Evolutionary Ancient Mechanism of Pollination by Flotation Has Been Lost in Many Conifers

A new study offers not only a sweeping analysis of how pollination has evolved among conifers, but also an illustration of how evolution -- far from being a straight-ahead march of progress -- sometimes allows for long-standing and advantageous functions to become irrevocably lost. Moreover, the authors show that the ongoing breakdown of the successful, but ultimately fragile, pollination mechanism may have led to a new diversity of traits and functions. Dr. Andrew Leslie, Assistant Professor of Ecology and Evolutionary Biology at Brown Univesity, and his co-authors studied more than 460 conifer species to order and trace the evolution of a trio of traits that provide an ancient function of pollination. Many pine and spruce species still exhibit these attributes: pollen grains that are buoyant because of structures called sacci (air-filled bladders), downward facing ovules, and the well-timed emission of a drop of liquid. For a few days a year, these trees send their pollen into the wind. The pollen grains that lands on the cone under the ovule becomes engulfed in the droplet and, because the pollen grains are buoyant, float up into the ovule. The process has the advantages of filtering out non-bouyant particles, and of guiding a concentration of pollen saccae to the otherwise well-shielded ovule. "People thought these traits were correlated," said Dr. Leslie, first author of the paper that was published online on April 22, 2015 in the journal Evolution. "What we did was put this in an evolutionary context." The article is titled “Trait integration and Macroevolutionary Patterns in the Pollination Biology of Conifers.” What the scientists found is that while the mechanism had apparently served the wide world of conifers well for hundreds of millions of years, it is gradually disappearing.
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