Outcross Pollen Research Articles

Relationships between population density, fine-scale genetic structure, mating system and pollen dispersal in a timber tree from African rainforests.

Owing to the reduction of population density and/or the environmental changes it induces, selective logging could affect the demography, reproductive biology and evolutionary potential of forest trees. This is particularly relevant in tropical forests where natural population densities can be low and isolated trees may be subject to outcross pollen limitation and/or produce low-quality selfed seeds that exhibit inbreeding depression. Comparing reproductive biology processes and genetic diversity of populations at different densities can provide indirect evidence of the potential impacts of logging. Here, we analysed patterns of genetic diversity, mating system and gene flow in three Central African populations of the self-compatible legume timber species Erythrophleum suaveolens with contrasting densities (0.11, 0.68 and 1.72 adults per ha). The comparison of inbreeding levels among cohorts suggests that selfing is detrimental as inbred individuals are eliminated between seedling and adult stages. Levels of genetic diversity, selfing rates (∼16%) and patterns of spatial genetic structure (Sp ∼0.006) were similar in all three populations. However, the extent of gene dispersal differed markedly among populations: the average distance of pollen dispersal increased with decreasing density (from 200 m in the high-density population to 1000 m in the low-density one). Overall, our results suggest that the reproductive biology and genetic diversity of the species are not affected by current logging practices. However, further investigations need to be conducted in low-density populations to evaluate (1) whether pollen limitation may reduce seed production and (2) the regeneration potential of the species.

The influence of herbivory on pre‐ and postzygotic stages of reproduction following open, self, and outcross pollination

Herbivory affects pollination success and reproductive output in plants. However, the different stages in the process from pollination to seed maturation have hardly been investigated within the context of herbivory. Herbivory might affect these stages via its effect on geitonogamous pollination and thereby the proportion of self pollen delivered to the stigma and/or via its effect on the nutritional capacity of the maternal plant. Plants of monocarpic Cynoglossum officinale were experimentally subjected to root herbivory and exposed to natural open pollination in combination with self and outcross hand pollination. We quantified pollen germination, pollen tube competition intensity, pollen tube attrition, fruit set, and seed initiation, abortion, and maturation. Although root herbivory did not affect pollen germination or pollen tube attrition, fruit set and seed initiation and maturation were negatively affected by herbivory, but for seed initiation only in the case of outcross- and open-pollinated flowers. The intensity of pollen tube competition positively affected seed initiation, but only in plants infested with the herbivore. Our study demonstrates that herbivory did not affect the early stages following pollination, but significantly impacted later postpollination stages such as fruit set and seed maturation and selection based on pollen tube competition intensity on zygote development. Our findings suggest that decreased nutritional capacity of the mother plant in response to root herbivory rather than herbivory effects on pollen quality was responsible for the lower fruit and seed production in infested plants.

Mitigation of pollen limitation in the lowbush blueberry agroecosystem: effect of augmenting natural pollinators

Growers of small fruit crops often supplement the natural pollinator community by introducing pollinators into commercial orchards and fields, but there are relatively few studies that test the extent to which such interventions increase fruit yield. To test whether plants are limited by pollen availability, inflorescences in 78 commercial lowbush blueberry fields during three years were hand‐pollinated either with supplemental outcross pollen, or marked and left as controls (open‐pollination). Maximum fruit set with supplemental pollination was in the range 50–65%, whereas with open‐pollination, it was slightly, but significantly lower, in the range of 47–57%, suggesting that pollen limitation can affect fruit set. In a two‐year experiment, we augmented native pollinators with introduced honey bees, bumble bees, and leaf cutter bees and all combinations in 48 fields. Stigmatic pollen loads were influenced by the total numbers of managed bees in a field in 2010 but not 2011. The presence of leaf cutter bees had a small but positive effect on seed set in 2010, and honey bees had a small but negative influence on seed set in 2011. There was a strong correlation between fruit set reproductive output of supplementally pollinated and open‐pollinated plants, suggesting that plant health or plant resources influence reproductive success. Temperature variation among regional groupings of fields was minimal and is unlikely to have accounted for differences among fields in fruit set. We propose several reasons why, despite pollen‐limitation of fruit and seed set of blueberry plants, augmentation of the pollinator community of lowbush blueberry did not significantly increase reproduction. These include pollinator‐mediated transfer of self‐pollen followed by subsequent fruit abortion due to inbreeding depression and resource limitation for fruit maturation. Management practices that focus on increasing outcross pollen receipt, or plant resources for fruit set may have greater overall benefit than pollinator augmentation alone.

