German cockroach, Blattella germanica
The origin of the so-called 'German' cockroach, which has nothing especially to do with Germany, has been something of a mystery, as it is unknown outside human habitation. It is what is known as an obligate, commensal species which, like the barn swallows, house mice, human lice and bed bugs, has co-evolved with human culture to live in and on humans and their buildings.
They neatly illustrate how speciation occurs when a population learns to exploit a new resource and then becomes genetically isolated from its parent population and so evolves over time into a new species.
Tell me about human commensal species, please. Human commensal species are organisms that live in close association with humans, benefiting from the relationship without causing harm or providing any significant benefit to humans. This commensalism can take various forms, including living on human skin, in the human gut, or in human habitats like homes and urban environments. Here are some examples of human commensal species:
Characteristics of Human Commensal Species
- Domestic Animals and Birds
- House Mouse (Mus musculus): These rodents thrive in human dwellings, benefiting from the food and shelter humans provide.
- Pigeons (Columba livia): Often found in urban areas, pigeons feed on human food waste and roost on buildings.
- Insects
- Cockroaches (Blattodea): Various species of cockroaches, such as the German cockroach (Blattella germanica), are common in human habitats where they find food and shelter.
- Houseflies (Musca domestica): These flies are found in homes and other buildings, feeding on human food and waste.
- Microorganisms
- Staphylococcus epidermidis: A bacterium that is part of the normal flora of human skin. It usually does not cause harm and benefits from the nutrients on the skin.
- Escherichia coli (E. coli): Certain strains of E. coli are part of the normal gut flora in humans, living in the intestines and benefiting from the nutrients there.
- Fungi
- Candida albicans: This yeast is a normal commensal of the human mouth, gut, and vagina. It typically does not cause disease unless the immune system is compromised or the microbial balance is disturbed.
- Arthropods
- Dust Mites (Dermatophagoides spp.): These tiny mites live in human homes, feeding on dead skin cells shed by people.
Impact on Humans
- Adaptability: These species are highly adaptable to human environments and can exploit resources provided by human activities.
- Opportunistic Behavior: Many commensal species can switch to different food sources or habitats if necessary.
- Rapid Reproduction: Many have high reproductive rates, allowing them to quickly colonize new areas.
While commensal species generally do not harm humans, they can sometimes become pests or cause indirect harm:
- Allergies and Asthma: Dust mites and cockroach allergens can trigger allergies and asthma in sensitive individuals.
- Food Contamination: Species like houseflies and cockroaches can contaminate food with pathogens, leading to foodborne illnesses.
Summary
Human commensal species are a diverse group of organisms that have adapted to live in close association with humans. While they typically do not harm their human hosts, some can become nuisances or pose health risks under certain conditions.
Asian cockroach, Blattella germanica
The probable ancestor of the German Cockroach"
The probable ancestor of the German Cockroach"
Now a team of scientists including Virginia Tech entomologist Warren Booth, believe they have solved the mystery - the German cockroach originated in Southeast Asia just 2,100 years ago, and spread around the globe riding on human trade routes.
The team’s findings have just been published in the online journal Proceedings of the National Academy of Sciences (PNAS) and explained in a Virginia Tech news release:
A team of international scientists, including Virginia Tech entomologist Warren Booth, have solved the 250-year-old origin puzzle of the most prevalent indoor urban pest insect on the planet: the German cockroach.
The team's research findings, representing the genomic analyses of over 280 specimens from 17 countries and six continents, show that this species evolved some 2,100 years ago from an outside species in Asia and were released this week in the Proceedings of the National Academy of Sciences journal.
One may think by its name that its origins are in Germany. But it is not native to any wilderness in that country. In fact, it doesn’t seem to have any home in the wild anywhere in the world. To date, populations have never been found outside of structures.
Following its evolution, the German cockroach spread from Southeast Asia, hitchhiking around the world in association with humans. In addition to rapid spread, it evolved a resistance to a variety of insecticides, making it extremely difficult to control using over-the-counter products.
According to Booth, the German cockroach is a major public health issue due to its links to disease spread, the contamination of food, and its role in triggering asthma and allergies.
AbstractAnother biological mystery solved and of course it turned out to be not whether but how, where and when one species gave rise to a daughter species which became the misnamed 'German cockroach', in a classic example of allopatric speciation. And the present degree of distance has evolved in just 2,100 years.
