Variation in the strength of reproductive interference from an alien congener to a native species in Taraxacum

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

Reproductive interference (RI), an interspecific mating interaction that reduces the fitness of at least one of the species involved, can lead to exclusive distributions in closely related species. A hypothesis previously proposed is that RI in plants may occur by ovule usurpation, in which pistils lack interspecific incompatibility and mistakenly accept heterospecific pollen, thereby losing an opportunity for conspecific pollen fertilization. However, few comparative studies have evaluated the consistency of the inferred mechanism within and among individuals and populations. We conducted hand-pollination experiments in six populations of three native Taraxacum species that suffered from different levels of RI from an alien congener, T. officinale, and compared pollen-pistil interactions among populations. We also investigated the interactions for eight individual T. japonicum plants whose response to heterospecific pollen deposition had been previously measured. Our results revealed that pollen tubes often penetrated native ovaries following heterospecific pollination in populations suffering from strong RI, whereas they seldom did in populations suffering from marginal RI. However, the relative frequency of the pollen tube penetration was not significantly related to the strength of alien RI. Not all pistils on an individual plant showed the same pollen receptivity following heterospecific pollination; rather, some accepted and some refused the pollen tubes. The relationship between pollen tube penetration following heterospecific pollination and the strength of the alien RI was also not significant among individuals. Our present results generally support the ovule usurpation hypothesis, but suggest that other factors, such as competition for pollinator services, variation in the effects of heterospecific pollen donors, and condition of the native inflorescences, might also affect the observed RI strength.

Hidden invasion and niche contraction revealed by herbaria specimens in the fungal complex causing oak powdery mildew in Europe

Deciphering the dynamics involved in past microbial invasions has proven difficult due to the inconspicuous nature of microbes and their still poorly known diversity and biogeography. Here we focus on powdery mildew, a common disease of oaks which emerged in Europe at the beginning of the twentieth century and for which three closely related Erysiphe species are mainly involved. The study of herbaria samples combined with an experimental approach of interactions between Erysiphe species led us to revisit the history of this multiple invasion. Contrary to what was previously thought, herbaria sample analyses very strongly suggested that the currently dominant species, E. alphitoides, was not the species which caused the first outbreaks and was described as a new species at that time. Instead, E. quercicola was shown to be present since the early dates of disease reports and to be widespread all over Europe in the beginning of the twentieth century. E. alphitoides spread and became progressively dominant during the second half of the twentieth century while E. quercicola was constrained to the southern part of its initial range, corresponding to its current distribution. A competition experiment provided a potential explanation of this over-invasion by demonstrating that E. alphitoides had a slight advantage over E. quercicola by its ability to infect leaves during a longer period during shoot development. Our study is exemplary of invasions with complexes of functionally similar species, emphasizing that subtle differences in the biology of the species, rather than strong competitive effects may explain patterns of over-invasion and niche contraction.

Comparison of mechanisms of reproductive interference in Taraxacum

BACKGROUND AND AIMS

Reproductive interference may reduce fitness of either of the involved species, with potentially important ecological and evolutionary consequences. Except for the effect of shared pollinators on reproductive success, however, mechanisms underlying reproductive interference have been little studied, even though the severity of its impact may depend on the specific mechanism. The aim of this study was therefore to explore the mechanisms of reproductive interference between Taraxacum japonicum (native to Japan) and Taraxacum officinale (alien).

METHODS

In a field survey, the association between alien species density and seed set in T. japonicum, and whether pollinator behaviour indicated a preference for the alien, were examined. Effects of heterospecific pollen deposition were measured in a series of hand pollination experiments, including mixed pollination experiments in which the order of application of conspecific and heterospecific pollen was varied. Finally, to investigate hybridization frequency, the parentage of seedlings produced following natural, mixed or heterospecific pollination was compared.

KEY RESULTS

Alien species density did not negatively affect native seed set, nor did pollinators appear to have a preference for alien flowers. The hand pollination experiments showed that heterospecific pollen deposition adversely affected native seed set, especially when alien pollen was applied before conspecific pollen. No viable hybrids were found following natural pollination, which suggests that hybridization might be a rare event.

CONCLUSION

Among the examined mechanisms, heterospecific pollen deposition might have the largest deleterious effect on the native species. This effect is frequency dependent; thus, a positive feedback loop may cause the effect on the population dynamics to increase over time, with the result that the alien might eventually displace the native in a population. Effects of the examined mechanisms on population dynamics should be investigated further to improve understanding of the impact of reproductive interference on the structure of plant communities.