of smell in the wild are relaxed in the controlled environment of indoor breeding. In the wild, a strong sense of smell is crucial for finding food, mates, and suitable egg-laying sites. However, in domestication, these needs are met by humans, reducing the necessity for a highly sensitive olfactory system. This relaxed selection allows for the accumulation of genetic changes that reduce olfactory sensitivity, as these changes no longer negatively impact survival and reproduction.