Scientists Solve Century-Old Supergene Mystery

Scientists have solved the century-old mystery of a supergene that causes efficient cross-pollination in flowers. 

The Long and Short of a Supergene for Efficient Pollination

Researchers have solved the century-old mystery of a supergene that causes efficient cross-pollination in flowers. The results reveal that sequence length variation at the DNA level is important for the evolution of two forms of flowers that differ in the length of their sexual organs.

Since the 1500s, botanists and gardeners have known that some plant species have two forms of flowers that differ reciprocally in the length of their male and female sexual organs. Charles Darwin first proposed that such distylous flowers promoted efficient cross-pollination by insect pollinators. Early geneticists showed that the two forms of flowers were controlled by a single chromosomal region that likely harbored a cluster of genes, a supergene. Until recently, however, this supergene had never been sequenced.

Pollinator Visiting Linum tenue

Pollinator visiting Linum tenue. Credit: Juanita Gutiérrez-Valencia

Now, researchers have solved the mystery of the supergene. They studied a system where already Darwin described distyly, wild flaxseed species, Linum, and used modern DNA sequencing methods to identify the supergene. The study by scientists at Stockholm University, together with partners at Uppsala University, Durham University, University of Granada, and University of Seville, is published today (September 9, 2022) in the journal Current Biology.

Surprisingly, the researchers discovered that the supergene responsible for differing lengths of male and female sexual organs itself varied in length. Specifically, the dominant form of the supergene contained about 260,000 base pairs of DNA that were missing from the recessive form. The 260,000 base pair stretch of DNA harbored several genes likely to cause length variation in sexual organs.

Linum tenue flowers photographed in the field in Spain. Credit: Benjamin Laenen and Aurélie Désamoré

”These results were really surprising to us, because a similar genetic makeup of the supergene that governs distyly has previously been identified in another system, primroses, where it evolved completely independently,” said Tanja Slotte, senior author of the study and Professor in Ecological Genomics at Stockholm University.

”Not only has evolution repeatedly led to similar variation in the flowers of primroses and flaxseed species, it has also relied on a similar genetic solution to achieve this feat,” said Juanita Gutiérrez-Valencia, PhD, first author of the study and student at Stockholm University.

The two forms of flowers of fine flax, Linum tenue, differ reciprocally in the length of their sexual organs. The top panel shows entire flowers whereas the bottom part shows the reproductive structures within the flowers, with arrows indicating male and female sexual organs. Credit: Juanita Gutiérrez-Valencia

These discoveries provide new insights into the remarkable power of evolution to find convergent solutions to widespread adaptive challenges such as the need for flowering plants to be cross-pollinated.

”Distyly is ultimately a mechanism for efficient cross-pollination. Understanding pollination mechanisms is particularly important today given climate change and challenges faced by both plant and insect pollinator populations,” said Professor Tanja Slotte.

Reference: “Genomic analyses of the Linum distyly supergene reveal convergent evolution at the molecular level” 9 September 2022, Current Biology.
DOI: 10.1016/j.cub.2022.08.042

Stockholm University

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