House Dust Mites Prove Evolution Can Be Reversible? Sorry, Darwin!

Science has long determined that evolution only works in one direction: forward. This rule, known as Dollo's law, states this fact.

Evolution is irreversible, according to Dollo's law.

But, is it really? The "law" has long been a topic for debate and now two researchers from the University of Michigan have discovered that the common house dust mite could be proof that evolution can go backwards.

"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," Michigan Department of Ecology and Evolutionary Biology denizens Pavel Klimov and Barry O'Connor said as relayed by Science Daily. 

The team's paper (Is permanent parasitism reversible? -- Critical evidence from early evolution of house dust mites), which was scheduled for publication on Friday, March 8 in the journal Systematic Biology, illustrates the house dust mite's having evolved from parasites which in turn evolved from free-living organisms that existed millions of years ago.

Though O'Connor's and Klimov's paper takes on a controversial issue in disproving Dollo's law, they also confess that the house dust mite itself is still a widely obscure organism whose origins are themselves hotly debated. They say that there are no less than 62 hypotheses published that deliberate over whether the mites evolved from parasites or a free-living predecessor.

O'Connor and Klimov dissected all 62 hypotheses in their study, which also involved DNA sequencing, the construction of phylogenies (ornate evolutionary trees) and what Science Daily refers to as "sophisticated statistical analyses to test the hypotheses about the ancestral ecology" of the house dust mites.

"This result was so surprising that we decided to contact our colleagues to obtain their feedback prior to sending these data for publication," Klimov  says.

The excitement resulted from the notion that the researchers' findings are contrary to the belief that specialized parasites cannot once again become free-living (living without a host) as had lived their forebears.

House dust mites thrive in the mattresses, carpets and sofas of "even the cleanest homes" and thus exist in this free-living way.

O'Connor and Klimov point to several factors that allowed the dust mites to abandon their parasitic ways. These include:

  • 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
  • Low host specificity with frequent shifts to unrelated hosts

The team believes that better understanding of the dust mites will have significant applications in our world, as the mites have a terrible history of causing health problems for the humans in whose homes they inhabit.

"Our study is an example of how asking a purely academic question may result in broad practical applications," O'Connor 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."

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