Is permanent parasitism reversible? – Critical evidence from early evolution of house dust mites

Long-term specialization may limit the ability of a species to respond to new environmental conditions and lead to a higher likelihood of extinction. For permanent parasites and other symbionts, the most intriguing question is whether these organisms can return to a free-living lifestyle and, thus, escape an evolutionary ‘dead end’. This question is directly related to Dollo's law, which stipulates that a complex trait (such as being free-living vs. parasitic) cannot re-evolve again in the same form. Here we present conclusive evidence that house dust mites, a group of medically important free-living organisms, evolved from permanent parasites of warm-blooded vertebrates. A robust, multigene topology (315 taxa, 8,942 nt), ancestral character state reconstruction, and a test for irreversible evolution (Dollo's law) demonstrate that house dust mites have abandoned a parasitic lifestyle, secondarily becoming free-living, and then speciated in several habitats. Hence, as exemplified by this...