Better common than rare? Effects of low reproductive success, scarce pollinator visits and interspecific gene flow in threatened and common species of Tibouchina (Melastomataceae)

AbstractThe reasons for plant rarity have been the focus of many studies, especially because rare species are more prone to extinction than common species. Under the same habitat conditions, rare plants are expected to attract fewer flower visitors and to show some limitation in their reproductive success. Here, using one of the most emblematic Neotropical plant genus (Tibouchina) we tested whether narrow endemic and threatened species in Ecuador have a lower reproductive success or are visited by fewer pollinators than common species, in 13 populations monitored from 2011 to 2013. We also assessed whether interspecific gene flow could be considered a threat to the rare species. However, contrary to expectations, we found that few pollinators visited the flowers, independently of species rarity. Natural outcross pollinations were always very low in all small‐size populations, leading to high levels of pollen limitation. Interspecific crossing experiments also revealed weak reproductive barriers in some species. This study reveals that both narrow and common species of Tibouchina have similar reproductive and pollinator patterns in Ecuador and, therefore, other causes of the rarity of these species should be considered.

Postpollination discrimination between self and outcross pollen covaries with the mating system of a self-compatible flowering plant.

Variation in the mating system of hermaphroditic plant populations is determined by interactions between genetic and environmental factors operating via both pre- and postmating processes. Models predicting the maintenance of intermediate outcrossing rates in animal-pollinated plants often assume that the mating system is primarily controlled by floral morphology and pollinator availability, but rarely has the influence of postpollination processes on variation in outcrossing been examined. We investigated the influence of stylar discrimination between illegitimate and legitimate pollen-tube growth and the pollen-load capacity of stigmas on mating-system variation in the annual, tristylous species Eichhornia paniculata using controlled crosses and genetic markers. This species exhibits an exceptionally broad range of outcrossing rates in natural populations. There was significant variation among populations in the pollen-load capacity of stigmas and the ability of styles to discriminate between illegitimate vs. legitimate pollen. There was strong correspondence between stylar-discrimination ability and variation in outcrossing rate among populations and style morphs. The combination of stigmatic pollen-load capacity and stylar discrimination explained more than 80% of the variation in outcrossing rates among populations. The finding that stigmatic pollen-load capacity and stylar-discrimination ability contributed significantly to explaining the wide range of outcrossing rates in E. paniculata suggests that postpollination mechanisms play an important role in governing mating patterns in this species. The difference in levels of stylar discrimination between outcrossing and selfing populations may reflect a trade-off between selection for increased outcrossing and greater reproductive assurance.

Reproductive Assurance through Autogamous Self-pollination across Diverse Sexual and Breeding Systems

Pollination becomes a constraint when conspecific plants and/or their pollinators become scarce. Many plant species have evolved autogamous self-pollination as a means of reproductive assurance (RA) under pollination-uncertain environments. So far RA has been studied and discussed largely with reference to self-compatible species producing bisexual flowers. RA seems to have evolved across all other sexual and breeding systems - monoecy, dioecy and self-incompatibility (SI). Both monoecy and dioecy produce bisexual flowers (andro/gyno-monoecy, andro/gynodioecy and polygamous conditions) which may provide RA. Similarly, most of the SI species are leaky and do set some seeds upon self-pollination. This phenomenon termed 'partial self-compatibility' is quite common and does provide RA in SI species. Although dioecy and SI have evolved as obligate outbreeding systems, they seem to have reached an evolutionary dead end because of the constraints for outcross pollination. In the light of habitat destruction leading to a reduction in the diversity and density of native pollinators, it is likely that many of the obligate outbreeders tend to shift to mixed mating system in the coming decades. Similarly, obligate mutualism in which each plant species is dependent on one animal species for pollination also seems to have reached a dead end and the trend is to abandon such obligate mutualism as a survival strategy. In the absence of such a change, obligate out breeders and those with highly specialized pollination system are likely to become endangered or even extinct.

