How can sterility make a plant more fertile?

Brian Park and colleagues have been examining a peculiar fact. A plant can increase its reproductive success by producing sterile flowers. Not all the flowers are sterile. Viburnum lantanoides, (Adoxaceae) has enlarged sterile flowers at the edge of clusters of flowers. The reason is that these flowers are all about display. They pull the pollinators in and, it is thought, improve the outcrossing of genes.

What Park and his team have been studying is how this shift in display comes about. What are the benefits of sterile flowers when they first start to evolve? If every plant has sterile flowers, then no one is at a disadvantage, but was this true for the first plants to develop sterile flowers?

Park and colleagues ran experiments over three years. The first experiment shows that V. lantanoides does produce more fruit when it has external pollen. Next, they compared two populations of plants. One had sterile marginal flowers (SMFs), and the other didn’t have these sterile flowers. The team made sure one group didn’t have sterile flowers by removing them. What the authors found was that the plants left intact did better than the stripped population. Finally, they tried an experiment to reconstruct the first plants to use sterile flowers.

*Viburnum lantanoides*. (A) Plants flower for 5–7 d in early spring prior to canopy closure, as shown by a fisheye photograph taken from above one of our focal plants. (B) Genets can be composed of multiple flowering ramets; larger plants can produce >100 inflorescences. (C) Inflorescences bear ~165 fertile flowers on five inflorescence rays, and eight SMFs (two on each of the four lateral rays). (D) The presence of SMFs increases the overall width of an inflorescence by a factor of 2. (E) Flowers are visited by a variety of insects but are most effectively pollinated by early-emerging Andrena bees. (F) Drupe fruits mature to the red-coloured phase ~2 months after flowering and then asynchronously turn black at maturity.

They had a stand of V. lantanoides plants. For all but a few, they removed the sterile flowers. This was to make the plants with the sterile flowers sit among many more that didn’t put energy into growing and maintaining these displays. The plants with the sterile flowers attracted twice the insect visits. The same plants also produce more fruit. But isn’t that the other plants having few flowers? Park and colleagues say not: “There was no significant effect of the number of inflorescences or fertile flowers on insect visitation or fruit set, indicating that the presence of sterile marginal flowers accounted for these differences.”

They conclude these sterile flowers significantly increased pollinator attraction and female reproductive success. This was true both for current and ancestral conditions. This, the authors say, shows that directional selection drove the evolution when the sterile flowers first appeared. Once they became common, stabilizing selection made sure the plants kept them.