Comparisons among populations and individuals to evaluate pollen-pistil interaction as a mechanism of reproductive interference in Taraxacum

Pollinator peaking earlier than flowering is more detrimental to plant fecundity

Climate change has caused asynchronous phenological shifts between most plants and their pollinators, resulting in an earlier or later appearance of peak flowering relative to peak pollinator abundance. The fitness impact of these two mismatch patterns may not be simply equivalent, but the information has so far been limited. To explore how differently plant fitness responds to the distinct mismatch patterns, we conducted a seed-setting comparative study at the individual level in an alpine grassland community in the Qilian Mountains of China. By monitoring flowering abundance and insect visits, we measured the phenological matching relationship between plants and their key pollinators, and evaluated the impact of mismatches on plant productivity. We found that the pattern of "pollinator peaks earlier" accounted for a relatively high proportion in the natural community, with a significantly stronger fitness impact on plants than that of the "flower peaks earlier" pattern. The asymmetry in the fitness impacts between phenological mismatch patterns is related to the length of flowering period. Specially, the shorter the flowering duration, the greater the difference in influence between the two patterns. Our results suggest that plants with shorter flowering periods may be confronted with more severe pollination limitations if climate warming cause insects to forage further ahead. Therefore, the asymmetric effects of phenological mismatch patterns should be considered in phenological models to improve the predictive performance of plant responses to climate change.

The cost of self-promotion: ecological and demographic implications of the mentor effect in natural plant populations

Under the mentor effect, compatible heterospecific pollen transfer induces self-pollen germination in otherwise self-incompatible plants. The mentor effect could be considered a novel mode of reproductive interference if it negatively impacts fitness. Yet to date, this phenomenon has predominately been investigated under experimental conditions rather than in situ. We address this gap in natural populations of the self-incompatible native dandelion, Taraxacum ceratophorum, where selfing only occurs in association with hybridization from exotic Taraxacum officinale. We tested whether self-fertilization rate increases in the hybrid zone, as predicted due to the mentor effect. Using results from these investigations, we created an exponential growth model to estimate the potential demographic impacts of the mentor effect on T. ceratophorum population growth. Our results demonstrate that the strength of the mentor effect in Taraxacum depends on the prevalence of pollinator-mediated outcross pollen deposition rather than self-pollination. Demographic models suggest that reduced outcrossing in T. ceratophorum under exotic invasion could negatively impact population growth through inbreeding depression. We demonstrate the mentor effect is rare in natural populations of T. ceratophorum due to masking by early life cycle inbreeding depression, prevalent outcrossing, and ovule usurpation by heterospecific pollen.

Conspecific pollen advantage mediated by the extragynoecial compitum and its potential to resist interspecific reproductive interference between two Sagittaria species

The extragynoecial compitum formed by the incomplete fusion of carpel margins, while allowing intercarpellary growth of pollen tubes in apocarpous angiosperms, may also increase the risk of reproductive interference caused by heterospecific pollen (HP) deposition. In Sagittaria, congeneric HP tubes grow via different paths and enter the ovules later than conspecific pollen (CP) tubes. However, it is unclear how the growth advantage of the CP tube helps ensure reproductive success when HP is deposited on the stigmas. We performed molecular characterization of interspecies-pollinated seeds to examine the consequences of interspecific pollen deposition between Sagittaria pygmaea and S. trifolia. We also conducted CP-HP (1:1) mixed pollination and delayed CP pollination treatments to explore the seed-siring abilities of CP and HP. Our results showed that although HP could trigger the development of fruits, the interspecies-pollinated seeds contained partially developed embryos and could not germinate. More than 70% of the embryos in these seeds were molecularly identified as hybrids of both species, suggesting that HP tubes could enter the ovules and fertilize the egg cells. Moreover, CP could sire more offspring (≥70%) after the CP-HP (1:1) mixed pollination treatment, even when HP reached the stigma 0.5-1 h earlier than CP (≥50%). Following adequate CP vs. HP (1:1) pollination on carpels on two sides of the apocarpous gynoecium, both species produced > 70% conspecific seeds, indicating that the CP tubes could occupy ovules that should be occupied by HP via the extragynoecial compitum. Our results reveal that in Sagittaria, pollen deposition from co-existing congeneric heterospecies leads to interspecific seed discounting. However, the CP advantage mediated by the extragynoecial compitum is an effective strategy to mitigate the effects of interspecific pollen deposition. This study improves our understanding of how apocarpous angiosperms with an extragynoecial compitum can maintain species stability and mitigate the negative reproductive interference effect from sympatrically distributed related species.