Nectarless flowers with deep corolla tubes inPedicularis: does long pistil length provide an arena for male competition?

Pollinator-mediated selection does not seem to have a direct influence on the evolution of a long corolla tube in a nectarless flower. We hypothesized that the long pistil length of the nectarless flower with a deep corolla tube provided an opportunity for male competition. Pedicularis siphonantha, a nectarless and partially self-incompatible lousewort with substantial variation in corolla tube length, was used to test the hypothesis. We tested whether and how corolla tube length affected seed production per capsule and seed germination rate with different pollination treatments. Flowers were hand-pollinated with pollen from one self donor and one outcross donor and mixed pollen grains consisting of equal amounts from the two donor types, respectively. Additionally, seeds from open-pollinated flowers with different corolla tube lengths were collected separately for measurement of germination rate. Pollination treatment and corolla tube length significantly affected number of seeds per capsule. Moreover, a significant positive relationship between seeds per capsule and corolla tube length was found when mixed hand pollination was conducted. Seeds of self hand-pollinated flowers had a lower germination rate than those from outcross-pollinated flowers. Under open pollination, seeds from flowers with longer corolla tubes tended to have higher germination rate. In P. siphonantha, outcross pollen may have a higher probability of contributing to the next generation when transferred to flowers with longer corolla tubes. The pistil length, therefore, should be seen as a female choice mechanism, which provides an arena for male-to-male competition. © 2015 The Linnean Society of London, Botanical Journal of the Linnean Society, 2015, 179, 526–532.

Population genetics of self-incompatibility in a clade of relict cliff-dwelling plant species

The mating systems of species in small or fragmented populations impact upon their persistence. Small self-incompatible (SI) populations risk losing S allele diversity, responsible for the SI response, by drift thereby limiting mate availability and leading to population decline or SI system breakdown. But populations of relict and/or endemic species have resisted these demographic conditions over long periods suggesting their mating systems have adapted. To address a lack of empirical data on this topic, we studied the SI systems of three relict cliff-dwelling species of Sonchus section Pustulati (Asteraceae): S. masguindalii, S. fragilis and S. pustulatus in the western Mediterranean region. We performed controlled pollinations within and between individuals to measure index of SI (ISI) expression and identify S alleles in multiple population samples. Sonchus masguindalii and S. pustulatus showed strong SI (ISI = 0.6-1.0) compared to S. fragilis (ISI = 0.1-0.7). Just five S alleles were estimated for Spanish S. pustulatus and a moderate 11-15 S alleles for Moroccan S. pustulatus and S. fragilis, respectively. The fact that autonomous fruit set was generally improved by active self-pollination in self-compatible S. fragilis suggests that individuals with weak SI can show a wide range of outcrossing levels dependent on the degree of self or outcross pollen that pollinators bear. We conclude that frequent S allele dominance interactions that mask the incompatibility interactions of recessive S alleles leading to higher mate availability and partial breakdown of SI leading to mixed mating, both contribute to reproductive resilience in this group.

Differential selection on pollen and pistil traits in relation to pollen competition in the context of a sexual conflict over timing of stigma receptivity.

Sexual conflict and its evolutionary consequences are understudied in plants, but the theory of sexual conflict may help explain how selection generates and maintains variability. Here, we investigated selection on pollen and pistil traits when pollen arrives sequentially to partially receptive pistils in relation to pollen competition and a sexual conflict over timing of stigma receptivity in the mixed-mating annual Collinsia heterophylla (Plantaginaceae). In this species the conflict is generated by early fertilizing pollen that reduces seed production, which is counteracted by delaying receptivity in the recipient. We performed sequential two-donor pollinations at early floral developmental stages involving two pollen deposition schedules (with or without a time lag of 1 day), using only outcross or self and outcross pollen. We investigated pollen and pistil traits in relation to siring success (male fitness) and seed production (female fitness). In contrast to previous findings in receptive pistils in C. heterophylla and in other species, last arriving pollen donors showed highest siring success in partially receptive pistils. The last male advantage was weaker when self pollen was the first arriving donor. Two measures of germination rate (early and late) and pollen tube growth rate of first arriving donors were important for siring success in crosses with a time lag, while only late germination rate had an effect in contemporary crosses. Curiously, late stigma receptivity was negatively related to seed production in our contemporary crosses, which was opposite to expectation. Our results in combination with previous studies suggest that pollen and pistil traits in C. heterophylla are differentially advantageous depending on stage of floral development and varying pollen deposition schedules. Variation in success of these traits over floral development time may result from sexually antagonistic selection.

