Chloe Thangavelu

Mentor: Gregory Davis

Biology

 

In-situ Hybridization of Differentially Expressed Genes in Sexually and Asexually Fated Pea Aphid Embryos

 

The aim of this research project is to investigate the developmental switch that underlies the reproductive polyphenism in aphids. Polyphenism is a form of plasticity in which different phenotypes are formed response to environmental variation. Because they display some of the most distinctive and classic examples of polyphenism, aphids serve as a model system for studying the mechanisms of phenotypic plasticity and its evolution. In the reproductive polyphenism, aphids alternate between two different reproductive phenotypes, a sexual morph and an asexual morph, which differ phenotypically and reproductively.

 

The sexual morph reproduces oviparously by laying eggs that must be fertilized by male sperm, whereas the asexual morph forgoes fertilization through parthenogenesis and is viviparous, so oocytes within the asexual morph enter embryogenesis and complete their development within the mother before being born. Asexual and sexual morphs can be distinguished by the presence of either smaller, diploid, developing embryos or larger, haploid, yolk-filled oocytes within the ovaries of the aphid.

 

The decision to produce asexual or sexual morphs is dependent on the mother’s exposure to sunlight during a critical period, beginning in late embryogenesis and continuing through adulthood. Exposure to long days and short nights results in the development of mothers that produce asexual morphs. Exposure to long nights results in the development of mothers, called sexuparae, that produce sexual morphs. Specifically, the goal of this project is to understand the molecular mechanism within the embryo that results in the specification of either the sexual or asexual fate, as mediated by the mother’s photoperiod.

 

In situ hybridization (ISH) will be used to look at genes that are expressed at different levels in sexually versus asexually fated embryos at different points in specification and differentiation.  The ISH protocol will involve tissue sample preparation, probe synthesis, hybridizing the probe to the sample, and probe visualization.

 

We have recently obtained a large transcriptonomic data set that Gemma Johnson is currently examining for candidate genes that are differentially expressed between sexually and asexually fated embryos and may contribute to the mechanism. Meanwhile, I will develop the technical ISH skills to further analyse the expression of any emerging candidates. Candidate genes of particular interest are those possibly involved in hormone production, which is a suspected contributor to specification of fate, or genes expressed in tissues that develop differently in sexuals and asexuals and are therefore likely to play a role in differentiation.

 

My hope is that ISH will narrow the list of candidates by providing information about the spatial and temporal expression of these genes. ISH can help identify genes that are strong candidates, such as those expressed in tissues that develop differently in sexuals and asexuals. Contrasting the expression of candidate genes in the asexually versus sexually fated embryos should provide insight into the nature of the molecular switch governing aphid reproductive polyphenism. Understanding the mechanism of the switch is also a prerequisite to understanding how this environmental response has evolved in cases where aphid populations have compromised responses, or lost sex altogether.