It has been a popular assumption in evolutionary biology that when an organism develops very highly specialized traits that it cannot return to the lifestyle that its predeccessors lived. But newly released research on the evolutionary path of dust mites is offering a strong argument against that idea. The research has found that the ancestors of modern dust mites were originally free-living animals, that then evolved to become full-time parasites, before returning to a free-living lifestyle once again. This is the first evidence that an organism living a fully parasitic lifestyle can evolve to become a free-living lifestyle.
The popular idea that evolution is “irreversible” with regards to high-specialization is known as “Dollo’s law”. This “law” though, isn’t actually a scientific law, there has debate about the truth of it for some years. And this new research is the first to truly offer a strong argument against it.
Researchers from the University of Michigan made the discovery about the evolutionary path of dust mites while performing a large-scale genetic study on them. The genetic study clearly shows that even with high-specialization it is possible for an animal to return to a more generalist lifestyle.
“The study shows that tiny free-living house dust mites, which thrive in the mattresses, sofas and carpets of even the cleanest homes, evolved from parasites, which in turn evolved from free-living organisms millions of years ago.”
“All our analyses conclusively demonstrated that house dust mites have abandoned a parasitic lifestyle, secondarily becoming free-living, and then speciated in several habitats, including human habitations,” according to Pavel Klimov and Barry OConnor of the U-M Department of Ecology and Evolutionary Biology.
Dust mites, for those that don’t know, are a very diverse group of arachnids related to spiders, many species of which eat dead human skin. They are one of the most common causes of allergic symptoms in humans, current estimates are that the affect the health of roughly 1.2 billion people worldwide.
Even though they are a significant health problem for many humans, the scientific understanding of the evolutionary history of these animals has been very limited. “According to Klimov and OConnor, there are 62 different published hypotheses arguing about whether today’s free-living dust mites originated from a free-living ancestor or from a parasite — an organism that lives on or in a host species and damages its host.”
For the new research, Klimov and OConnor thoroughly investigated all 62 of the hypotheses. “Their project used large-scale DNA sequencing, the construction of detailed evolutionary trees called phylogenies, and sophisticated statistical analyses to test the hypotheses about the ancestral ecology of house dust mites.”
“On the phylogenetic tree they produced, house dust mites appear within a large lineage of parasitic mites, the Psoroptidia. These mites are full-time parasites of birds and mammals that never leave the bodies of their hosts. The U-M analysis shows that the immediate parasitic ancestors of house dust mites include skin mites, such as the psoroptic mange mites of livestock and the dog and cat ear mite.”
“This result was so surprising that we decided to contact our colleagues to obtain their feedback prior to sending these data for publication,” stated Klimov, the lead author of the new paper and an assistant research scientist in the Department of Ecology and Evolutionary Biology.
It was a “surprising” result because it was in such stark contrast to many entrenched ideas, but primarily to the idea that highly specialized parasites are unable to revert to the free-living lifestyle.
“Parasites can quickly evolve highly sophisticated mechanisms for host exploitation and can lose their ability to function away from the host body,” Klimov said. “They often experience degradation or loss of many genes because their functions are no longer required in a rich environment where hosts provide both living space and nutrients. Many researchers in the field perceive such specialization as evolutionarily irreversible.”
According to the researchers, the new findings have important implications for human-health. “Our study is an example of how asking a purely academic question may result in broad practical applications,” OConnor said. “Knowing phylogenetic relationships of house dust mites may provide insights into allergenic properties of their immune-response-triggering proteins and the evolution of genes encoding allergens.”
“The project started in 2006 with a grant from the National Science Foundation. The first step was to obtain specimens of many free-living and parasitic mites — no simple task given that some mite species are associated with rare mammal or bird species around the world.”
“The research team relied on a network of 64 biologists in 19 countries to obtain specimens. In addition, Klimov and OConnor conducted field trips to North and South America, Europe, Asia and Africa. On one occasion, it took two years to obtain samples of an important species parasitizing African birds.”
In total, about 700 species of mites were gathered for the study. “For the genetic analysis, the same five nuclear genes were sequenced in each species. How might the ecological shift from parasite to free-living state have occurred?”
“There is little doubt that early free-living dust mites were nest inhabitants — the nests of birds and mammals are the principal habitat of all modern free-living species in the family Pyroglyphidae. Klimov and OConnor propose that a combination of several characteristics of their parasitic ancestors played an important role in allowing them to abandon permanent parasitism: tolerance of low humidity, development of powerful digestive enzymes that allowed them to feed on skin and keratinous (containing the protein keratin, which is found in human hair and fingernails) materials, and low host specificity with frequent shifts to unrelated hosts.”
“These features, which occur in almost all parasitic mites, were likely important precursors that enabled mite populations to thrive in host nests despite low humidity and scarce, low-quality food resources, according to Klimov and OConnor. For example, powerful enzymes allowed these mites to consume hard-to-digest feather and skin flakes composed of keratin.”
But with the rise of human civilization, the very attractive environments of human dwellings became an option. “Nest-inhabiting pyroglyphids could have shifted to human dwellings from the nests of birds and rodents living in or around human homes. Once the mites moved indoors, the potent digestive enzymes and other immune-response-triggering molecules they carry made them a major source of human allergies.”
The new research was published March 8 in the journal Systematic Biology.
In a way, the climate controlled living conditions of human dwellings are a lot like the body of a host animal. They provide shelter, food, and warmth, allowing parasitic organisms to have a comparatively “easy” lifestyle, when compared to their free-living ancestors. The rise of human civilization has also resulted in significant losses of natural habitat through the widespread human activity of deforestation and its associated processes. It is very possible that animals such as dust mites shifted to living in human settlements in response to the loss of their previous living environments.
Source: University of Michigan
Image Credits: G. Bauchan and R. Ochoa