Pollen limitation and effects of local patch density on reproductive success in the alpine herb Inula royleana (Asteraceae)

In alpine species depending on insects for pollination, the decrease of pollinator abundance and activity, due to more hostile weather and climatic conditions, could limit reproductive success because of failures in pollination. We examined the breeding system and pollen limitation (PL) using self- and cross-pollinations and pollen supplementations in Inula royleana in alpine Kashmir Himalaya. In addition, we also investigated how natural variation in local patch density and floral neighborhood affect species reproductive success. Supplemental hand-pollinations with outcross pollen resulted in 19 % increase in seed output above natural levels, thus demonstrating PL in this species. The breeding system in I. royleana was self-incompatible as caged flower heads, and the selfed?? capitula produced no or very few seeds compared to seed set in open-pollinated capitula. Consistent with the conspecific density in flowering plants, percent seed set differed significantly among patches. Seed set increased with local density of conspecifics, indicating that the reduced plant density in sparse patches may reduce plant reproductive success. Further in patches of low density, number of flowering conspecifics in the immediate vicinity of a focal plant (positive relationship) and distance to the nearest flowering conspecific plant (negative relationship) was found linearly related with greater PL resulting in reduced seed set and fecundity in isolated plants.

Pollen Dispersal and Pollination Patterns Studies in Pati Kopyor Coconut using Molecular Markers

Parentage analysis has been used to evaluate pollen dispersal in Kopyor coconut (Cocos nucifera L.). Investigations were undertaken to elucidate (i) the dispersal of pollen, (ii) the rate of self and out-crossing pollination, and (iii) the distance of pollen travel in Pati kopyor coconut population. The finding of this activities should be beneficial to kopyor coconut farmers to increase their kopyor fruit harvest and to support breeding of this unique coconut mutant. As many as 84 progenies were harvested from 15 female parents. As many as 95 adults coconut provenances surrounding the female parents were analyses as the potential male parents for the progenies. The adult coconut palms were mapped according to their GPS position. All samples were genotyped using six SSR and four SNAP marker loci. Parentage analysis was done using CERVUS version 2.0 software. Results of the analysis indicated that evaluated markers were effective for assigning candidate male parents to all evaluated seedlings. There is no specific direction of donated pollen movement from assigned donor parents to the female ones. The donated pollens could come from assigned male parents in any directions relative to the female parent positions. Cross pollination occured in as many as 82.1% of the progenies analyzed. Outcrossing among tall by tall (TxT), dwarf by dwarf (DxD), hybrid by hybrid (HxH), TxD, DxT, TxH, DxH, and HxD were observed. Self-pollination (TxT and DxD) occurred in as many as 17.9% of the progenies. The dwarf coconut was not always self pollinated. The presence of DxD, TxD, and HxD outcrossing was also observed. The donated pollens could come from pollen donor in a range of at least 0-58 m apart from the evaluated female recipients. Therefore, in addition to the wind, insect pollinators may have played an important role in Kopyor coconut pollination.

High‐elevation range limit of an annual herb is neither caused nor reinforced by declining pollinator service

