MCB and Butterfly lilies wing their way to Science

The common butterfly lily features on the landing page in Science this week.

The inner genetic secrets of how butterfly lilies break left-right symmetry are revealed in a paper published in Science this week. MCB honours and graduate students working with Prof Nicola Illing and Associate Professor Robert Ingle made several important contributions to this research.

First, some background. The body plans of most animals and the flowers of plants are either radially, or bilaterally symmetrical.  In the latter case, the left-hand side is a mirror image of the right-hand side.   An intriguing problem is how organisms which are mainly symmetrical, can break symmetry.   For example, in humans, left-right symmetry is broken when your heart develops on the left hand side of your body.  This phenomenon occurs in flowers too, when the female style is deflected either to the left or right of the midline, a feature known as enantiostyly in the botanical world.  In most cases, flowers with styles deflected to the left, or to the right, are found on the same plant. However, in the case of butterfly lilies, which are endemic to the Western  and Eastern Cape, and in a sister genus, Barberetta aurea found in Kwa-Zulu Natal and the Eastern Cape, all the flowers on a single plant have styles pointing to the left (while the central stamen is deflected to the right), or vice versa.   This ingenious strategy separates the style from the stamen, minimizing the risk of self pollination, while facilitating the efficient transfer of pollen to the style of a recipient flower of the opposite orientation (illustrated in this video).

In collaboration with researchers at the University of Potsdam and Wagenenin, and with support from the Human Frontier Science Programme and South Africa 1KSA project,  the UCT team set out to identify the genetic and cellular mechanisms by which butterfly lilies belonging to the genus Wachendorfia and B. aurea consistently develop either left- or right morph flowers. In doing so, the researchers addressed a classic evolutionary curiosity noted by Charles Darwin nine days before his death, in his last letter to the naturalist James E. Todd in America.
 

Several challenges were solved by the UCT team, starting with how to isolate high quality genomic DNA and RNA from these plants which are rich in gloopy carbohydrates.  Through sheer tenacity, Kelly Shepherd solved this problem during her MSc, developing a protocol to successfully isolate high quality DNA and RNA from flower buds.  By comparing the DNA signatures of hundreds of left-morph and right-morph plants, across the four Wachendorfia species and B. aurea,  the team were able to identify a stretch of DNA, the hemizygous R-locus, that is only present in right-morph plants (Figure 2). 
They then examined which genes in this stretch of DNA were active in developing styles and stamens before these organs began to bend.  The work identified two closely linked genes, YUC-R and miR156 that switch the flower’s default left-morph developmental programme into a right morph programme.

Demonstrating that these candidate genes performed the predicted functions presented further challenges.  UCT Honours students Oliver Marketos and Anand Shankar, conducted experiments showing that the Wachendorfia miR156 gene had similar functional properties to miR156 from the model plant Arabidopsis thaliana. Their results confirmed the biochemical role of miR156 as a regulatory, non-coding RNA.
An idea to search for individual plants which might have natural mutations in these genes, led to a frenzied hunt across the Western Cape looking for unusual looking butterfly lily flowers.  Genetic analysis by Olivia Page-MacDonald revealed that the mutant flowers provided a natural test of the handedness genes’ function. When YUC-R was defective, the central stamen switched position from left to right, while if miR156 was defective, the style switched positions from the right to the left (Figure 2).
Serendipity also played an important role.  While the team was attempting to record the development of a style and stamen, a flower bud was accidentally placed upside down. To our surprise, the developing flower flipped the orientation of the style and stamen.  This key observation made by MSc student Caroline Robertson, revealed that gravity provides an important directional cue that helps orient the style and stamen as the flower develops, rather than guidance from an internal axis.

An animation of the model of how chirality and gravity and the presence/absence of the hemizygous, R-locus supergene cue the development of right- and left-morph flowers is illustrated in this video.

This Spring, look out for the yellow blooms of the four Wachendorfia species on which this research was based and take a close look at their remarkable floral structures. The tall yellow flower spikes of the marsh butterfly lily, Wachendorfia thyrsiflora, are a common sight along stream banks in Cape Town and in botanical gardens around the world. Wachendorfia multiflora and Wachendorfia brachyandra can be seen flowering on Rondebosch Common from August to September. Wachendorfia paniculata is widespread in the Western Cape and can be seen flowering from Table Mountain to Cape Point between September and November.

See the full article on UCT's news page.