Study Sheds Light on Origins of Unique Silkworm Strain
By Ed Ricciuti
Since its discovery by the outside world in 1986, a silkworm raised only in 29 villages of China’s Nandan County, Guangxi Province, has been suspected of being as taxonomically unique as the Baiku Yao people who farm it are culturally and ethnically distinct.
But, new research by Chinese scientists published this month in the Journal of Insect Science says it isn’t so. Even though legends that the Yao silkworm was gifted upon the Baiku Yao by mountain gods suggested it might have a different origin than the domestic silkworm (Bombyx mori), it turns out to be the same species, although a very primitive form, perhaps transitional from the wild silkworm B. mandarina.
The researchers found enough genetic similarities between strains of the Yao silkworm and those of its domestic counterpart to declare that they are basically the same critter and share a common ancestor, although the Yao variety, raised for at least a millennium, is an ancient population—a “living fossil,” in the words of the research team. The findings were based upon comparison of the mitochondrial genomes, or “mitogenomes,” of Yao silkworms with those of domestic strains from the same geographic area. The mitogenome is the maternally transmitted genetic information within the chromosome of the mitochondrion, the energy-generating structure in cellular cytoplasm.
“Analysis of the mitogenomes of five Yao silkworm strains revealed a great similarity with … domestic silkworm strains in genome size, gene composition, and genomic organization and a large difference with the wild silkworm B. mandarina,” write the researchers.
Even so, the Yao is somewhat different. “Our study evidenced that the Yao silkworm is a lineage of the domestic silkworm … but exhibits a distinct variation pattern from the other domestic silkworm strains,” says Yan-Qun Liu, Ph.D., of the Shenyang Agricultural University. “In this study, through comparative mitochondrial genomics, we revealed that the Yao silkworm is an evolutionary intermediate or a living fossil from the wild moth to the domestic silkworm.”
All domestic silkworms descend from B. mandarina, one of half a dozen so-called silk months in the genus Bombyx. Of all, only the domestic species, B. mori, provides silk suitable for mass production. The caterpillar larvae of the Yao silkworm resemble those of the wild species but, even so, produce manufacturable silk. Unlike the silk from other strains of B. mori, however, silk from the Yao strain does not have to be reeled through machinery to process into thread but can be spun directly from the cocoons.
Baiku Yao women incubate the Yao silkworm eggs on their bodies, close to their skin, to promote hatching. The larvae are then reared in home for less than two months. The Yao people live mostly in southern China and neighboring Vietnam. The particular group that cultures the Yao silkworm is called the Baiku Yao, or “White-Pants Yao,” due to the knee-length white trousers traditionally worn by its men. Most live in China’s Guangxi Province, an autonomous region because of its ethnic composition and which includes Nandan County.
During their study, the researchers compared the genome architecture—the three-dimensional arrangement of genes and other functioning elements in the genome—of five Yao strains with those of 10 other domestic strains. The architectural similarities and other genetic elements in common were sufficient to convince the researchers that the Yao moth is the domestic species.
“The five Yao silkworm mitogenomes exhibited genome architectures identical to typical set of 37 mitochondrial genes … and a high level of genome sequence similarity with the domestic silkworm,” the researchers write. Comparison of portions of DNA segments of the Yao and traditional domestic silkworms relate to the primitive nature of the former, say the researchers.
The research team plans to extend the study. “In the next step,” says Liu, “we will sequence the whole genome of the Yao silkworm. Through comparative genomics between the Yao silkworm and the other domestic silkworm strains available, we could identify the nucleotide variants related to artificial domestication and formulate the strategies for improving breeding varieties of the domestic silkworm.”
Beyond that, silkworms are important in various areas of scientific research. Bombyx mori, the domesticated silkworm, is the leading genetic and genomic model for the order to which moths and butterflies belong, Lepidoptera. With more than 350 mutations, the silkworm has some genes similar to those related to hereditary diseases in humans, giving it a role in biomedical research.
Journal of Insect Science
Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments 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 at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.