Why are the seed cones of conifers so diverse at pollination?

Wind-pollinated plants rely on the same breezes to deliver pollen. If the problem's the same for all of them, why do conifers have different shaped cones to catch pollen?

When we think of pollination we think of flowers or blossoms, but this is only part of the story. Many plants pollinate without flowers, such as conifers. They produce male and female cones. Pollen blows from the male cones to land in the female cones. That takes a lot of pollen, and explains why the hayfever season starts with tree pollen for many people.

The problem for female cones is the same for any conifer, how to catch pollen on the breeze. It’s the same problem for all gymnosperms, yet they have various solutions. You can recognise what cones belong to what species.

Morphological development in Abies koreana and Picea jezoensis seed cones.
Morphological development in Abies koreana and Picea jezoensis seed cones. (A) A. koreana cones around bud break but before pollination; bract scales are the only visible structures. (B) P. jezoensiscones following bud break, with both bract scales and ovuliferous scales visible. (C) A. koreana cones at pollination, showing gaps between bract scales where pollen can enter the cone. (D) P. jezoensiscone at pollination; red structures are ovuliferous scales and the much smaller bract scales are no longer visible. (E) A. koreana after pollination; ovuliferous scales have expanded to fill spaces between bracts, which are now visible only as thin pointed structures. (F) P. jezoensis cone following pollination; growth of the ovuliferous scale bases has closed the gaps between them, while the entire cone has also shifted position.

Losada and Leslie use conifer species in the fir (Abies) and spruce (Picea) lineages of the pine family (Pinaceae) to investigate the evolutionary mechanisms leading to reproductive diversity among wind-pollinated plants. Abies and Picea are thought to draw in pollen in slightly different ways. Abies may use rainwater to move pollen into its ovules following pollination while Picea ovules exude an aqueous pollination drop to move the pollen.

The authors used Abies koreana and Picea jezoensis cones from trees growing in the Arnold Arboretum of Harvard University. They tested whether cone morphology influences pollination function through natural and artificial pollination experiments. For natural pollination, they sampled cones every other day during the pollination period, dissected them, and recorded the position of pollen grains. For artificial pollination experiments, they first collected cones that had opened but had not yet received pollen and then artificially pollinated them in a wind tunnel.

Wind tunnel analyses using these cones suggest that different morphologies are equally effective in capturing pollen, demonstrating how simple differences in development can give rise to equally functional morphologies. Losada and Leslie conclude: “In the absence of clear functional differences among cones, neutral variation in cone scale development appears to be the primary reason for diverse pollination-stage morphologies exhibited by Pinaceae.”

Reference List

Losada, J. M., & Leslie, A. B. (2018). Why are the seed cones of conifers so diverse at pollination? Annals of Botany, 121(7), 1319–1331. https://doi.org/10.1093/aob/mcy029