The left‑right asymmetry of South African butterfly lilies (genus Wachendorfia) has been regarded as a classic evolutionary puzzle for more than a century. Due to the mirror-image arrangement of the floral parts, the plants achieve efficient cross pollination. The molecular basis of this arrangement has now been uncovered by an international team led by the University of Potsdam. Their study in collaboration with the University of Cape Town and Wageningen University has been published in "Science".
The researchers show that in Wachendorfia species a "supergene" determines whether plants consistently develop styles and stamens that are bent to the left or to the right. This supergene contains two key factors, which independently control the orientation of stamens and styles and thus enable the precise, mirror image positioning of the reproductive organs.
"Our results show that two tightly linked genes act together to ensure that pollen is deposited at exactly those positions on the bodies of pollinators from which it can be picked up again by the respective mirror image floral form," explains first author Dr Haoran Xue, bioinformatician at the Institute of Biochemistry and Biology at the University of Potsdam. "This supergene couples the orientation of style and stamen so tightly that the plants remain reliably dependent on cross pollination – an elegant mechanism to promote genetic diversity."
The team was able to show that the styles rotate and subsequently continue to grow asymmetrically under the influence of gravity to the left or right from the floral midline. Biomechanical models and cultivation experiments demonstrate that both components – rotation and gravity guided growth – are necessary to produce the pronounced deviation from the midline that is observed. "What is particularly exciting is that left and right here are not defined relative to internal body axes, as in many animals, but relative to an external cue – gravity," says Xue.
For the cell biological investigations, the team used plants of the species Wachendorfia thyrsiflora that are cultivated in the Botanical Garden of the University of Potsdam. "Without the living collections in the Botanical Garden, we would not have been able to carry out many of the microscopic and developmental analyses," Xue emphasises. "The close integration of the Botanical Garden, experimental plant biology and modern genomics is a real asset of the University of Potsdam."
The newly published study is the result of a long standing collaboration: Researchers in Cape Town contributed their expertise in fieldwork and molecular analyses in South African populations of Wachendorfia, while the team in Wageningen developed mathematical and biomechanical models to explain the organ movements. "The combination of fieldwork and laboratory experiments in South Africa, theoretical modelling in Wageningen, and genomic and cell biological analyses in Potsdam was crucial for finally solving the riddle of mirror image flowers," says senior author Prof Michael Lenhard from the University of Potsdam.
"Our findings underline further, that supergenes are a recurring principle whenever multiple traits need to be perfectly coordinated – in this case the orientation of style and stamen," Xue concludes. "Understanding such systems helps us grasp how plants 'program' their pollinators with high precision and thereby drive speciation and ecological adaptation."
Link to Publication: Haoran Xue, Marco Saltini, Nicola Illing, Kelly Shepherd, Olivia Page-Macdonald, Oliver Marketos, Caroline Robertson, Anand Shankar, Sarah Süß, Christian Kappel, Saleh Alseekh, Eva E. Deinum, Robert A. Ingle, Michael Lenhard: Supergene control of chiral development in mirror-image flowers, Science: https://doi.org/10.1126/science.aeb1157
Image 1: Arrangement of the floral parts of South African butterfly lily Wachendorfia thyrsiflora. The pink arrowheads point to the styles, and the blue arrowheads point to the stamens. Photo: Genetics working group, University of Potsdam.
Image 2: Prof. Michael Lenhard and Dr. Haoran Xue in front of the plant collections of Wachendorfia thyrsiflora in the Botanical Garden of the University of Potsdam. Photo: Genetics working group, University of Potsdam.
