Geitonogamous Selfing Research Articles

IMPACT OF INSECT POLLINATOR GROUP AND FLORAL DISPLAY SIZE ON OUTCROSSING RATE

Despite the strong influence of pollination ecology on the evolution of selfing, we have little information on how distinct groups of insect pollinators influence outcrossing rate. However, differences in behavior between pollinator groups could easily influence how each group affects outcrossing rate. We examined the influence of distinct insect pollinator groups on outcrossing rate in the rocky mountain columbine, Aquilegia coerulea. The impact of population size, plant density, size of floral display, and herkogamy (spatial separation between anthers and stigmas) on outcrossing rate was also considered as these variables were previously found to affect outcrossing rate in some plant species. We quantified correlations between all independent variables and used simple and two-factor regressions to determine direct and indirect impact of each independent variable on outcrossing rate. Outcrossing rate increased significantly with hawkmoth abundance but not with the abundance of any of the other groups of floral visitors, which included bumblebees, solitary bees, syrphid flies, and muscidae. Outcrossing rate was also significantly affected by floral display size and together, hawkmoth abundance and floral display size explained 87% of the variation in outcrossing rate. None of the other independent variables directly affected the outcrossing rate. This is the first report of a significant impact of pollinator type on outcrossing rate. Hawkmoths did not visit fewer flowers per plant relative to other pollinator groups but preferred visiting female-phase flowers first on a plant. Both the behavior of pollinators and floral display size affected outcrossing rate via their impact on the level of geitonogamous (among flower) selfing. Given that geitonogamous selfing is never advantageous, the variation in outcrossing rate and maintenance of mixed mating systems in populations of A. coerulea may not require an adaptive explanation.

The evolution of self‐fertilization and inbreeding depression under pollen discounting and pollen limitation

We model the evolution of plant mating systems under the joint effects of pollen discounting and pollen limitation, using a dynamic model of inbreeding depression, allowing for partial purging of recessive lethal mutations by selfing. Stable mixed mating systems occur for a wide range of parameter values with pollen discounting alone. However, when typical levels of pollen limitation are combined with pollen discounting, stable selfing rates are always high but less than 1 (0.9<s<1 in most cases); in this situation, complete selfing does not evolve because pollen discounting becomes very large at high selfing rates, so that the automatic advantage of selfing changes to a disadvantage. These results suggest that mixed mating systems with high selfing rates can be maintained by selection, whereas mixed mating systems with low to moderate selfing rates are more likely attributable to unavoidable geitonogamous selfing.

Reproductive ecology of the Qinghai-Tibet Plateau endemic Gentiana straminea (Gentianaceae), a hermaphrodite perennial characterized by herkogamy and dichogamy

The combined occurrence of both herkogamy and dichogamy in a hermaphrodite species has been considered to strongly favour outcrossing. In this study, we investigated in detail the reproductive ecology of Gentiana straminea Maxim. (Gentianaceae), a hermaphrodite perennial endemic to the Qinghai-Tibet Plateau. In a series of observations and experiments over four consecutive years, we examined whether the combination of dichogamy and herkogamy in individual flowers completely prevents geitonogamous pollen transfer in this species. The mode of floral development clearly indicates that autonomous self-pollination is completely avoided through herkogamy and dichogamy in individual flowers. This implication was confirmed by the breeding experiments, since no seed was produced when flowers were isolated. However, this gentian proved to be highly self-compatible when geitonogamous selfing was artificially induced. Many flowers opened simultaneously on individual plants, the ratio of male to female phase flowers was close to 2:1 in each inflorescence, at the full anthesis phase, and they were randomly distributed amongst the upper, middle and lower parts of each stem's inflorescence. On average, Bombus sushikini Skorikov, the most frequent visitor and only legitimate pollinator of G. straminea, visited nearly two flowers per inflorescence, and four flowers per plant. Among the pollinators' foraging bouts, the proportions of geitonogamous visits to inflorescences or flowers within an individual plant were 29% and 37%, respectively. Therefore, despite the strict dichogamous and herkogamous characteristics of the individual flowers, geitonogamous selfing might still prevail in G. straininea because of the size of its floral displays and the continuous visiting behavior of B. sushkini. (C) 2005 Elsevier SAS. All rights reserved.

