Monday, March 11, 2013

Barcoding parasitic worms

Acanthocephalan with hooks
Acanthocephalans are intestinal parasites of vertebrates. Adult endoparasites inhabiting the digestive tract of  fish, marine mammals, birds amphibians, and reptiles. Larval development takes place in intermediate arthropod hosts typically crustaceans (amphipod, copepod or decapod). Human infections are considered to be rare

The name "Acanthocephala" is derived from Greek roots meaning "spiny head" and refers to the set of hooks on one end of the worm. Most acanthocephalans are less than 20 cm long, although a few exceed 60 cm. Females are generally larger than males. As a consequence of their lifestyle, the digestive tract has been completely lost and most other organ systems are notably reduced, with the exception of the reproductive system.  

Acanthocephalan cystacanth in an amphipod
The life cycle of most species is poorly understood but, apparently, eggs are released from the definitive hosts with the faeces. Once in the water, eggs are eaten by an arthropod that represents the intermediate host, where up to three larval stages are developed (acantor, acanthella and cystacanth). Aquatic vertebrates (such as fish) feed upon the arthropods and the last larval stage the cystacanth re-encysts. The life cycle is completed when these hosts are eaten by other vertebrates (e.g. fish-eating birds) and the adult parasites establish in their intestines.

Similarly to other endoparasites the taxonomy of acanthocephala is based on adult morphological traits. Larval forms cannot be assigned to species. As a result we have only little understanding of the life cycle of most parasite species let alone what their hosts are.

A group of Mexican Researchers has now shown how DNA Barcoding can help to assign larval stages to adult forms. They successfully linked larval stages of the acanthocephalan species Polymorphus brevis found in freshwater fishes with adults that usually parasitize other aquatic and terrestrial vertebrates, most commonly fish-eating birds. They sequenced cystacanths from freshwater fishes collected across central Mexico and adults obtained from fish-eating birds, to determine whether they were conspecific. To corroborate the molecular results, they conducted a morphometric analysis using a software developed to detect heterogeneity in otherwise morphologically similar acanthocephalans based on the multivariate statistical analysis of hook dimensions. Both methods showed that larvals stages infecting freshwater fishes in central Mexico belong to a single species, Polymorphus brevis. 

...the development of a library of COI sequences will be a valuable resource for better understanding the life cycles and intermediate and definitive host spectra for helminths (parasitic worms) across large spatial scales, and a broad variety of host taxa. 

Dirk Steinke (2013). Barcoding parasitic worms

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