SummaryPollination failure has been proposed to be an important determinant of plant species’ range limits if pollinator activity declines along an environmental gradient, directly limiting plant populations, or if plant populations decline along an environmental gradient and subsequently fail to attract sufficient visitation. Both mechanisms predict reduced pollinator visitation, increased pollen limitation and decreased seed production towards range limits, and the first additionally predicts declining pollinator abundance independent of any particular plant species. However, many self‐compatible species have some capacity for autonomous self‐fertilization, which may buffer reproductive success from declining pollinator visitation if inbreeding depression is mild. Thus pollinator‐mediated limits may also predict selection for reduced reliance on pollinators towards range limits.We tested these predictions towards the high‐elevation limit of the self‐compatible, bumblebee (Bombus)‐pollinatedRhinanthus minor,along two elevation transects in the Rocky Mountains of Alberta, Canada.Bombusabundance was highest at mid‐ (range‐centre) and high‐elevation (range limit) sites, so declining pollinator abundance is unlikely to impose high‐elevation limits for bumblebee‐pollinated species in this area.Flowers per plant and per m2declined at upper range limits, potentially rendering edge populations less attractive. However, visitation rate did not decline towards the range limit at either transect. Stigmatic pollen receipt declined with increasing elevation, but seed set did not, nor did outcross pollen supplementation increase seed set at any site.Investment in floral attractiveness (corolla area/ovary area) increased towards range limits, but capacity for high‐quality autonomous seed set and adult inbreeding coefficients inferred from genetic markers were uniformly high, suggesting frequent self‐fertilization and weak inbreeding depression throughout the range.Synthesis. We found no evidence for pollination failure towards the upper range limit ofR. minor. Moreover, unlike some species with a capacity for autogamy, autonomous selfing makes a major contribution toR. minor's mating system and demography, and likely buffers reproductive success from stochasticity in pollination. Continued investment in floral attractiveness despite high autonomous selfing suggests some evolutionary benefit to pollinator‐mediated outcrossing, rather than ecological benefits via increasing seed quantity or quality. Given that >50% of angiosperms are self‐compatible, the reproductive assurance provided by selfing may reduce the importance of pollination in limiting plant distributions compared to other biotic interactions.

Flower colour polymorphism in Lysimachia arvensis: How is the red morph maintained in Mediterranean environments?

The maintenance of floral polymorphisms within natural populations is enigmatic, because directional selection imposed by biotic or abiotic selective agents combined with random genetic drift should lead to the rapid loss of floral variation. Lysimachia arvensis is a polymorphic species with red and blue flowers, red colour being genetically dominant. Both morphs show differences in fitness due to climatic factors, and the blue performs better in xeric Mediterranean environments where red-flowered plants persist in low proportion. Although the role of abiotic factors as selective agent has been studied, the selection mediated by pollinators has still not been considered. Experimental manipulations of morph frequencies were made to measure the effect of flower colour on pollinator visitation. Moreover, selection analyses on both male and female fitness were used in three populations with different morph proportions. Blue flowers are perceived by pollinators as UV-blue while red flowers are perceived as UV, but the contrast between blue flowers and the background is stronger than that of red flowers. Solitary bees were the main pollinators and exerted phenotypic selection on flower colour (quantified by bee vision) but not on flower size. Blue flowers were visited at a higher rate than expected, based on their frequency in populations. In the three populations studied, male success was markedly higher in the blue morph but colour had no consistent effect on female success. We found strong directional phenotypic selection for blue flowers through male function in the two blue-biased populations, and through female function only in the most blue-biased population. Therefore, in Mediterranean areas, not only is the blue morph favoured by climatic factors, but it is also selected by pollinators. Thus, selection by pollinators or climatic factors does not explain the maintenance of red flowered plants in Mediterranean areas. The delayed selfing mechanism of L. arvensis could offer reproductive assurance when outcross pollen is limited, mitigating the effects of both biotic and abiotic selective agents and protecting the red morph.

Relationship of stigma behaviors and breeding system in three Mazus (Phrymaceae) species with bilobed stigma

AbstractSensitive stigma has been recognized to facilitate outcrossing. We hypothesized that species with different levels of sensitivity might have corresponding differences in components of their breeding system. In this study, three Mazus species with bilobed stigmas were used to test the hypothesis. We explored stigma behaviors of the species in reaction time, recovery time, permanent closing time, and the minimum pollen load causing permanent closure. We investigated floral traits, pollinator type and behavior, pollination intensity, and natural schedule of pollen deposition on stigma. Moreover, we evaluated the mating system of the species by checking seed set after controlled pollination treatments, namely, natural flowers with open pollination, enclosed flowers without pollination, and enclosed flowers with self and outcross hand pollination. Results indicated that stigma of M. pumilus (N. L. Burman) Steenis was not sensitive, whereas stigmas of M. miquelii Makino and M. stachydifolius (Turcz.) Maxim. closed and reopened quickly in response to pollination. Accordingly, hand pollination treatments revealed that seed set of self‐spontaneous pollination in M. pumilus was similar to the other treatments. For M. miquelii, outcross pollen resulted in significantly higher seed set than self‐pollen. Mazus stachydifolius was self‐incompatible. Additionally, the corresponding characteristics in other components of the breeding system for each species were found. Our study indicated that the sensitivity of bilobed stigma might be linked with floral traits and the mating system in a given species. Sensitive stigma should be regarded as an evolutionary mechanism for enhancement of outcrossing.