Effects of pollen shortage and self-pollination on seed production of an endangered tree, Magnolia stellata

Pollen limitation is a significant determinant of seed production, and can result from both insufficient pollen quantity (pollen shortage) and quality (mainly relating to self-pollination). For animal-pollinated tree species with large floral displays, pollen limitation may be determined by a balance between increased pollen quantity due to increased attractiveness for pollinators, countered by increased self-pollination due to increased geitonogamy. The contributions of pollen shortage and self-pollination on seed production were quantitatively examined in the natural pollination of an insect-pollinated, dichogamous, endangered tree, Magnolia stellata, which has a large, showy floral display. Manual self- and cross-pollinations were conducted to determine the effects of selfing on seed production. The outcrossing rate was measured using microsatellite analyses of open-pollinated seeds, and the embryo mortality rate caused by self-pollination was indirectly estimated. The frequency of ovule mortality due to pollen shortage was also inferred using the embryo mortality and ovule survival rates from natural pollination. The average fruit set, seed set per fruit, and ovule survival rate per tree from hand cross-pollination were 1.37, 3.15, and 3.34 times higher than those from hand self-pollination, respectively, indicating that self-pollination causes inbreeding depression for fruit and seed set. The multilocus-outcrossing rate (t(m)) was intermediate, 0.632, and the primary selfing rate was 0.657. This indicates that frequent geitonogamous selfing occurs. The ovule mortality rate due to pollen shortage and the embryo mortality rate due to self-pollination were estimated to be 80.8 % and 45.9 %, respectively. It is concluded that seed production of M. stellata is strongly limited by both pollen shortage and self-pollination. Inefficient beetle-pollination and the automimicry system via asynchronous flowering might be responsible for the high level of pollen shortage and frequent geitonogamy. This is despite a large, showy floral display and the dichogamous system of the species.

Seed production from the mixed mating system of Chesapeake Bay (USA) eelgrass (Zostera marina; Zosteraceae)

In monoecious plants, gametes can be exchanged in three ways: among unrelated genets (outbreeding), with close relatives (inbreeding), or within individuals (geitonogamous selfing). These different mating systems may have consequences for population demography and fitness. The experiment presented herein used artificial crosses to examine the mating system of Chesapeake Bay, Virginia, USA eelgrass (Zostera marina L; Zosteraceae), a bisexual submerged aquatic plant that can outbreed, inbreed, and self. Genetic data indicate severe heterozygosity deficiencies and patchy genotype distribution in these beds, suggesting that plants therein reproduce primarily by vegetative propagation, autogamy, or geitonogamy. To clarify eelgrass reproductive strategies, flowers from three genetically and geographically distinct beds were hand-pollinated in outbred, inbred, and selfed matings. Fertilization success and seed production, life history stages which contribute greatly to the numeric maintenance of populations, were monitored. We found no evidence that inbreeding had negative consequences for seed production. On the contrary, selfed matings produced seeds significantly more frequently than outcrossed matings and produced significantly larger numbers of seeds than either inbred or outbred matings. These results contrast with patterns for eelgrass in other regions but might be expected for similar populations in which pollen limitation or a short reproductive season renders selfing advantageous.

Effects of Geitonogamy on the Seed Set of Magnolia obovata Thunb. (Magnoliaceae)

The effects of geitonogamy on the seed set of a protogynous hermaphroditic tree, Magnolia obovata Thunb., were investigated by electrophoretic allozyme analysis and pollination experiments. The multilocus outcrossing rates (tm) estimated in 1995 and 1997 by using four polymorphic loci were 0.39 and 0.42, respectively, indicating that more than half of the seeds are produced by geitonogamous selfing. Manual pollination demonstrated that the follicle set per fruit (frequency of ripe carpels) and seed set per fruit from self‐pollination were 2.6 and 3.8 times less than those from cross‐pollination, respectively, and the frequency of aborted seeds from self‐pollination was significantly higher than that from cross‐pollination. There was no significant difference between self‐ and cross‐pollination in the fruit set and the frequency of remnant ovules (ovules unfertilized or aborted before endosperm development). These results indicate that inbreeding depression is a primary factor reducing the seed set of self‐pollinated fruits. The fruit set, follicle set per tree, and seed set per tree from natural pollination were significantly lower than those from the manual pollinations. From these results, we estimated that 91% of ovules in M. obovata were not fertilized owing to pollen shortage, and 65% of embryos failed to develop because of geitonogamous selfing. The high embryo mortality rate from geitonogamy in M. obovata can be regarded as a cost of this tree’s pollination system, which is based on automimicry via asynchronous flowering.

The effect of protandry on siring success in Chamerion angustifolium (Onagraceae) with different inflorescence sizes.

