In many organisms females directly or indirectly select mates (or sperm) and potentially influence the fitness of their offspring. Mate choice and sexual selection in plants is more complex in some ways than in animals because plants are sessile organisms and often have to rely on external vectors, such as animals, for pollen transport. As such, there is only so much a plant can do to affect the timing of pollen arrival, or the size and diversity of deposited pollen. But can a plant control which pollen grains, of the hundreds that land on their stigmas, make it to the ovules?
Åsa Lankinen from the Swedish University of Agricultural Sciences, whose research focuses on mate choice and sexual selection in plants, teamed up with Josefin Madjidian from Lund University, Sweden, to see if they could determine whether a mechanism that plants may use to exert female choice was delayed stigma receptivity in the pistil (the female part of the flower). Delaying receptivity to pollen grains may affect aspects of pollen competition such as siring ability, paternal diversity, and offspring fitness. Their study can be found online in the July issue of the American Journal of Botany.
"Our study contributes to our general understanding of how and why plants choose their mates when pollen arrive repeatedly to the stigma over a period of time," said Lankinen.
Lankinen and Madjidian hand-pollinated flowers of an annual herb, Collinsia heterophylla (Plataginaceae), in a controlled greenhouse setting in a series of crosses that simulated delayed stigma receptivity.
The authors found that when stigmas were receptive, pollination arrival sequence was important: pollen from the donor that was applied first sired more offspring (up to 74%) than the second donor, even when there was only a few minutes of time lag between their depositions. This was also the case for pollinations that were done with a 24-hour lag between them. However, even in the latter scenario, a surprising proportion of the seeds were still sired by the second donor.
Interestingly, when a mixture of the two donors was applied once, all at the same time, simulating delayed stigma receptivity (because all the pollen start to grow toward the ovules at the same time), there was greater offspring diversity compared to two separate pollination events, one from each donor. In other words, the proportion of offspring sired by each donor was more even in the single pollination event of mixed pollen, rather than being dominated by a single donor.
Moreover, high paternal diversity was also associated with increased seed production, indicating a fitness advantage to delaying stigma receptivity.
"Our results indicate that in our study species the ability to enhance pollen competition between competing mates by delaying stigma receptivity is beneficial because it increases paternal diversity of the brood, which in turn is positively connected to number of seeds produced," Lankinen said. "Even though many studies have shown that enhanced pollen competition can be beneficial for the female reproductive function, the underlying reason for this association is not clear."
Given the benefit of delaying stigma receptivity, Lankinen notes that she and her colleagues "are puzzled by the fact that variation in timing of stigma receptivity is very large in wild populations," and concludes that it would be of interest to understand more about this trait under natural conditions.
Of additional interest is the potential occurrence of a sexual conflict over timing of stigma receptivity. "We have detected that some pollen donors can fertilize ovules ahead of others, presumably securing paternity by avoiding competition with later arriving pollen," Lankinen added. "Such early fertilization seems to be disadvantageous for the female function in terms of reduced seed set. Because our current study showed a benefit of delaying stigma receptivity, this further points to the presence of a sexual conflict."
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