Epigenetic Control of Sexual vs. Asexual Development in the Pea Aphid

Posted June 23rd, 2010 at 1:29 pm.

Abstract: Amelie Raz
Mentor: Dr. Davis
This summer, I will be researching sexual polyphenism in aphids and its possible epigenetic consequences. Aphids have a highly unusual method of reproduction; there are parthenogenic female morphs that, after several generations, give rise to generations of sexual morphs, who in turn produce another all-female population. The transformation from an asexual to a sexual population is governed by environmental triggers, specifically changing day length: the asexually reproducing aphids are found in the spring and summer, while the sexual aphids do not appear until the fall. Given that the two morphs are genetically identical, the differences are most likely caused by non-sequence DNA alteration, specifically differential methylation and histone modification. Thus far, all analysis on the difference in expression of these genes between sexual and asexual aphids has been qualitative. Last year, students in Dr. Davis’ lab performed in situ hybridization experiments to initially determine possible differences in gene expression. They discovered that the genes in the Torso-like developmental pathway, specifically torso, torso-like, rolled, and capicua, are differentially expressed between sexual and asexual aphids, making them possible candidates for epigenetic modification. Extending this analysis, I would use quantitative polymerase chain reaction, or qPCR, to repeat their qualitative experiments quantitatively.

Once genes that exhibit quantitative differential expression have been identified, I will attempt to determine if there is epigenetic modification and, if so, what sort of modification. Should I discover differential modification, two possible causes are DNA methylation or histone modification. Methylation occurs when a methyl group is added to the nucleotides cytosine or adenine in the DNA sequence. I will be looking specifically in methylation of CpG islands (stretches of DNA with many occurances of a cytosine next to a guanine). To test for methylation in the target genes, I will use bisulfite conversion, by which all non- methylated cytosines located in CpG islands will be converted to thymine. Methylated cytosines, however, will not be converted. A failure to convert thus indicates methylation. By comparing the amount of unconverted cytosines in sexual aphids to the number of unconverted cytosines in the asexual aphids, I may be able to determine if there is differential methylation of the candidate genes. Second, the epigenetic variation could be caused by histone modification. Depending on how the DNA is wrapped around histone organizing proteins, there can be variation in translation and thus gene expression. Using technologies employed in Dr. Tamara Davis’ lab, I will test for variations in histone function between the two morphs.

Filed under: 2010,Davis, Dr. Gregory,Raz, Amelie Tags: by Lisa Klinman

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