Pollination success increases with plant diversity in high-Andean communities

Pollinator-mediated plant–plant interactions have traditionally been viewed within the competition paradigm. However, facilitation via pollinator sharing might be the rule rather than the exception in harsh environments. Moreover, plant diversity could be playing a key role in fostering pollinator-mediated facilitation. Yet, the facilitative effect of plant diversity on pollination remains poorly understood, especially under natural conditions. By examining a total of 9371 stigmas of 88 species from nine high-Andean communities in NW Patagonia, we explored the prevalent sign of the relation between conspecific pollen receipt and heterospecific pollen diversity, and assessed whether the incidence of different outcomes varies with altitude and whether pollen receipt relates to plant diversity. Conspecific pollen receipt increased with heterospecific pollen diversity on stigmas. In all communities, species showed either positive or neutral but never negative relations between the number of heterospecific pollen donor species and conspecific pollen receipt. The incidence of species showing positive relations increased with altitude. Finally, stigmas collected from communities with more co-flowering species had richer heterospecific pollen loads and higher abundance of conspecific pollen grains. Our findings suggest that plant diversity enhances pollination success in high-Andean plant communities. This study emphasizes the importance of plant diversity in fostering indirect plant–plant facilitative interactions in alpine environments, which could promote species coexistence and biodiversity maintenance.

More examples of breakdown the 1:1 partner specificity between figs and fig wasps

Background The obligate mutualism between fig trees (Ficus, Moraceae) and pollinating fig wasps (Agaonidae) is a model system for studying co-evolution due to its perceived extreme specificity, but recent studies have reported a number of examples of trees pollinated by more than one fig wasp or sharing pollinators with other trees. This will make the potential of pollen flow between species and hybridization more likely though only few fig hybrids in nature have been found. We reared pollinator fig wasps from figs of 13 Chinese fig tree species and established their identity using genetic methods in order to investigate the extent to which they were supporting more than one species of pollinator (co-pollinator). Results Our results showed (1) pollinator sharing was frequent among closely-related dioecious species (where pollinator offspring and seeds develop on different trees); (2) that where two pollinator species were developing in figs of one host species there was usually one fig wasp with prominent rate than the other. An exception was F. triloba, where its two pollinators were equally abundant; (3) the extent of co-pollinator within one fig species is related to the dispersal ability of them which is stronger in dioecious figs, especially in small species. Conclusions Our results gave more examples to the breakdown of extreme specificity, which suggest that host expansion events where pollinators reproduce in figs other than those of their usual hosts are not uncommon among fig wasps associated with dioecious hosts. Because closely related trees typically have closely related pollinators that have a very similar appearance, the extent of pollinator-sharing has probably been underestimated. Any pollinators that enter female figs carrying heterospecific pollen could potentially generate hybrid seed, and the extent of hybridization and its significance may also have been underestimated.

Pollen-Pistil Interactions as Reproductive Barriers

Pollen-pistil interactions serve as important prezygotic reproductive barriers that play a critical role in mate selection in plants. Here, we highlight recent progress toward understanding the molecular basis of pollen-pistil interactions as reproductive isolating barriers. These barriers can be active systems of pollen rejection, or they can result from a mismatch of required male and female factors. In some cases, the barriers are mechanistically linked to self-incompatibility systems, while others represent completely independent processes. Pollen-pistil reproductive barriers can act as soon as pollen is deposited on a stigma, where penetration of heterospecific pollen tubes is blocked by the stigma papillae. As pollen tubes extend, the female transmitting tissue can selectively limit growth by producing cell wall–modifying enzymes and cytotoxins that interact with the growing pollen tube. At ovules, differential pollen tube attraction and inhibition of sperm cell release can act as barriers to heterospecific pollen tubes.

Community level individual-based plant-pollinator networks unveil pollen flow dynamics and plant reproductive success

Ecological networks are a widely used tool to understand the dynamics of ecological communities in which plants interact with their pollinator counterparts. However, while most mutualistic networks have been defined at the species level, ecological processes, such as pollination, take place at the individual level. This recognition has led to the development of individual-based networks, yet current approaches only account for individuals of a single plant species due to conceptual and mathematical limitations. Here, we introduce a multilayer framework designed to depict the conspecific and heterospecific pollen flows mediated by floral visitors among plant individuals belonging to different species. Pollen transfer is modeled as a transport-like system, where an ensemble of conspecific plant-pollinator “circuits” are coupled through pollinators. With this physical conceptualization of ecological processes, we investigate how the reproductive success of plant individuals is affected by the overall dynamics of the whole multilayer network (macrostructure), as well as by their local position within the network (mesostructure). To illustrate this multiscale analysis, we apply it to a dataset of nine well-resolved individual plant-pollinator interaction networks from annual plant grasslands. Our results show that the resulting individual-based networks are highly modular, with insect visitors effectively connecting individuals of the same and different plant species. We also obtain empirical evidence that network structure is critical for modulating individual plant reproduction. In particular, the mesoscale level is the best descriptor of plant reproductive success, as it integrates the net effect of local heterospecific and conspecific interactions on seed production of a given individual. We provide a simple, but robust set of metrics to scale down network ecology to functioning properties at the individual level, where most ecological processes take place, hence moving forward the description and interpretation of multitrophic communities across scales.

