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Genetic study of house dust mites demon­strates reversible evolution

pub­lished 09 March 2013 | mod­i­fied 08 March 2014

house-dust-miteIn evo­lu­tion­ary biol­ogy, there is a deeply rooted sup­po­si­tion that you can’t go home again: Once an organ­ism has evolved spe­cialised traits, it can’t return to the lifestyle of its ances­tors. There’s even a name for this per­va­sive idea. Dollo’s law states that evo­lu­tion is uni­di­rec­tional and irre­versible. But this “law” is not uni­ver­sally accepted and is the topic of heated debate among biol­o­gists.

Now a research team led by two Uni­ver­sity of Michi­gan (U-​M) biol­o­gists has used a large-​scale genetic study of the lowly house dust mite to uncover an exam­ple of reversible evo­lu­tion that appears to vio­late Dollo’s law.

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
Pavel Klimov and Barry Ocon­nor, Uni­ver­sity of Michi­gan Depart­ment of Ecol­ogy and Evo­lu­tion­ary Biol­ogy »

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.

Their paper, “Is per­ma­nent par­a­sitism reversible? — Crit­i­cal evi­dence from early evo­lu­tion of house dust mites,” has been pub­lished online as a cor­rected proof on Feb­ru­ary 15 in the jour­nal Sys­tem­atic Biol­ogy.

Mites are arach­nids related to spi­ders (both have eight legs) and are among the most diverse ani­mals on Earth. House dust mites, mem­bers of the fam­ily Pyro­glyphi­dae, are the most com­mon cause of aller­gic symp­toms in humans, affect­ing up to 1.2 bil­lion peo­ple world­wide.

Despite their huge impact on human health, the evo­lu­tion­ary rela­tion­ships between these speck-​sized crea­tures are poorly under­stood. 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. In their study, Klimov and OCon­nor eval­u­ated all 62 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,” said Klimov, the first author of the paper and an assis­tant research sci­en­tist in the Depart­ment of Ecol­ogy and Evo­lu­tion­ary Biol­ogy. The result was so sur­pris­ing largely because it runs counter to the entrenched idea that highly spe­cialised par­a­sites can­not return to the free-​living lifestyle of their ances­tors.

“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 spe­cial­i­sa­tion as evo­lu­tion­ar­ily irre­versible.”

The U-​M find­ings also have human-​health impli­ca­tions, said OCon­nor, a pro­fes­sor in the Depart­ment of Ecol­ogy and Evo­lu­tion­ary Biol­ogy and a cura­tor of insects and arach­nids at the U-​M Museum of Zool­ogy:
“Our study is an exam­ple of how ask­ing a purely aca­d­e­mic ques­tion may result in broad prac­ti­cal appli­ca­tions,” he 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. A total of around 700 mite species were col­lected 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.

With the advent of human civil­i­sa­tion, 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.

(Source: Uni­ver­sity of Michi­gan news release, 08.03.2013)

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