Development of molecular markers for western honey bee (Apis mellifera L.) subspecies of regulatory concern in the United States

This paper presents the development and validation of a rapid, cost-effective, and accurate molecular toolkit using SNP-based qPCR and RFLP assays to specifically identify and differentiate African-derived honey bees and *Apis mellifera capensis* for regulatory purposes in the United States.

The Gist

Imagine the world of honey bees as a massive, global family reunion. While they all look and act somewhat similar, they belong to different "branches" of the family tree. Some branches are friendly and gentle, while others are known for being highly defensive and aggressive. In the United States, regulators need to know exactly which branch a bee belongs to because some specific types are considered "regulatory concerns"—essentially, they are the ones that might cause trouble if they get out of hand or enter the country.

This paper is like a team of bee detectives developing a new, high-tech ID card scanner to quickly and cheaply identify these specific, potentially troublesome bee branches.

Here is how they did it, explained through simple analogies:

The Problem: The "Look-Alike" Mix-Up

For a long time, telling these bees apart was like trying to identify a specific person in a crowd just by looking at their shoes. Scientists used to measure wing shapes (like checking shoe size) or use expensive, slow DNA tests (like sending a sample to a high-end lab for a full background check).

The problem is that bees travel. Bees from Africa, Europe, and Asia have mixed and mated over the years. In the US, there are bees that look like the gentle European types but might have "aggressive" African ancestors hidden in their family tree. Regulators need a way to spot these hidden ancestors quickly, especially at ports of entry or when a beekeeper reports a colony that is stinging people.

The Solution: A Three-Step "Security Check"

The researchers created a step-by-step testing kit, similar to a security checkpoint at an airport. You don't need to run a full background check on everyone; you just run a few quick scans to see if they match a specific "watch list."

Step 1: The "A-Lineage" Detector (The General Alarm)

• What it does: This is the first scan. It asks a simple question: "Does this bee have African ancestry?"
• The Analogy: Think of this as a metal detector at an airport. It doesn't tell you what the metal is, just that there is metal there. If the alarm goes off, the bee belongs to the "African family branch" (A-lineage). If it doesn't, the bee is likely from Europe or Asia, and they can move on.
• The Tech: They look at a tiny genetic "barcode" in the bee's mitochondria (the bee's energy battery) called the Cytb gene.

Step 2: The "Africanized" Detector (The Specific Threat)

• What it does: If Step 1 says "Yes, African," this step asks: "Is this the specific aggressive type known as the Africanized Honey Bee (AHB)?"
• The Analogy: This is like a facial recognition camera that only triggers if it sees a specific person on a "most wanted" list. Not all African bees are aggressive, but this specific type (which originated in South Africa and spread through Brazil to the US) is. This test looks for a tiny genetic "typo" (a SNP) in the Cytb gene that is unique to this aggressive group.
• The Result: If this test is positive, regulators know they are dealing with the specific bees that are banned or heavily regulated in many US states because of their stingy behavior.

Step 3: The "Cape" Detector (The Special Intruder)

• What it does: This test looks for a different specific bee: Apis mellifera capensis.
• The Analogy: Imagine a VIP guest who isn't on the "most wanted" list but is still not allowed in the building because they have a special power that causes chaos. This bee, from South Africa, has a superpower called "thelytoky"—it can clone itself without a father. If it gets into a US hive, it can take over the whole colony. This test checks a different gene (ND4) to see if this specific "cloner" is present.

The Backup Plan: The "Paper Cut" Test (RFLP)

The researchers also created a low-tech, low-cost backup test called an RFLP assay.

• The Analogy: Imagine you have a long piece of string (the bee's DNA). You have a pair of scissors (an enzyme) that only cuts the string if it sees a specific knot.
  • If the string has the "African" knot, the scissors won't cut it. The string stays long.
  • If the string is from a "European" bee, the scissors cut it into two smaller pieces.
• Why it matters: Not every lab has expensive high-tech scanners (qPCR). This "scissors test" can be done with basic equipment, making it accessible to more people who need to check for these bees.

Why This Matters

This new toolkit is like giving beekeepers and government inspectors a smartphone app instead of a library of encyclopedias.

1. Speed: It takes hours, not weeks.
2. Cost: It's much cheaper than full genome sequencing.
3. Accuracy: It reduces the chance of false alarms.

By using these tests, the US can better protect its beekeeping industry. They can stop the "aggressive" bees from spreading further and keep the "cloning" bees from accidentally entering the country, ensuring that the honey bees we rely on for pollination and honey remain safe and manageable.

Technical Summary

1. Problem Statement

Accurate, rapid, and cost-effective identification of Apis mellifera subspecies is critical for regulatory purposes in the United States, particularly for subspecies considered invasive or of high risk. Two primary subspecies pose regulatory concerns:

• African-derived Honey Bees (AHBs): Primarily derived from A. m. scutellata, these bees exhibit heightened defensive behavior and pose risks to human safety and managed European honey bee (EHB) colonies. They are established in several US states.
• Cape Honey Bees (A. m. capensis): Native to South Africa, this subspecies is listed in US federal regulations (7 CFR §322.1) due to the risk of accidental introduction. It possesses the unique ability of thelytokous parthenogenesis (workers laying diploid eggs), which can cause colony collapse in other subspecies.

Current Limitations:

• Morphometrics: Traditional wing-vein measurements are time-consuming and often fail to distinguish hybrids or low-level introgression.
• Genomic Methods: Whole-genome SNP approaches are accurate but too expensive, time-consuming, and technically demanding for routine regulatory screening (e.g., port intercepts or apiary inspections).
• Existing Molecular Assays: Previous assays could identify the broad "A-lineage" (African ancestry) but could not distinguish specific invasive mitotypes (like AHBs) from other African subspecies, nor could they specifically identify A. m. capensis in the presence of A. m. scutellata.

