A CRAL-TRIO domain gene involved in butterfly vision

This post is written by UCI grad student Aide Macias

Graduate student Aide Macias presenting a poster at SMBE 2017

Butterflies have extremely diverse wing color patterns which cause us to wonder, what do these brightly colored insects see? The Briscoe lab at the University of California, Irvine aims to study the evolution of vision in butterflies. As a graduate student in Adriana Briscoe’s lab my projects have involved quantifying the expression of vision-related genes. In my latest research project, I used a variety of methods to identify a gene that encodes a chromophore-binding protein.

Cartoon of a photoreceptor cell membrane with a rhodopsin before and after light absorption

In vision, light information is converted into an electrical signal by a process called phototransduction. Photoreceptor cells have a light-sensitive receptor called rhodopsin which is made up of an opsin protein bound to a retinal molecule known as chromophore. The vertebrate chromophore is 11-cis-retinal, while the Drosophila chromophore is 11-cis-3-hydroxyretinal. When light is absorbed by rhodopsin, the chromophore undergoes a configurational change from 11-cis to all-trans, which triggers the phototransduction signaling cascade. In vertebrates and Drosophila, chromophores are transported by a group of proteins that have a structural region in common, a CRAL-TRIO domain. A previous study investigating the CRAL-TRIO domain gene family in insect genomes found many copies of these genes in moths and butterflies, but none with a common evolutionary history to those found in Drosophila (Smith and Briscoe 2015). Do any of these moth and butterfly gene copies have the same chromophore binding function?

In order to identify a candidate CRAL-TRIO domain gene involved in butterfly chromophore transport, we explored the evolution and expression of this gene family in the butterfly Heliconius melpomene. H. melpomene is a good species to use because it has a reference genome and a lot of sequencing data is available for this species.

Heliconius butterfly

We began our study by searching for CRAL-TRIO domain genes in genomes of the butterfly H. melpomene. We found 43 CRAL-TRIO domain genes in the H. melpomene reference genome, but we also found that other genomes had more or fewer copies of the genes, termed “copy number variation”. To see which of these copies functions in vision, we looked at their expression in four different tissues: head, antennae, legs and mouth parts. We hypothesized that genes involved in vision would be expressed more highly in heads. We found only one gene that had high expression in butterfly heads. To visualize where this gene was expressed, we designed an antibody against the gene encoded protein. We looked at sections of the butterfly eye for fluorescence showing that an antibody is binding the protein target. The protein stain showed that the product of our candidate gene was expressed in the primary and secondary pigment cells surrounding the photoreceptor cells. Thus, we were able to provide the first evidence for a visual function for any member of this large and rapidly-evolving gene family in butterflies. Details of this study are published in Macias-Muñoz et al. (2017)

Graphical abstract summarizing results

References:

Smith G, Briscoe AD. 2015. Molecular evolution and expression of the CRAL_TRIO protein family in insects. Insect Biochem. Mol. Biol. 62:168–173.

Macias-Muñoz A, McCulloch KJ, Briscoe AD. 2017. Copy number variation and expression analysis reveals a non-orthologous pinta gene family member involved in butterfly vision. Genome Biology and Evolution 9:3398-3412.

 

 

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