Abstract
BackgroundTuna species of the genus Thunnus, such as the bluefin tunas, are some of the most important and yet most endangered trade fish in the world. Identification of these species in traded forms, however, may be difficult depending on the presentation of the products, which may hamper conservation efforts on trade control. In this paper, we validated a genetic methodology that can fully distinguish between the eight Thunnus species from any kind of processed tissue.MethodologyAfter testing several genetic markers, a complete discrimination of the eight tuna species was achieved using Forensically Informative Nucleotide Sequencing based primarily on the sequence variability of the hypervariable genetic marker mitochondrial DNA control region (mtDNA CR), followed, in some specific cases, by a second validation by a nuclear marker rDNA first internal transcribed spacer (ITS1). This methodology was able to distinguish all tuna species, including those belonging to the subgenus Neothunnus that are very closely related, and in consequence can not be differentiated with other genetic markers of lower variability. This methodology also took into consideration the presence of introgression that has been reported in past studies between T. thynnus, T. orientalis and T. alalunga. Finally, we applied the methodology to cross-check the species identity of 26 processed tuna samples.ConclusionsUsing the combination of two genetic markers, one mitochondrial and another nuclear, allows a full discrimination between all eight tuna species. Unexpectedly, the genetic marker traditionally used for DNA barcoding, cytochrome oxidase 1, could not differentiate all species, thus its use as a genetic marker for tuna species identification is questioned.
Highlights
The genus Thunnus, which belongs to the family Scombridae, is comprised of eight species that are commonly known as tunas [1,2,3]
The genetic marker traditionally used for DNA barcoding, cytochrome oxidase 1, could not differentiate all species, its use as a genetic marker for tuna species identification is questioned
Validation of genetic marker for species identification The comparison of intraspecific genetic variability for each species among molecular markers (Table 1) indicates that the mtDNA Control Region (CR) has about ten-fold greater nucleotide diversity than the other two markers
Summary
The genus Thunnus, which belongs to the family Scombridae, is comprised of eight species that are commonly known as tunas [1,2,3]. The three bluefin tuna species look very similar, Atlantic and Pacific bluefin tuna, but they are distinguishable from bigeye, yellowfin, albacore and skipjack based on external attributes (body shape and other morphometrics, characteristics of the fins, number of gill rakers, etc.). Identification of these species in traded forms, which are typically dressed, gilled and gutted, or loin and belly meat, and either fresh/ chilled or frozen, is difficult. We validated a genetic methodology that can fully distinguish between the eight Thunnus species from any kind of processed tissue
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