Pollen limitation and the contribution of autonomous selfing to fruit and seed set in a rewarding orchid

• Although rewarding orchids are believed to have a high pollination efficiency, pollination success is often low, suggesting that rewarding orchids may be prone to pollen limitation. Assuming that selfing contributes significantly to fruit and seed set (i.e., reproductive assurance) and that the quality of selfed seeds is high (i.e., low inbreeding depression), it can be hypothesized that under these circumstances populations evolve to facultative or even complete self-pollination.• In this study, we performed emasculation and pollen supplementation experiments in the field to assess the extent of outcross pollen limitation and the contribution of autonomous self-pollination to fruit and seed set in the rewarding orchid Epipactis palustris. Hand-pollinations using cross and self-pollen were performed in the greenhouse to investigate the impact of selfing on seed set and seed quality and to assess the magnitude of inbreeding depression.• Fruit set under natural conditions was high: ca. 70% of all flowers set fruit. Percentage fruit set of emasculated plants was 56%, implying that fruit set in about 14% of all flowers was the result of autonomous self-pollination. Pollen supplementation significantly increased fruit and seed set, indicating strong outcross pollen limitation. Hand-pollination with cross pollen resulted in significantly higher seed set and seed quality compared with manual self-pollination, indicating high levels of early inbreeding depression (δ = 0.46 ± 0.08).• Overall, these results indicate that, despite the high pollination efficiency, populations of this rewarding orchid species were strongly pollen limited. Costs of early inbreeding depression, on the other hand, were high, which may prevent rapid evolution of complete selfing.

Intrinsic and extrinsic factors acting on the reproductive process in alpine‐snowbed plants: roles of phenology, biological interaction, and breeding system

AbstractPollinator activity and competition for pollinators lead to quantitative and qualitative pollen limitations on seed production and affect the reproductive success of plant species, depending on their breeding system (e.g., self‐compatibility and heterospecific compatibility) and genetic load (e.g., inbreeding depression and hybrid inviability). In alpine ecosystems, snowmelt regimes determine the distribution and phenology of plant communities. Plant species growing widely along a snowmelt gradient often grow with different species among local populations. Their pollinators also vary in their abundance, activity, and behavior during the season. These variations may modify plant–pollinator and plant–plant interactions. We integrated a series of our studies on the alpine dwarf shrub, Phyllodoce aleutica (Ericaceae), to elucidate the full set of intrinsic (species‐specific breeding system) and extrinsic factors (snow condition, pollinator activity, and interspecific competition) acting on their reproductive process. Seasonality of pollinator activity led to quantitative pollen limitation in the early‐blooming populations, whereas in the late‐blooming populations, high pollinator activity ensured pollination service, but interspecific competition for pollinators led to qualitative and quantitative pollen limitation in less competitive species. However, negative effects of illegitimate pollen receipt on seed‐set success might be reduced when cryptic incompatibility systems (i.e., outcross pollen grains took priority over self‐ and heterospecific pollen grains) could effectively prevent ovule and seed discounting. Our studies highlight the importance of species‐specific responses of plant reproduction to changing pollinator availability along environmental gradients to understand the general features of pollination networks in alpine ecosystems.

Heterospecific pollen receipt affects self pollen more than outcross pollen: implications for mixed‐mating plants

Flowering plants often occur in mixed‐species groups where interactions between them can occur both pre‐ and post‐pollination. In post‐pollination interactions, conspecific (CP) and heterospecific pollen (HP) interact on the style and previous work indicates that these interactions, often, but not always, lead to reduced fitness. However, over half of insect‐pollinated species display a mixed mating system, where both self and outcross CP could interact with HP, yet no study has evaluated the effects on both types of CP. We present and test hypotheses for the differential effects of HP depending on CP source and timing of HP arrival with a mixed‐mating plant, Mimulus guttatus, and one of its HP donors (Helianthus exilis). We found H. exilis pollen reduced tube growth and fertilization success of M. guttatus self pollen to a greater degree than outcross pollen, but only when CP and HP were applied simultaneously. Our results indicate that for mixed‐mating plants the fitness consequences of HP receipt can be more detrimental than previously thought. Furthermore, our findings suggest that HP receipt, which can be high in biodiverse areas, has the potential to influence mating system and genetic diversity of the plant populations within them.

