Researchers have discovered a gene that lowers the number of pollen grains produced by a plant's flower.

Pollen is to plants what sperm is to animals. The numbers of these two kinds of male gametes vary from plant to plant and animal to animal.

It seems logical that a larger number of male gametes would be advantageous, but many domesticated plants produce lower concentrations of pollen. Scientists have hypothesized that for species and varieties with high levels of self-fertilization and inbreeding, it might make sense to save resources by producing lower pollen numbers.

However, the hypothesis remains unproven. Studies have failed to produce corroborating evidence.

"So far, there has been little evidence to support this idea, because the production of male gametes is a complex trait affected by many genes with small effects and its molecular basis remained unknown," Kentaro Shimizu, professor of evolutionary biology and environmental studies at the University of Zurich in Switzerland, said in a news release.

To better understand the variation in pollen numbers observed among plants, researchers focused on the model plant Arabidopsis thaliana, which mostly self-fertilizes and produces fewer pollen grains than its wild relatives.

Scientists collected dozens of plants, each with distinct genetic lineages, and documented the number of pollen grains produced by their flowers. They observed a wide variety of pollen densities, with levels ranging between 2,000 and 8,000 pollen grains per flower.

Next, researchers surveyed the genomes of the nearly 150 plants to isolate genetic differences that could explain why some plants produce more pollen than others.

With the help of sophisticated computer algorithms, scientists identified a gene, the RDP1 gene, that influences the amount of pollen produced by a plant's flowers. Using CRISPR-Cas9, researchers engineered plants with several different mutations of the RDP1 gene, both in plants with lower and higher pollen levels. Then, scientists cross-bred the plants to produce hybrid offspring.

Their experiments, described Monday in the journal Nature Communications, confirmed that RDP1 works to reduce the amount of pollen produced by a plant's flowers.

Statistical models designed to analyze the evolution of genetic traits showed that the prevalence of RDP1 isn't random, and that lower pollen numbers have been positively selected for by Arabidopsis thaliana plants.

"The evidence supports the theoretical prediction that reduced investment in male gametes is advantageous," said Shimizu. "This is not only important for evolutionary biology but also for the practice of plant breeding and domestication in general."

"Many crop plants have a reduced number of pollen due to domestication. Lowering the cost of producing pollen may increase crop yield," he said. "On the other hand, too few pollen grains might be an obstacle to breeding and seed production. Our study opens the way for molecular breeding of the optimal pollen number."

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