The origin of the German cockroach, Blattella germanica, is enigmatic, in part because it is ubiquitous worldwide in human-built structures but absent from any natural habitats. The first historical records of this species are from ca. 250 years ago (ya) from central Europe (hence its name). However, recent research suggests that the center of diversity of the genus is Asian, where its closest relatives are found. To solve this paradox, we sampled genome-wide markers of 281 cockroaches from 17 countries across six continents. We confirm that B. germanica evolved from the Asian cockroach Blattella asahinai approximately 2,100 ya, probably by adapting to human settlements in India or Myanmar. Our genomic analyses reconstructed two primary global spread routes, one older, westward route to the Middle East coinciding with various Islamic dynasties (~1,200 ya), and another younger eastward route coinciding with the European colonial period (~390 ya). While Europe was not central to the early domestication and spread of the German cockroach, European advances in long-distance transportation and temperature-controlled housing were likely important for the more recent global spread, increasing chances of successful dispersal to and establishment in new regions. The global genetic structure of German cockroaches further supports our model, as it generally aligns with geopolitical boundaries, suggesting regional bridgehead populations established following the advent of international commerce.
The rise of human civilization has triggered the evolution and spread of commensal species adapted to urban environments. Some species have become invasive pests, with serious implications for human well-being and economic prosperity (1). Many of these pest species have spread globally, facilitated by human technological innovations, which include advancements in transportation and housing, notably plumbing and heating (2). A deeper understanding of the factors that facilitate the spread of pest species can help prevent further spread and mitigate future economic losses (3). However, tracing the spread of pests based on historical records is often obscured by a dearth of accurate record-keeping, especially prior to the 1950s (4). Advances in population genomics help trace origins and reconstruct spread routes when historical records are limited and geographically biased (4). The German cockroach, Blattella germanica L., the world’s most prevalent cockroach pest, is ubiquitous in human buildings globally but not outdoors; it imposes significant social, medical, and economic costs (5) due to prevalent insecticide resistance allowing it to outcompete ~40 known pest cockroach species in buildings (6).
The origin and spread of the German cockroach are shrouded in mystery. Described by Linnaeus in 1776 about a decade after the Seven Years’ War, historical records have suggested a global spread of German cockroaches from Europe between the late 19th to early 20th centuries (6). However, the German cockroach has no close relatives in Europe; those are in Africa and Asia. The ancestral species was suggested to be the Asian cockroach, Blattella asahinai Mizukubo (7, 8), native to the Bay of Bengal in Asia (east India, Myanmar, and nearby islands), and invasive in agricultural landscapes in the southern United States (7). The paradox of a European beginning but Asian phylogenetic affinity is likely due to the almost complete lack of systematic entomological knowledge across the world prior to the 20th century. To help fill the knowledge gap and solve this paradox, we used genome-wide markers from 281 samples from 17 countries around the world (Fig. 1), from which we described the genetic structure and reconstructed spread routes of the German cockroach.
Fig. 1.
Population genetic structure and global spread routes of the German cockroach, Blattella germanica. (A) Median joining haplotype network based on the COI gene (1,536 bp). (B) Principal component analysis based on 158,216 genome-wide single nucleotide polymorphisms. (C) Bar plot (Top), split into region-specific bar plots mapped to sampling sites, derived from maximum likelihood estimation of ancestry (ADMIXTURE) at the optimal number of ancestral clusters (K = 6). The diagram on top of the bar plot indicates nucleotide diversity (π) per locus at each sampling site. The inferred timing and routes of global spread are mapped with black arrows. (D) Divergence of key populations presented in the form of an allele-frequency-based unrooted tree with residuals of drift parameters mapped as migration edges. (E) A zoomed-in view of (C) focused on Asia. The natural range of the ancestor, B. asahinai, is shown in green.
Tang, Qian; Vargo, Edward L.; Ahmad, Intan; Jiang, Hong; Varadínová, Zuzana Kotyková; Dovih, Pilot; Kim, Dongmin; Bourguignon, Thomas; Booth, Warren; Schal, Coby; Mukha, Dmitry V.; Rheindt, Frank E.; Evans, Theodore A.
Solving the 250-year-old mystery of the origin and global spread of the German cockroach, Blattella germanica
Proceedings of the National Academy of Sciences 121(22) e2401185121; DOI: 10.1073/pnas.2401185121
Copyright: © 2024 The authors.
Published by PNAS. Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)
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