CLEMENT F. KENT, AMER ISSA, ALEXANDRA C. BUNTING and AMRO ZAYED
Background on Vitellogenin
Vitellogenin is the term used for both the gene and the protein it expresses. This protein is considered a glycolipoprotein, since it has the properties of sugar, fat, and proteins. Vitellogenin is a precursor protein for egg yolk and is expressed in about all species that lay eggs (Chen et al. 1997; Tufail & Takeda 2008).
It has been known to play a role in honeybees as both a food storage reservoir and antioxidant affecting lifespan. In honeybee colonies, the nurse bees have higher reserves of vitellogenin than the foragers (they get this vitellogenin through feeding of the protein). This is thought to be an effective strategy for utilizing vitellogenin because the foragers have a more dangerous job (leaving the hive) and are, therefore, only supplied with the necessary amount to keep them working. The vitellogenin levels are important for determining the division of labor in the nest. Vitellogenin (Vg) also plays a role in the fecundity of the queen (“Vitellogenin,” Wikipedia: The Free Encyclopedia).
What effect do these Vg precursors have on traits in the honey bee?
Methods and Procedures
Apis mellifera (as well as a few other honey bee species from the genus Apis) were sampled from Africa, Eastern and Western Europe (Apis mellifera) and Thailand (A. cerana, A. florea and A. dorsata). DNA from the thoracic tissue was extracted with a Qiagen DNeasy blood and tissue kit. PCR reactions were conducted with primers used to target exons in the honey bee genome. Sequencing was done on the purified products and trimmed of low-quality bases. Heterozygous sites were detected with a threshold of 34% and using a Sequencer’s Call Secondary Peaks command. The Apis mellifera genome was used as a reference when aligning the sequences.
The researchers then tested to be sure that the three distinct A. mellifera populations were detectable and found that it was consistent in showing A. mellifera of three populations (Africa, Western Europe, Eastern Europe).
They found that ‘Vg exhibits high levels of adaptive evolution in the genus Apis‘. It was found that Vg has the highest levels of nucleotide diversity in A. mellifera compared to other genes. McDonald-Kreitman testing was done, which tests for ancient selection over extended periods of time rather than testing for mutations occurring over time without selection (McDonald & Kreitman, 1991). MK tests of Vg from A. mellifera and A. cerana proved to be highly significant leading to the knowledge that there is, in fact, a significant difference between Vg from one subspecies and the other. Vitellogenin was found to be positively selected for (shown in the figure below).
The researchers selected for the African, Western and Eastern European populations because Apis mellifera originated in Africa and then colonized both halves of Europe in two separate expansion events. (Whitfield et al. 2006; Zayed & Whitfield 2008). The researchers found that there were many sites with significant linkage disequilibrium in Vg in both East European and Western European Apis. This linkage disequilibrium is shown in the following figure.
- The evidence for positive selection acting on Vg demonstrates that the gene affects fitness in honey bees.
- “The large proportion of nonsynonymous polymorphisms fixed by selection in Vg suggests that functional mutations in the gene do not often have antagonistic effects or that their net effect on fitness across queens and workers are positive.”
- Social pleiotropy does not necessarily limit adaptive protein evolution in social insects.
Strengths/Weaknesses of Paper
Vg is fast evolving in non-eusocial lineages, thus one cannot separate ancestral from proximate evolutionary effects as the paper (Elango et al., 2009).
The large picture implications from looking at this one protein are a bit stretched. The paper gives shows interesting results, however, it makes more inferences about what this could mean than is actually possible. The researchers suggest their finding is more important than it actually is to social insect biologists.
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Kent, Clement F., Amer Issa, Alexandra C. Bunting, and Amro Zayed. “Adaptive Evolution of a Key Gene Affecting Queen and Worker Traits in the Honey Bee, Apis Mellifera.” Molecular Ecology 20 (2011): 5226-235. Print.
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