The Extent of Pollinator Sharing Among Fig Trees in Southern China

Background: The obligate mutualism between fig trees (Ficus, Moraceae) and pollinating fig wasps (Agaonidae) is a model system for studying co-evolution due to its perceived extreme specificity, but recent studies have reported a number of examples of trees pollinated by more than one fig wasp or sharing pollinators with other trees. This makes pollen flow between species and hybridization more likely. We reared pollinator fig wasps from figs of 13 Chinese fig tree species trees and established their identity using genetic methods in order to investigate the extent to which are they were supporting more than one species of pollinator.Results: Our results showed 1) pollinator sharing was frequent among closely-related dioecious species (where pollinator offspring and seeds develop on different trees), but not monoecious species and 2) that where two pollinator species were developing in figs of one host species there was usually one fig wasp that was far rarer than the other. An exception was F. triloba, where its two pollinators were equally abundant. Conclusions: Our results suggest that host expansion events where pollinators reproduce in figs other than those of their usual hosts are not uncommon among fig wasps associated with dioecious hosts. Because closely related trees typically have closely related pollinators that have a very similar appearance, the extent of pollinator-sharing has probably been underestimated. Any pollinators that enter female figs carrying heterospecific pollen could potentially generate hybrid seed, and the extent of hybridization and its significance may also have been underestimated.

Asymmetrical reproductive barriers in sympatric jewelflowers: are floral isolation, genetic incompatibilities and floral trait displacement connected?

Floral visitors influence reproductive interactions among sympatric plant species, either by facilitating assortative mating and contributing to reproductive isolation, or by promoting heterospecific pollen transfer, potentially leading to reproductive interference or hybridization. We assessed preference and constancy of floral visitors on two co-occurring jewelflowers [Streptanthus breweri and Streptanthus hesperidis (Brassicaceae)] using field arrays, and quantified two floral rewards potentially important to foraging choice – pollen production and nectar sugar concentration – in a greenhouse common garden. Floral visitors made an abundance of conspecific transitions between S. breweri individuals, which thus experienced minimal opportunities for heterospecific pollen transfer from S. hesperidis. In contrast, behavioural isolation for S. hesperidis was essentially absent due to pollinator inconstancy. This pattern emerged across multiple biotic environments and was unrelated to local density dependence. S. breweri populations that were sympatric with S. hesperidis had higher nectar sugar concentrations than their sympatric congeners, as well as allopatric conspecifics. Previous work shows that S. breweri suffers a greater cost to hybridization than S. hesperidis, and here we find that it also shows asymmetrical floral isolation and floral trait displacement in sympatry. These findings suggest that trait divergence may reduce negative reproductive interactions between sympatric but genetically incompatible relatives.

Heterospecific pollination by an invasive congener threatens the native American bittersweet, Celastrus scandens

Invasive plants have the potential to interfere with native species’ reproductive success through a number of mechanisms, including heterospecific pollination and hybridization. This study investigated reproductive interactions between a native North American woody vine (American bittersweet,Celastrus scandens) and an introduced congener (oriental bittersweet,C.orbiculatus). The decline ofC.scandensin the eastern portion of its range is coincident with the introduction and spread ofC.orbiculatus, and the two species are known to hybridize. The relationship between proximity and floral production of conspecific and heterospecific males on fertilization and hybridization rates was measured at a field site in northwestern Indiana, USA where both species occur and reproduce. We found that the invasive vine had an extreme advantage in both male and female floral production, producing nearly 200 times more flowers per staminate plant and 65 times more flowers per pistillate plant than the native. Using nuclear microsatellite DNA markers we found that hybridization rates were asymmetric; 39% of theC.scandensseeds tested were hybrids, compared to only 1.6% ofC.orbiculatusseeds. The asymmetric hybridization rates were likely not solely due to greater abundance ofC.orbiculatuspollen because experimental hand crosses revealed thatC.scandenshad a higher rate (41%) of heterospecific fertilization thanC.orbiculatus(2.4%). We previously reported that few hybrids were observed in the wild, and hybrids had greatly reduced fecundity. Thus, in our system, the threat posed by heterospecific pollen is not replacement by hybrids or introgression, but rather asymmetric reproductive interference. Reproductive interference extended to distances as great as 100 meters, thus, efforts to conserve the native species must reduce its exposure toC.orbiculatusover a relatively large spatial scale.