Self-pollinated plant shows rapid loss of genetic variation

Without the bumblebees, a flowering plant that can self-pollinate lost substantial genetic variation in just nine generations, according to an experimental study.

A group of “self-contained” monkey-flowering plants lost 13% to 24% of their genetic variation compared to another group propagated by bumblebees. This loss could rob plants of their ability to adapt to environmental challenges, according to the study published in the journal Evolution. With bee populations declining in character, the results point to serious problems for wild plants and crops that rely on these pollinators.

“We found that in a very short time, there were major consequences for the genomes of plants when they had to adopt self-fertilization,” said Jeremiah Busch, an evolutionary biologist at Washington State University and lead author of the study.

Pollinators like bees are important to biodiversity as a whole, Busch added, but the study says their decline will also have potentially devastating effects on plants, and quickly.

“If pollinators are lost, it won’t just be a problem for pollinators: plant populations will lose genetic variation over dozens of generations — not thousands, but tens,” Busch said.

While scientists knew that embracing self-pollination can endanger the very long-term survival of a plant species, they weren’t sure exactly how it worked genetically or at what speed.

Busch’s colleagues rented a controlled greenhouse experiment using yellow monkey flower plants, a common wildflower found in the western United States, in which a group of plants was isolated from their bumblebee pollinators. At first, plants other than bees produced few seeds, then they produced many seeds by adapting to self-pollination. The flowers have also changed with their male and female reproductive organs, the tops of their stamens and pistils, moving closer together to allow easier transfer of pollen.24

While the self-pollinated plants continued to reproduce, they lost their genetic variation compared to a control group visited by bumblebees.

Adaptation is key to explaining these startling declines, Busch said. In self-fertilized populations, a favored genotype will spread if it has an advantage, but so will any other mutations it carries, simply because they are likely to reside in that plant’s genome. This phenomenon of “genetic self-cease” is much less pronounced when bees visit plants because the offspring are a mixture of the genetic variability of their parents.

“High inbreeding fundamentally altered the consequences of adaptation,” he said.

Future research should follow plants over a longer period of time to see if and when loss of genetic variation leads to population collapse, Busch said.

“A really important next step is to see how quickly groups highly inbred will see their viability decline over time – to know how quickly these populations will disappear,” he said. “We really need to understand what the consequences of losing pollinators are. This will matter to wild populations of plants and crops. Many crops depend on bees.”

This work was supported by the National Science Foundation as well as a sabbatical from WSU at Busch which allowed him to work with the co-authors John Kelly and Sharif Tusuubira at the University of Kansas as well as with Sarah Bodbyl-Roels at the Colorado Faculty of Mines.

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