DNA barcodes point to differences between disease-carrying insects
Culicoides are tiny biting flies, about 1 mm long, which are known to carry more than a dozen viruses which they transmit to livestock. One species, Culicoides brevitarsis, is the principal vector (organism that transmits infections from one host to another) of Bluetongue and Akabane viruses in Australia. Its main hosts are cattle, and it breeds only in their dung.
My colleagues and I got the first hint that not all was as it seemed with this species when we produced “DNA barcodes” (short mitochondrial DNA sequences diagnostic for species) for specimens from across the species range in Australia and overseas. Comparison of DNA barcodes from Australian and Japanese specimens showed two distinct barcodes, and at some sites, such as in East Timor, both forms were found. Did we have two species and not one?
Due to this puzzling result, we extended our work, collaborating with colleagues from Japan, South Africa and France, to include as many closely related species as possible, and looked at multiple lines of evidence, including characteristics of the flies’ wings, eyes and mouthparts, and sequences of other genes.
Results confirmed our initial suspicions, and we have described the Japanese form, now known to occur more widely in Asia but not in Australia, as a new species Culicoides asiatica.
Our results, and others like it, call into question what we think we know about significant pests and diseases. With insect-borne diseases, our understanding of the insect vectors plays a crucial role in our ability to combat the disease. For example, the distribution of C. brevitarsis in Australia is restricted by colder temperatures, and it doesn’t survive where winter temperatures dip below about 7 degrees Celsius. Why would that be the case if the insect can survive Japanese winters? Now we know that the Japanese form is actually a different species, C. asiatica, we are not so surprised that it tolerates different conditions.
Now we might also consider whether all of the 19 different viruses associated with C. brevitarsis are really found in this species, or might some of them be restricted to C. asiatica?
And whereas previously we would not have noticed C. asiatica if it were intercepted by quarantine authorities, because morphological differences between it and C. brevitarsis are subtle, DNA barcodes provide the means to rapidly detect either species, even in bulk trap samples.
Dr Andrew Mitchell
Senior Research Scientist
Bellis, G., Dyce, A., Gopurenko, D., Yanase, T., Garros, C., Labuschagne, K & Mitchell, A. (2014) Revision of the Culicoides (Avaritia) Imicola complex Khamala & Kettle (Diptera: Ceratopogonidae) from the Australasian region. Zootaxa 3768 (4): 401-427.
This study is part of an ongoing collaboration between the Northern Australia Quarantine Strategy (part of the Australian Department of Agriculture), University of Queensland, NSW Department of Primary Industries and Australian Museum Research Institute scientists to build a DNA barcode database for the Culicoides species of Australasia. The study was funded by the Australian Biological Resources Study and the NSW BioFirst Initiative.