Molecular Biology and Species Identification: A Study of Seafood Fraud via DNA Barcoding

Overview of Seafood Fraud and Project Objectives

• Definition of Seafood Fraud: Seafood fraud is defined as any illegal action that misrepresents a species of fish that is being sold to consumers. Typically, this involves selling a less expensive or more common fish as a more desirable, high-value species (e.g., selling Cod or Grouper as North Atlantic Salmon).

• Economic and Consumer Incentives: Fraud is prevalent because once fish is prepared and cooked, it becomes physically difficult for the average consumer to distinguish between species. Only individuals with a very "educated palette" are likely to identify a discrepancy in the species served.

• Prevalence Statistics:     - Surveys indicate that rates of counterfeit fish range from $10\%$ to $75\%$ of fish sold to consumers.     - Sushi Restaurants: These establishments act as the primary "trouble spot," with nearly $75\%$ of the fish sold found to be misrepresented.     - Regional Context: Monitoring has been conducted in Denver, but local areas such as Pueblo and Colorado Springs have not yet been systematically monitored, making the class project a novel investigation into local honesty.

• Scope of Business: Fraud encompasses grocery stores and food chains, though the rates of counterfeit fish in grocery stores are generally slightly lower than in restaurants.

• Attribution of Fault: It is important to note that the immediate seller may not be the perpetrator of the fraud; they may have been deceived by suppliers further up the supply chain. Identifying where the "lie" occurred involves tracing sequences back through the distribution chain.

Applications of Molecular Identification Techniques

• General Utility: The molecular techniques used in this project are standard across diverse fields, including wildlife biology, molecular biology, and any discipline requiring the exact identification of an organism's genus and species.

• Pathogen Identification: These methods are used to identify unknown viruses and bacteria.

• Endangered Species Management:     - If a species is suspected of being endangered, researchers must first definitively prove its identity to distinguish it from closely related, non-endangered species.     - Confirmation of the species identity is a prerequisite for legal habitat protection.

• Evolutionary Biology and Phylogeny:     - Techniques allow for distinguishing between two closely related species.     - Researchers can examine evolution at multiple taxonomic levels: species, genus, family, order, and phylum.

The Scientific Workflow for Fish Identification

• Week 1: DNA Collection: The initial step involves extracting the total DNA from a given fish sample.

• Week 2: DNA Amplification: Between the first and second weeks, researchers use Polymerase Chain Reaction (PCR) to make billions of copies of a specific Gene of Interest.     - Definition of PCR: Fundamentally, PCR is "DNA replication in a test tube."

• DNA Sequencing: Once amplified, the DNA is sent for sequencing to determine the exact order of nucleotides.

• Database Comparison: The resulting sequence is searched against a global database of known species sequences. A match provides a conclusive identification of the fish sample. If the result differs from the labeled species, it confirms a case of misrepresentation.

The Gene of Interest: Cytochrome C Oxidase Subunit 1 ($CO1$)

• Genomic Location: This gene is not part of the nuclear genome; it is located within the circular DNA found inside the mitochondria.

• Protein Function: The gene codes for a protein that is part of the electron transport chain involved in cellular respiration.

• Biological Role of $CO1$:     - It is a subunit of Complex IV, located in the inner membrane of the mitochondria.     - Specifically, it accepts electrons from Cytochrome C (oxidizing it).     - It then passes those electrons to Oxygen, which acts as the final electron acceptor to produce water ($H_2O$).

• Rationale for Selection:     - Ubiquity: It is nearly universal in eukaryotes, including plants, fungi, lizards, humans, and fish. Anything with a mitochondria contains this gene.     - Extensive Database: Because it is so widespread, there is a humongous existing database of $CO1$ sequences for comparison.     - Molecular Clock: This gene acts as a molecular clock because it accrues mutations at a steady, predictable rate over evolutionary time (tens to hundreds of millions of years).

• Target Sequence Size: While the entire gene is long, the project only requires a stretch of approximately $700$ base pairs ($bp$) for identification.

• Evolutionary Distance Application: By plotting the number of mutations in the $CO1$ gene, scientists can determine how long ago different species diverged. For example, it is used to calculate the divergence time between Humans and Neanderthals, or between proto-humans and chimpanzees.

Laboratory Procedure for Total DNA Isolation

• Step 1: Sample Preparation:     - Take a frozen fish sample and slice/chop it finely using a razor blade.     - Place the sample into a $1.5\,ml$ microfuse tube.     - Optionally, grind the sample further using a micro-pestle.

• Step 2: Lysis and Protein Digestion:     - Add $180\,\mu l$ of ATL Buffer (a lysis buffer) to dissolve the plasma membrane, outer mitochondrial membrane, and inner mitochondrial membrane to release the DNA.     - Add $20\,\mu l$ of Proteinase K to digest proteins.     - Enzyme Inhibition: This step is crucial to disable lysosomal enzymes that would otherwise degrade the DNA molecules.

• Step 3: Incubation:     - Incubate the mixture at $56^\circ C$.     - This temperature physically melts lipid membranes (fats), denatures extraneous proteins/enzymes, frees DNA from protein associations, and provides the optimal environment for Proteinase K activity.

• Step 4: DNA Conditioning:     - Mix $200\,\mu l$ of Ethanol with $200\,\mu l$ of Buffer AT.     - This mixture conditions the DNA, making it "sticky" so it will bind to the filter membrane.

• Step 5: DNA Binding:     - Pipette the mixture onto a DNeasy Mini Spin Column containing a fibrous membrane.     - Centrifuge the column. The DNA binds to the filter, while unwanted cellular debris and contaminants pass through into the collection tube.

• Step 6: Washing:     - Wash 1: Add AW1 Wash Buffer and spin to remove salts and residual ethanol.     - Wash 2: Add AW2 Wash Buffer and spin to remove remaining proteins, lipids, and other contaminants.     - Drying Spin: Centrifuge at $14000$ (rpm) for $3$ minutes to ensure the membrane is completely dry.

• Step 7: Elution:     - Place the filter into a clean, new $1.5\,ml$ microfuse tube.     - Add $100\,\mu l$ of Molecular Biology Grade Water. This water is ultra-pure, more filtered than distilled water, and guaranteed to be "nuclease-free."     - Incubate for $5$ minutes to allow the water to release the DNA from the membrane.     - Centrifuge one final time to collect the purified DNA in the tube, then discard the filter.

• Next Steps: The purified DNA will be used to set up PCR reactions following the DNA collection process.