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Genomes of Placid Killer Bees in Puerto Rico Show High Levels of Genetic Diversity

Africanized honey bee

NEW YORK (GenomeWeb) – The Africanized honeybees, or killer bees, that were accidentally released in Puerto Rico in the 1990s are not only relatively gentle, but also genetically distinct from other Africanized honeybees, new research suggests.

A team from the US, China, and Denmark did genome sequencing on European honeybees and unusually gentle Africanized honeybees from populations in Puerto Rico. Comparison with genome sequences from Africanized honeybees at a site in North America where the Puerto Rican killer bees are thought to have originated pointed to several sites of selection in the genomes of Africanized honeybees currently found in Puerto Rico.

In particular, the researchers identified apparent signatures of selection that may affect hundreds of genes. Still, representatives from the Puerto Rican honeybee colonies considered had high levels of genetic diversity, they reported today in Nature Communications, suggesting that they may be able to withstand some diseases and environmental challenges that have negatively affected European honeybees. Available evidence suggests Africanized honeybees are resistant to the Varroa mite parasite, for example.

"Genetically diverse gentle honeybees could help secure agricultural production by providing pollinators more resistant to threats such as parasites and diseases," University of Illinois at Urbana-Champaign genomic biology researcher Arian Avalos and Chinese Academy of Sciences genetic and evolution researcher Hailin Pan, the study's co-first authors, and their co-authors wrote. "Evolution of [gentle Africanized honeybees] also provides a paradigm for the adaptation of social organisms to new environments."

Hybridized honeybees representing a cross between European honeybees and African honeybees first escaped from a breeding program in Brazil in the 1950s, the team explained. Though the Africanized honeybees or killer bees, as they are called, are typically aggressive, the Puerto Rican populations that have persisted since an accidental release in the mid-1990s appear to have adapted and become relatively docile within a couple of decades.

To try to tease out genetic factors that might render Puerto Rico's Africanized bees more mild-mannered than their North American counterparts, Avalos, Pan, and their colleagues collected individual gentle Africanized drone bees from 30 unrelated colonies in Puerto Rico and nearby islands. Similarly, 30 European honeybees and 30 Africanized honeybee representatives came from unrelated colonies or research apiaries in the US or Mexico.

Collaborators at BGI in Hong Kong used Illumina HiSeq 2000 instruments to do whole-genome paired-end sequencing on all 90 drone bees, generating 20-fold average coverage of each honeybee genome.

When they compared the genomes to one another and to a honeybee reference genome from BeeBase, the researchers identified more than 2.8 million bee SNPs. Their extended haplotype homozygosity-based analysis picked up nearly 28,100 SNPs showing signs of selection in and around 250 genes. Though Puerto Rico's placid Africanized honeybees have retained high levels of genetic diversity, the team reported that they cluster genetically in a group that is distinct from other European and the Africanized honeybees.

"Evolution involves changes in the frequency of gene variants across a population, and that's what we're seeing in Puerto Rico," co-senior author Gene Robinson, a genomic biology researcher at the University of Illinois, Urbana-Champaign, said in a statement. "Now we know that these gentle Africanized bees can be genetically distinguished both from other Africanized honey bees and from European honey bees."