Does sexual reproduction matter for a rare clonal species in frequently disturbed habitats?1

Sexual reproduction is often considered essential for evolutionary change, with many plants possessing mechanisms to promote outcrossing and produce genetically variable offspring. Many plant species, however, are also capable of asexual reproduction, but its relative role in population dynamics is often overlooked. Given that sexual reproduction depends in part on mate availability and therefore plant density, would asexual reproduction predominate in rare species, especially those found in highly disturbed habitats? To explore this question, we used field and genetic techniques to investigate the reproductive system of a rare, clonal shrub, Spiraea virginiana Britton, which inhabits the frequently disturbed riparian zone. We performed controlled hand pollinations within and among individuals both within and across populations. We also quantified allelic richness and heterozygosity at a locus purported to control self-incompatibility (SI). Hand pollinations resulted in low total fruit production but more infructescences were produced with outcross pollen (20%) than with self-pollen (2.3%). Low allelic richness (2–7 alleles/population) was found at the putative SI-locus; variation at this locus did not correspond directly with the pollination results, suggesting either the SI system is leaky or that the examined SI-locus is not primarily responsible for incompatibility in this species. This study shows that rarity combined with clonality within a highly disturbed habitat may promote a combination of asexual and sexual reproduction. Although sexual reproduction can occur, it may be limited by a scarcity of available mates for S. virginiana, thereby resulting in primarily asexual reproduction in the species.

Floral display size, conspecific density and florivory affect fruit set in natural populations of Phlox hirsuta, an endangered species.

Natural variation in fruit and seed set may be explained by factors that affect the composition of pollen grains on stigmas. Self-incompatible species require compatible outcross pollen grains to produce seeds. The siring success of outcross pollen grains, however, can be hindered if self (or other incompatible) pollen grains co-occur on stigmas. This study identifies factors that determine fruit set in Phlox hirsuta, a self-sterile endangered species that is prone to self-pollination, and its associated fitness costs. Multiple linear regressions were used to identify factors that explain variation in percentage fruit set within three of the five known populations of this endangered species. Florivorous beetle density, petal colour, floral display size, local conspecific density and pre-dispersal seed predation were quantified and their effects on the ability of flowers to produce fruits were assessed. In all three populations, percentage fruit set decreased as florivorous beetle density increased and as floral display size increased. The effect of floral display size on fruit set, however, often depended on the density of nearby conspecific plants. High local conspecific densities offset - even reversed - the negative effects of floral display size on percentage fruit set. Seed predation by mammals decreased fruit set in one population. The results indicate that seed production in P. hirsuta can be maximized by selectively augmenting populations in areas containing isolated large plants, by reducing the population sizes of florivorous beetles and by excluding mammals that consume unripe fruits.

Costs and benefits of the mixed-mating system ofNarcissus serotinus(Amaryllidaceae) in the conservation of small fragmented populations

Small fragmented populations often exhibit reduced plant–pollinator interactions and scarce outcrossing opportunities. In this context, mixed-mating systems can be advantageous since selfing can provide reproductive assurance, but they may also carry relevant costs such as those involved in inbreeding depression. This study examines the advantages and costs of selfing in Narcissus serotinus L., a mixed-mating geophyte that currently occurs in several fragmented populations in Portugal, that resulted from the construction of the largest European dam. Observation of pollinators revealed that cross-pollination is less frequent in small than in large populations. Manual self-pollinations significantly increased fruit and seed set in small-size populations, also suggesting limited insect pollination. The existence of selfing may provide reproductive assurance in small-size populations of N. serotinus where outcrossing pollination is reduced. Although floral biology and experimental pollinations showed that N. serotinus is capable of autonomous selfing, four of the six fitness traits studied showed significant inbreeding depression in all populations. The high levels of inbreeding depression found in N. serotinus suggest that the initial reproductive assurance advantage of selfing may be counterbalanced by lower survival of the resulting individuals and a decrease in the evolutionary potential of the populations.