Enhancer Trapping in the Ladybeetle, Harmonia axyridis

Marc Schetelig,

Marc Schetelig, Ph.D., Head of Emmy Noether and Fraunhofer Attract Group, Justus-Liebig-University Gießen

Nowadays, efforts to sequence genomes are routine experiments. But once it comes down to the bioinformatics and especially to understand the vast amount of data on a functional level – we are still struggling and good old techniques to find out about gene function are still the way to go.

Enhancer trapping is a functional genomics tool that has been around in model systems for decades.  Transposon-based enhancer trap systems require two components – a source of transposase and a mobile element sensitive to enhancers (sometimes called a ‘mutator’ element).

For the first time an enhancer-trap system was established in the ladybird beetle, Harmonia axyridis. Kuwayama et al (2014) developed this system using a single vector that contained two elements:

i) a so-called “mutator” cassette (the enhancer detector element)
ii) a transposase expressing cassette to catalyze “mutator” remobilization so the genome can be scanned for enhancers.

Kuwayama et al (2014) chose this strategy for constructing their vector  because they only have one transposon system (piggyBac) that they know functions in H. axyridis.  So how can you move the piggyBac transposase gene into the genome using a piggyBac vector and not have it keep jumping around and potentially out of the genome?  They used a method described by IGTRCN Participant Al Handler and colleagues (2004) that allows you to put a piggyBac element into a genome and have it become immobilized by removing one of the inverted repeats.  So, this is a clever strategy to create the two genetic lines needed by Kuwayama et al. using only one vector system.

Harmonia axyridis, various color patterns.

Harmonia axyridis, various color patterns.

After H. axyridis germline transformation using this vector with a success rate of 2.3%, the two cassettes were separated in subsequent generations through transposon remobilization of the mutator cassette, which at the same time stabilized the transposase-expressing cassette.

Basic strategy for enhancer trapping.  The reporter is on a transposon that can be moved around the genome

Basic strategy for enhancer trapping. The reporter is on a transposon that can be moved around the genome

Remobilization events were then identified by crossing transposase- and mutator-lines. These lines, carrying both cassettes but now unlinked in the genome, were exposed to a 40°C heat shock (to drive transposase expression) and the progeny were tested for remobilization events by looking for the respective fluorescent markers. The group was able to identify at least one new enhancer-trap event for each line tested and estimate that the rate of movement observed was comparable to that observed with similar systems in Drosophila melanogaster.

Kuwayama et al (2014) demonstrate a nice tool for studying new functional elements in Harmonia species.  Harmonia axyridis is the latest ‘non-model’ insect system for which an enhancer trap system has been developed.

Kuwayama H, Gotoh H, Konishi Y, Nishikawa H, Yaginuma T, et al. (2014) Establishment of Transgenic Lines for Jumpstarter Method Using a Composite Transposon Vector in the Ladybird Beetle, Harmonia axyridis. PLoS ONE 9(6): e100804. doi: 10.1371/journal.pone.0100804

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