Study sheds light on origins of unique silkworm strain

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Dubbed a “living fossil” by researchers, the Yao silkworm was domesticated in virtual isolation for a millennium by the Baiku Yao ethnic group in China. New genetic research on the strain reveals that the Yao silkworm (larvae pictured here) is a primitive form of the domesticated silkworm species Bombyx mori and more distant from other species of wild silkworms. (Photo courtesy of Yan-Qun Liu, Ph.D.)

By Ed Ricciuti

Ed Ricciuti

Ed Ricciuti

Since its discovery by the outside world in 1986, a silkworm bred only in 29 villages in China’s Nandan County, Guangxi Province, has been suspected to be as taxonomically unique as the Baiku Yao people who lived there. student is culturally and ethnically distinct.

But, new research from Chinese scientists published this month in the Journal of Insect Science says that is not the case. Even though the legends that the Yao silkworm was gifted to the Baiku Yao by the mountain gods suggest that it may have a different origin than the household silkworm (Bombyx mori), it turns out to be the same species, although a very primitive, possibly transitional form of the wild silkworm B. mandarin.

Researchers have found enough genetic similarities between strains of the Yao silkworm and those of its domesticated counterpart to state that they are basically the same creature and share a common ancestor, although the Yao variety, bred for at least a millennium , an ancient population… a “living fossil”, in the words of the research team. The results were based on comparing the mitochondrial genomes, or “mitogenomes”, of Yao silkworms with those of domestic strains from the same geographical area. The mitogenome is the genetic information passed down from the mother into the chromosome of the mitochondria, the energy-generating structure in the cell cytoplasm.

Yan-Qun Liu, Ph.D.

Yan-Qun Liu, Ph.D.

“Analysis of the mitogenomes of five strains of Yao silkworms revealed a great similarity with … strains of domestic silkworms in terms of genome size, gene composition and genomic organization and a great difference with the wild silkworm B. tangerine,write the researchers.

Even so, the Yao is somewhat different. “Our study highlighted that the Yao silkworm is a lineage of the domestic silkworm…but exhibits a distinct pattern of variation from other strains of domestic silkworms,” ​​says Yan-Qun Liu, Ph.D. ., from Shenyang Agricultural University. “In this study, through comparative mitochondrial genomics, we revealed that the Yao silkworm is an evolutionary intermediate or living fossil from the wild moth to the domesticated silkworm.”

All domestic silkworms are descended from B. tangerine, one of half a dozen so-called silk months in the genus bombix. Of all of them, only the domestic species, B. mori, provides silk suitable for mass production. Yao silkworm caterpillar larvae resemble those of wild species but, even so, produce manufacturable silk. Unlike the silk of other strains of B.mori, however, silk from the Yao strain does not need to be wound in machines to be made into yarn, but can be spun directly from the cocoons.

Baiku Yao women incubate Yao silkworm eggs on their bodies, close to their skin, to encourage hatching. The larvae are then reared at home for less than two months. The Yao people live mainly in southern China and neighboring Vietnam. The particular group that cultivates the Yao silkworm are called the Baiku Yao, or “White-Pants Yao”, because of the knee-length white pants traditionally worn by its men. Most live in the Chinese province of Guangxi, an autonomous region due to its ethnic composition and which includes the county of Nandan.

During their study, the researchers compared the genome architecture – the three-dimensional arrangement of genes and other functional elements in the genome – of five Yao strains with those of 10 other house strains. Architectural similarities and other common genetic elements were enough to convince researchers that the Yao butterfly is the domestic species.

“All five Yao silkworm mitogenomes exhibited genomic architectures identical to the typical set of 37 mitochondrial genes…and a high level of genome sequence similarity to the domesticated silkworm,” the researchers write. According to the researchers, the comparison of portions of DNA segments of domestic Yao and traditional silkworms is related to the primitive nature of the former.

The research team plans to expand the study. “In the next step,” says Liu, “we will sequence the entire genome of the Yao silkworm. Through comparative genomics between the Yao silkworm and other available domestic silkworm strains, we were able to identify the nucleotide variants related to artificial domestication and formulate the strategies to improve breeding varieties of the domestic silkworm.

Beyond that, silkworms are important in various fields of scientific research. Bombyx mori, the domesticated silkworm, is the primary genetic and genomic model of the order to which moths and butterflies belong, Lepidoptera. With more than 350 mutations, the silkworm has some genes similar to those linked to hereditary diseases in humans, giving it a role in biomedical research.

Ed Ricciuti is a journalist, author and naturalist who has been writing for over half a century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His missions have taken him around the world. He specializes in nature, science, conservation issues and law enforcement. A former curator at the New York Zoological Society, and now the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.

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