Barcoding Queensland Fruit Flies (Bactrocera tryoni): impediments and improvements

Abstract

Identification of adult fruit flies primarily involves microscopic examination of diagnostic morphological characters, while immature stages, such as larvae, can be more problematic. One of the Australia's most serious horticultural pests, the Queensland Fruit Fly (Bactrocera tryoni: Tephritidae), is of particular biosecurity/quarantine concern as the immature life stages occur within food produce and can be difficult to identify using morphological characteristics. DNA barcoding of the mitochondrial Cytochrome Oxidase I (COI) gene could be employed to increase the accuracy of fruit fly species identifications. In our study, we tested the utility of standard DNA barcoding techniques and found them to be problematic for Queensland Fruit Flies, which (i) possess a nuclear copy (a numt pseudogene) of the barcoding region of COI that can be co-amplified; and (ii) as in previous COI phylogenetic analyses closely related B.tryoni complex species appear polyphyletic. We found that the presence of a large deletion in the numt copy of COI allowed an alternative primer to be designed to only amplify the mitochondrial COI locus in tephritid fruit flies. Comparisons of alternative commonly utilized mitochondrial genes, Cytochrome Oxidase II and Cytochrome b, revealed a similar level of variation to COI; however, COI is the most informative for DNA barcoding, given the large number of sequences from other tephritid fruit fly species available for comparison. Adopting DNA barcoding for the identification of problematic fly specimens provides a powerful tool to distinguish serious quarantine fruit fly pests (Tephritidae) from endemic fly species of lesser concern.