No one wants German cockroaches, but almost everyone has them. As the largest number of cockroaches in the world, they only live among humans, and humans may also be responsible for its transmission. A genetic study published in the Proceedings of the National Academy of Sciences on May 20 found that medieval wars and colonial trade helped cockroaches spread all over the world.
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German cockroaches are not from Europe. Image source: SMUAY/GETTY IMAGES
Csamar Capinha, a biogeographer at the University of Lisbon, Portugal, who was not involved in the study, said that this discovery is striking, indicating that many species have left their habitats intentionally or unintentionally through travel and trade. He pointed out that a United Nations report in 2023 pointed out that humans had introduced more than 37000 species to new regions around the world, and more than 200 species would be introduced each year.
In 1776, Carl Linnaeus, a famous naturalist, formally described the German cockroach for the first time based on the specimens collected in Germany. But naturalists soon realized that it probably did not originate in Europe, because there were no similar cockroaches in Europe at that time. Since then, this insect has been common in all continents except Antarctica, and humans often regard it as a major pest.
"German cockroaches are very common, but people know little about them," said Qian Tang, a biologist at Harvard University's Roland Institute. So he and his colleagues tried to use DNA to find the answer.
Ten years ago, Qian Tangtu revealed the history of cockroaches by analyzing DNA fragments called microsatellites. However, these tiny genetic materials are not enough to explain how different populations of German cockroaches are related to each other. Now, since it is easy to sequence the entire genome, Qian Tang decided to re-examine his original specimen.
By comparing mitochondrial genes, researchers found that German cockroaches and Asian cockroach cockroaches had the closest genetic relationship. The cockroach was first described in Japan and later spread to the United States. In order to track the path of German cockroaches leaving Asia, scientists mapped the genome similarity of 281 samples from 17 countries where each human inhabits the continent. This correlation pattern shows that cockroaches played a dominant role in the world in the two migration waves, both of which may be realized with the help of human beings.
Qian Tang said that about 2100 years ago, probably in India or Myanmar, German cockroaches differentiated from Blattella assassina by adapting to human food, and then began to expand. About 1200 years ago, it moved westward to the Middle East, which coincided with the expansion of trade and war between the two Islamic caliphs, Umayyad and Abbas. Researchers believe that these insects hitchhike across the desert through bread baskets.
About 390 years ago, the species continued to expand eastward to Southeast Asia, coinciding with the establishment of colonial trade between the Netherlands and the British East India Company. Within about a century, trade ships brought them to Europe. According to Qian Tang, steam boats and improved heating and plumbing systems in homes have created warm and humid conditions that cockroaches like, enabling this mobile insect to spread around the world.
Cleo Bertelsmeier, a conservation biologist at the University of Lausanne, Switzerland, said: "Researchers have shown that adapting to human lifestyles is the key to the spread of German cockroaches. I think this is really original."
"This paper does a good job in matching patterns with actual historical events," Capinha added. But he wanted to know whether the genome data was consistent with the historical records of cockroaches arriving at different places.
As for Qian Tang, he is going back to the past and using the DNA of museum specimens to better understand the evolution of cockroaches. "The genus itself has a history of 200 million years, but we know little about it," he said.
"Understanding the origin and genetic diversity of cockroaches can also help us find solutions to insecticide resistance. When they become pests in restaurants and hotels, this is very important." Qian Tang said, "The number of German cockroaches is growing very fast. If they are not controlled, it will be a huge number in a few months."
Relevant paper information: https://doi.org/10.1073/pnas.2401185121
