German cockroach specimens collected about a century ago at the Harvard Museum of Comparative Zoology. Credit: Tang Qian
Now a commonly detested sight in urban homes, German cockroaches (Blattella germanica) appeared in Europe in the 1760s before scuttling across the world between the late 19th and early 20th centuries, but the origin of this resilient, nocturnal insect is Asia1.
An international team of scientists has used DNA sequencing to trace the species’ origins to East India and Bangladesh. The German cockroach’s sequence was almost identical to that of Blattella asahinai, a species native to the Bay of Bengal, from East India to Bangladesh and Myanmar.
“We don’t know of any naturally wild populations of this cockroach which is also a scavenger in human-dominated landscapes, and that makes it ecologically interesting to study,” says Pilot Dovih at Trivedi School of Biosciences, Ashoka University in Haryana, India.
Tang Qian, the study’s first author, from the National University of Singapore, says that understanding how the cockroach evolved to be more resistant to insecticides than most other insect species could help design better pest management strategies.
The team collected DNA samples from 281 cockroaches from 57 sites in 17 countries, including India, and sequenced mitochondrial genomes for 53 individuals to acquire the mitochondrial cytochrome oxidase subunit 1 (COI) gene, a reliable DNA barcoding gene whose sequence is highly consistent within a species.
They then compared the German cockroach with anatomically similar species from Asia. More than 80% of German cockroach samples matched perfectly.
The German cockroaches (B. germanica) had a small difference of 9 base pairs (about 0.59%) from the 'Asian cockroach' (B. asahinai). In contrast, they differed by 92 base pairs from B. bisignata and by 84 base pairs from B. lituricollis, says co-author, Theo Evans at the University of Western Australia in Australia.
The research suggests that around 2,100 years ago, this ancestor had two domesticated lineages in South Asia: one living in agricultural areas, and the other surreptitiously adapted to buildings after farmers cleared its natural habitat. India represents the source population with the highest genetic diversity, indicating a greater capacity to cope with change.
"Genomic data can be quantified, as we and others have done for various invasive species. Most invasive species have known native ranges, with known or strong suspected routes of introduction and spread. The German cockroach did not have a native range or routes of spread," says Qian.
Out of Asia
To reconstruct the cockroaches' voyage from the Bay of Bengal to Europe, the team identified 158,216 DNA sequences known as single nucleotide polymorphisms, the most common type of genetic variation in a genome.
Before their global march, the insect took two routes – west and east of the origin of India or Myanmar. They spread westwards to the Middle East around 1,200 years ago probably hitching a ride in the bread baskets of soldiers. In the more recent trail, European colonial commercial activities between South and Southeast Asia probably moved them eastwards 390 years ago.
According to the reconstruction, they arrived in Europe around 270 years ago, which matches historical accounts. “It was only after we completed our analyses, we compared our estimate of divergence of the Western European populations of about 270 years ago with historical records – the first was in 1763 (so 261 years ago), with the Swedish biologist, Carl Linnaeus, describing it in 1767. So a remarkable match,” says Evans.
Because this cockroach is physically different from other household cockroach pest species (it is smaller, more pale in colour), people could easily and quickly realise they had a new cockroach species in buildings. Therefore, they collected specimens and sent them to museums or universities for identification and then into zoological collections, the researchers say.
Except for those that found warmth in heated buildings, most of the insects perished in cold European winters far from their warm native habitats. Over time, the isolated indoor population evolved into a distinct indoor species2, relying on humans and their buildings, stealthily moving into other urban areas. The rise of European countries as global powers probably bolstered their spread to their colonies. By the late 19th century they were crawling all over Europe and most continents.
Evans says they evolved to be nocturnal, stayed away from open spaces, and stopped flying, even though they retained wings, to go undetected. After discovering related species in Africa and Asia, scientists suggested an African or Asian origin of the German cockroach.
