Growing a fruit is only one way of passing your genes on to the next generation if you’re a plant. You could also send pollen to another plant. But what factors influence siring success? Dorothy Christopher and colleagues have been reviewing the interaction between pollination and post-pollination processes in reproductive success.
To successfully father a new plant, you have two problems. One is the difficulty of getting your pollen to a recipient. This can depend on pollinators getting the pollen safely from your anthers to another plant’s stigma. Once the pollen arrives it’s in competition with pollen from other plants.
Getting the pollen to a pollinator
Christopher and colleagues consider flower shape and colour. The accessibility of nectar and pollen depends on the shape of the flower. Flowers can discriminate between certain pollinators by holding back or presenting rewards to place pollen onto pollinators bodies. There is not always a simple correlation between flower and pollinator. The authors refer to work on Polemonium brandegeei that attracts both hummingbirds and hawkmoths. These are two different bodies and so push the flowers to adapt in two different ways.
Colour can also make a difference. Pollinators prefer Raphanus raphanistrum when it’s yellow. Additionally, floral displays can make a bigger impact on pollinators when they’re bigger.
The team also examines research into pollination intensity. “When pollinators are rare, and thus pollinators are not competing for floral resources, trait preferences should be strong because the pollinator will preferentially visit the most rewarding phenotypes. Under these circumstances seed production (and pollen dispersal) will be greatest for plants that receive more, or more effective, visits,” write Christopher and colleagues.
“By contrast, when pollen is not limiting there will be little variation in female reproductive success (all plants are at capacity), and male reproductive success should largely reflect pollen abundance. In this case there may not be a consistent relationship between floral traits and male fertility if most pollen has been removed from each pollen donor. However, donors might still differ in male reproductive success if they differ in post-pollination success.”
Pollen versus Pollen competition
When pollen arrives at the stigma, a race is on to fertilize the eggs in the ovule. Understandably, getting there first gives a big advantage. A later pollen will have to develop a pollen tube and fight through layers of other pollen to get to the recipient plant. It isn’t just competitor pollen that a pollen grain has to contend with. Some unlucky grains from other species may be on the plant, interfering with the reproduction process. The plant may also be open to self-fertilization say Christopher and colleagues. “Lower age may influence pollen germination, especially in self-incompatible species. In self-incompatible Leptosiphon jepsonii, self pollen cannot sire seeds on the first day of stigma receptivity, but can on the second day.”
Growing a pollen tube to reach the ovary is not a simple task, say the authors. “Pollen tubes are generally not able to reach the ovules without drawing resources from the style, and crowded pollen grains can compete for access to resources, while maternal plants may choose to provision some pollen tubes over others.”
Once an ovule is fertilized, the competition isn’t over. Plants can selectively abort ovules or entire fruits. Christopher and colleagues say there’s plenty of evidence that this is not a random occurrence.
Siring success is a combination of factors
The combination of pollination and post-pollination competition means that it can sometimes be hard to see what the effects of some factors are simply by looking at the end result. Instead, Christopher and colleagues argue that both aspects need to be considered. They conclude, “There are no studies that have varied the intensity of pollen delivery and have measured the variance in male success in both the pollination and post-pollination phases. Such studies would provide insight concerning the ecological conditions that favour selection on floral traits such as nectar production and flower size, and on physiological traits such as pollen tube growth rate.”