2. Methodology

The authors developed a stepwise series of molecular assays based on Single Nucleotide Polymorphisms (SNPs) in mitochondrial genes. The workflow increases in specificity:

A. Assay Design & Targets:

1. qPCR Assay I (Existing): Targets a SNP in the Cytochrome b (Cytb) gene to differentiate A-lineage (African) from non-A-lineage (European/Asian) bees.
2. qPCR Assay II (New - AHB): Targets a specific SNP at position 146 in the Cytb gene. This marker distinguishes AHBs (specifically those with the mitotype originating from South Africa, moving through Brazil to the US) from other A-lineage bees. It uses a dual-labeled Locked Nucleic Acid (LNA) probe.
3. qPCR Assay III (New - A. m. capensis): Targets a SNP in the NADH dehydrogenase 4 (ND4) gene. This assay specifically identifies A. m. capensis (or its introgression) from the indigenous Cape region of South Africa, distinguishing it from A. m. scutellata and other subspecies.
4. RFLP Assay (New - ACMS Clade): A low-cost alternative for labs without qPCR. It targets a SNP in the NADH dehydrogenase 2 (ND2) gene.
   • Mechanism: Uses the restriction enzyme PacI.
   • Differentiation: The "ACMS clade" (A. m. adansonii, A. m. capensis, A. m. monticola, A. m. scutellata) possesses a C-allele that prevents PacI cutting (uncut band ~646bp). Non-ACMS subspecies possess a T-allele, creating a restriction site that cuts the DNA into smaller fragments (281bp and 365bp).

B. Sample Collection & Validation:

• Dataset: 145 samples representing 16 subspecies from the US (Florida, Arizona, Texas, Puerto Rico), South Africa, Europe, and North Africa.
• Validation: All qPCR results were validated against Sanger sequencing of the Cytb and ND4 genes. The RFLP results were validated against known lineage assignments.
• Phenotype Correlation: Samples included bees with reported "defensive" behaviors (stinging incidents) and "gentle" phenotypes to correlate genetic markers with behavioral traits.

3. Key Contributions

• Specific AHB Marker: Development of a qPCR assay that identifies a specific mitotype associated with the invasive AHB population in the Americas, moving beyond broad A-lineage detection.
• Capensis Specificity: Creation of the first targeted qPCR assay for A. m. capensis using the ND4 gene, capable of distinguishing it from the sympatric A. m. scutellata in South Africa.
• Low-Cost Alternative: Development of a PacI-based RFLP assay for the ACMS clade, providing a viable option for resource-limited laboratories to screen for African maternal ancestry.
• Comprehensive Workflow: A tiered diagnostic approach (A-lineage $\rightarrow$ AHB $\rightarrow$ capensis) that maximizes specificity while minimizing false positives from North African or Mediterranean A-lineage subspecies.

4. Results

• qPCR Assay I (A-lineage): Successfully differentiated 102 A-lineage samples from 30 non-A-lineage samples.
• qPCR Assay II (AHB):
  • Detected the specific AHB SNP in 32 Florida samples collected from stinging incidents.
  • Identified the marker in 8 US samples (Arizona/Florida) and 18 South African samples.
  • Crucial Finding: The marker was negative in samples from Puerto Rico (gentle Africanized bees) and some defensive samples in Texas/Arizona, indicating these bees possess different maternal lineages (likely North African or different South African mitotypes).
  • False Positives: None observed; Sanger sequencing confirmed all qPCR results.
• qPCR Assay III (A. m. capensis):
  • Identified 20 samples with the capensis T-allele (undetermined Ct), all from South Africa.
  • Specificity: None of the 19 presumed A. m. scutellata samples tested positive.
  • False Positive: One sample of A. m. litorea (East African) tested negative by qPCR but positive by sequencing (A-allele), suggesting the assay is highly specific to the Cape region clade but may have limitations with A. m. litorea.
• RFLP Assay (ND2):
  • Successfully assigned 105/142 samples to the ACMS clade (uncut band).
  • Correctly identified non-ACMS samples via banding patterns (2–3 bands).
  • Confirmed that no non-A-lineage samples were assigned to the ACMS clade.

5. Significance and Discussion

• Regulatory Utility: These assays provide a rapid, cost-effective tool for APHIS and state inspectors to screen honey bees at ports of entry and apiaries. They allow for the specific identification of regulatory threats (A. m. scutellata derivatives and A. m. capensis) without the need for expensive whole-genome sequencing.
• Behavioral vs. Genetic Correlation: The study highlights that defensive behavior is not strictly linked to a single mitochondrial marker. Some defensive bees lacked the specific AHB SNP (suggesting nuclear genetic factors or different African mitotypes drive the behavior), while some "gentle" bees in Puerto Rico were non-A-lineage. This reinforces that mitochondrial markers track maternal ancestry but do not solely dictate phenotype.
• Limitations & Future Directions:
  • The AHB marker targets a specific mitotype; it may miss other AHB populations derived from different South African queens.
  • The A. m. capensis marker is specific to a clade in the southwestern Cape; further research is needed to cover the full diversity of capensis mitotypes.
  • Mitochondrial markers cannot detect nuclear introgression or specific behavioral alleles, which are critical for a complete risk assessment.
• Conclusion: The developed suite of assays represents a significant advancement in honey bee diagnostics, offering a scalable solution for distinguishing between invasive African lineages and native/regulatory subspecies, thereby enhancing biosecurity and apiary management in the US.