For decades, scientists interested in the evolution of plants have debated which came first: two-celled or three-celled pollen grains. Now, researchers at Osaka City University have shown that bicellular pollen evolved about 149 million years ago, approximately five million years before the first appearance of tricellular pollen. This discovery may shed light on how plants evolved from spore-bearing species and diversified into the flowering plants we see today.

Pollen grains are male gametophytes (the male sex cells) of seed plants. When pollen grains land on the female stigma, they form a pollen tube that delivers the sperm cells for fertilization of the egg cells.

In most flowering plants, pollen grains are tricellular, consisting of two sperm cells and one vegetative cell. The vegetative cell is responsible for the growth of the pollen tube. In contrast, the gymnosperms, or non-flowering seed plants (e.g., pines and cycads), bear pollen grains that are usually multicellular and often wind-dispersed.

For many years, it has been a topic of debate when the tricellular pollen typical of most angiosperms (flowering plants) appeared in the fossil record and how this relates to the evolution of double fertilization. Until now, the oldest fossil evidence of unequivocal tricellular pollen dated to the Jurassic period, around 144 million years ago. But, how this relates to the evolution of double fertilization has remained obscure, as the oldest fossils of angiosperm seeds are only slightly younger, around 140 million years old.

The team led by Dr. Ken'ichi Nishida, a professor in the Graduate School of Science of Osaka City University, studied fossil pollen grains from East Asia and Australia. The oldest fossil bicellular pollen grain the researchers found dates from the Late Jurassic period (around 149 million years ago). Based on this finding, bicellular pollen is likely to have originated at least 149 million years ago. "This means that the evolution of bicellular pollen preceded that of tricellular pollen by approximately five million years," says Prof. Nishida. "From this, we can deduce that the ancestor of flowering plants first produced bicellular pollen."

The researchers suggest that the evolution of pollen with a reduced number of cells may have been a key factor in the diversification and ecological success of flowering plants. "Reducing the number of cells would've accelerated pollen development, allowing plants to reproduce faster," explains Dr. Nishida. "In turn, this acceleration of the life cycle is inferred to be a key innovation for the subsequent ecological diversification and success of angiosperms."