Vitellogenin Precursors and the Effects on Traits in the Honey Bee

CLEMENT F. KENT, AMER ISSA, ALEXANDRA C. BUNTING and AMRO ZAYED

2012

Apis Mellifera

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).

Vitellogenin Protein Molecule

Main Question:

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).

Results

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).

Fig. 1

Evidence for positive selection on vitellogenin in honey bees. Mean c = 2Nes (±SD; positive values indicate positive selection) for (a) all studied genes, and (b) for distinct func- tional regions in vitellogenin and Erk7 (See Table S4, Support- ing information, for region definitions). Both vitellogenin (Vg) and Erk7 show significant gene-wide signatures of positive selection (a), although both Vg’s N-terminal and Erk7’s Kinase domain lack signs of positive selection (b).

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.

Fig. 2

Evidence for positive selection on vitellogenin within Apis mellifera’s geographic subspecies. Both Western European (a) and Eastern European bees (b) have regions of highly sig- nificant linkage disequilibrium (red, R2 > 85%) concentrated in C-terminal portions of the gene. (c) Candidate functional poly- morphisms experiencing selective sweeps in vitellogenin (Vg)—all sites are nonsynonymous polymorphisms with outlier levels of genetic differentiation and high levels of linkage dis- equilibrium. Ancestral alleles were determined by comparison to Apis cerana and Apis florea sequences. Sites are labelled by amino acid residue number; A, African; M, Western European; C, Eastern European. Note fixation of derived allele for site 1220 in Western European bees and for site 1536 in Eastern European bees.

Conclusions

  • 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.

References:

Chen JS, Sappington TW, Raikhel AS (1997) Extensive sequence conservation among insect, nematode, and vertebrate vitellogenins reveals ancient common ancestry. Journal of Molecular Evolution, 44, 440–451.

“COSMOS Magazine.” Bees Can Count up to Four, Study Shows. Luna Media Pty Ltd, 28 Jan. 2009. Web. 25 Apr. 2012. <http://www.cosmosmagazine.com/news/2496/bees-can-count-four-say-researchers>.

Elango N, Hunt BG, Goodisman MAD, Yi SV (2009) DNA methylation is widespread and associated with differential gene expression in castes of the honeybee, Apis mellifera. Proceedings of the National Academy of Sciences of the United States of America, 106, 11206–11211.

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.

Lomize, Andrei, Mikhail Lomize, and Irina Pogozheva. “Vitellogenin.” Orientation of Proteins in Membranes: Vitellogenin. University of Michigan, Ann Arbor, 2005. Web. 25 Apr. 2012. <http://opm.phar.umich.edu/protein.php?pdbid=1lsh>.

McDonald JH, Kreitman M (1991) Adaptive protein evolution at the Adh locus in Drosophila. Nature, 351, 652–654.

“McDonald–Kreitman Test.” Wikipedia. Wikimedia Foundation, 13 Apr. 2012. Web. 26 Apr. 2012. <http://en.wikipedia.org/wiki/McDonald–Kreitman_test>.

Tufail M, Takeda M (2008) Molecular characteristics of insect vitellogenins. Journal of Insect Physiology, 54, 1447–1458.

“Vitellogenin.” Wikipedia. Wikimedia Foundation, 8 Dec. 2011. Web. 25 Apr. 2012. <http://en.wikipedia.org/wiki/Vitellogenin>.

Whitfield CW, Behura SK, Berlocher SH et al. (2006) Thrice out of Africa: ancient and recent expansions of the honey bee, Apis mellifera. Science, 314, 642–645.

Zayed A, Whitfield CW (2008) A genome-wide signature of positive selection in ancient and recent invasive expansions of the honey bee Apis mellifera. Proceedings of the National Academy of Sciences of the United States of America, 105, 3421– 3426.

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