Studying house dust mites, two University of Michigan biologists say they have discovered evidence that contracts Dollo’s law – a long standing scientific belief claiming evolution is irreversible.
Scientists at the University of Michigan announced the discovery Friday, saying their research reveals the common house dust mite has undergone “reverse” evolution, changing from a parasitic life form to a free-living one. The study shows the familiar house dust mite evolved over millions of years from free-living organisms into parasites before evolving back into free-living organisms.
The theory of an organism reverting to the characteristics of its ancestors is known as Dollo’s law. The hypothesis, posed by Belgian paleontologist Louis Dollo, states that “an organism is unable to return, even partially, to a previous stage already realized in the ranks of its ancestors.” Although evolutionary biologists have disagreed about how the rule is applied in nature, they have been in general agreement on the general principle that the evolutionary process is irreversible.
“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 O’Connor of the University of Michigan Department of Ecology and Evolutionary Biology.
Mites, which are members of the arachnid (spider) family, are known for causing allergic symptoms in humans; They are poorly understood from an evolutionary standpoint. According to Klimov and O’Connor, there are 62 different published hypotheses relating to whether the free-living dust mites originated from a free-living ancestor or from a parasite.
In an effort to paint a more comprehensive picture of the mites evolution, Klimov and O’Connor evaluated all 62 hypotheses. Using DNA sequencing, detailed evolutionary trees called phylogenies, and statistical analyses to test their hypotheses, the pair of researchers eventually recreated the evolutionary history of house dust mites.
House dust mites appear within a lineage of parasitic mites, called Psoroptidia, which never leave the bodies of their hosts — mainly mammals and birds. The analysis shows that the immediate parasitic ancestors of house dust mites include skin mites, such as the 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,” said Klimov, the first author of the paper and an assistant research scientist in the Department of Ecology and Evolutionary Biology.
“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 evolutionary irreversible.”
The research team obtained specimens from 19 different countries, conducting field trips to North and South America, Europe, Asia and Africa. Approximately 700 mite species were collected for the study.
Klimov and O’Connor say a number of characteristics of the house dust mite’s parasitic ancestors played an important role in allowing them to abandon permanent parasitism. These include development of powerful digestive enzymes that allowed them to feed on skin, hair and fingernails, and a tolerance for low humidity.