Thursday, July 24, 2014

DNA Barcoding the oriental latrine fly

 Male and female adult of Chrysomya megacephala
The oriental latrine fly (Chrysomya megacephala), is a blowfly that prefers warmer climates. This fly can be a nuisance to humans and even cause accidental myiasis. Myiasis is a parasitic infestation by maggots growing inside a host while feeding on its tissue. 

The species originates from tropical forests on the South Pacific islands, like Samoa but has spread across vast regions of the world. It is particularly prevalent in the Oriental region and the Australasian region but has been found in Japan and the Palearctic regions as well. Today you can find it also in Africa and the Americas often conquering new territory by means of harbors and airports. Presently, three forms of Chrysomya megacephala are known, namely, the normal form, the synanthropic derived form. and a recently reported feral derived form. The synanthropic derived form is thought to have emerged from Papua New Guinea. Synanthropic means it is ecologically associated with humans and it is this form that spreads so successfully around the globe. It occurs on dead fish, sweets, carcasses, human excrement and fruits. The adult flies are vectors of a few infectious diseases of the digestive tract and reported to carry Morganella sp. (Enterobacteriales: Enterobacteriaceae), which causes summer diarrhoea. It is also associated with many microbial pathogens including polio virus. Besides, this fly has been reported as vector of enteric pathogens in malnourished individuals living under unsanitary conditions. The larvae of C. megacephala are parasitic on semi-dried fish, causing a major problem in the fish industry of Southeast Asia.

Chrysomya megacephala is commonly found in cadavers in many parts of the world, and therefore it is used in forensic investigations to determine post-mortem intervals. Post mortem interval determination is useful in cases of homicide, suicide and accidental or unattended death because of natural causes. The oriental latrine fly is considered one of the most important species of flies to forensic science because it is known to be one of the very first species to show up on a corpse. 

Morphological identification of flies is generally difficult and Chrysomya megacephala is no exception. The common way to identify flies is to examine the adult stage under a compound microscope looking for particular usually minute features. This also requires that any larvae collected needs to be reared until development is complete. Identification of larval flies is extremely difficult especially since critical characteristics are very small and any variation ever so slightly, can lead to misidentification. 

A group of researchers from the University of Madras in India now reports on their efforts to use DNA Barcoding to identify Chrysomya megacephala. They used freshly emerged adults from pupae to obtain identifications by both morphological and molecular studies. This is also the first report of the species in South-East India, so far it was only known from the north and south-west of the country. This comes to no surprise given how successful this species follows humans but in this case the fact that it is widely used as fish bait in India might have contributed substantially to the further spread. The DNA Barcodes can be found both on GenBank and BOLD. A quick look at the BIN page of this species on BOLD also reveals the difficulties of morphological identification as this BIN contains eight different species names, seven of which likely represent misidentifications or taxonomic issues. I leave it to the experts to resolve this but given that there are several DNA Barcode sequences (actually 191 on BOLD alone) available as reference it should be a big step forward for forensics as identification can be provided at every life stage. This means no more wait for the lengthy rearing process which in the case of Chrysomya megacephala can take 40 days. This is certainly good news for applications such as post mortem interval determination or disease vector control. 

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