Protandry, a form of temporal separation of gender within hermaphroditic flowers, may reduce the magnitude of pollen lost to selfing (pollen discounting) and also serve to enhance pollen export and outcross siring success. Because pollen discounting is strongest when selfing occurs between flowers on the same plant, the advantage of protandry may be greatest in plants with large floral displays. We tested this hypothesis with enclosed, artificial populations of Chamerion angustifolium (Onagraceae) by experimentally manipulating protandry (producing uniformly adichogamous or mixed protandrous and adichogamous populations) and inflorescence size (two-, six-, or 10-flowered inflorescences) and measuring pollinator visitation, seed set, female outcrossing rate, and outcross siring success. Bees spent more time foraging on and visited more flowers of larger inflorescences than small. Female outcrossing rates did not vary among inflorescence size treatments. However, seed set per fruit decreased with increasing inflorescence size, likely as a result of increased abortion of selfed embryos, perhaps obscuring the magnitude of geitonogamous selfing. Protandrous plants had a marginally higher female outcrossing rate than adichogamous plants, but similar seed set. More importantly, protandrous plants had, on average, a twofold siring advantage relative to adichogamous plants. However, this siring advantage did not increase linearly with inflorescence size, suggesting that protandry acts to enhance siring success, but not exclusively by reducing between-flower interference.

THE EFFECT OF PROTANDRY ON SIRING SUCCESS IN CHAMERION ANGUSTIFOLIUM (ONAGRACEAE) WITH DIFFERENT INFLORESCENCE SIZES

Protandry, a form of temporal separation of gender within hermaphroditic flowers, may reduce the magnitude of pollen lost to selfing (pollen discounting) and also serve to enhance pollen export and outcross siring success. Because pollen discounting is strongest when selfing occurs between flowers on the same plant, the advantage of protandry may be greatest in plants with large floral displays. We tested this hypothesis with enclosed, artificial populations of Chamerion angustifolium (Onagraceae) by experimentally manipulating protandry (producing uniformly adichogamous or mixed protandrous and adichogamous populations) and inflorescence size (two-, six-, or 10-flowered inflorescences) and measuring pollinator visitation, seed set, female outcrossing rate, and outcross siring success. Bees spent more time foraging on and visited more flowers of larger inflorescences than small. Female outcrossing rates did not vary among inflorescence size treatments. However, seed set per fruit decreased with increasing inflorescence size, likely as a result of increased abortion of selfed embryos, perhaps obscuring the magnitude of geitonogamous selfing. Protandrous plants had a marginally higher female outcrossing rate than adichogamous plants, but similar seed set. More importantly, protandrous plants had, on average, a twofold siring advantage relative to adichogamous plants. However, this siring advantage did not increase linearly with inflorescence size, suggesting that protandry acts to enhance siring success, but not exclusively by reducing between-flower interference.

The consequences of rewardlessness in orchids: reward‐supplementation experiments with Anacamptis morio (Orchidaceae)

Pollinators are expected to respond to low reward availability in an inflorescence by visiting fewer flowers before departure, thus potentially causing reduced visitation, but also reduced geitonogamous selfing. I tested this hypothesis using Anacamptis morio, an orchid that does not reward its pollinators. Supplementation of inflorescences with artificial nectar did not result in an increase in fruit set on supplemented inflorescences compared to control inflorescences and tended to reduce pollinia removal. Supplementation resulted in reduced fruit quality, but there was no evidence that this was as a result of inbreeding depression. Behavioral experiments showed that pollinating bumble bees, as predicted, visited more flowers on supplemented inflorescences. Bumble bees also deposited more self-pollen on supplemented inflorescences, but this was marginally significant. Bumble bee queens removed significantly more pollinia from control inflorescences, while Bombus terrestris and B. lucorum workers did not. I conclude that while pollinators behaved as predicted, there was weak evidence that pollinia removal, pollen deposition, and fruit set followed the predictions of the hypothesis. I argue that this was probably because some pollinators were more efficient at removing and depositing pollen on control inflorescences, while others were not.

Protogyny, Pollination, and Sex Expression of AndromonoeciousPseudocymopterus montanus(Apiaceae, Apioideae)

Studies of dichogamy seldom address its possible effects on other floral traits. An exception has been work on the apioid umbellifers (Apiaceae, subfamily Apioideae), which typically exhibit andromonoecy and both intra‐ and interfloral dichogamy. Several students of apioid umbellifers have suggested that variation in proportions of perfect and staminate flowers among sequentially blooming inflorescences (umbels) of individual plants has evolved in response to dichogamy. Here, we report a 2‐yr study of dichogamy, pollination, and sex expression in Pseudocymopterus montanus, a common wildflower of the Rocky Mountains. We recorded floral phenology, determined the timing of pollination, collected and identified floral visitors, documented variation in sex expression and fecundity within and among plants, and examined possible reasons why some perfect flowers did not set fruit. Floral visitors included andrenid and halictid bees and anthomyid, muscoid, syrphid, and tachinid flies. Autogamy was prevented by strong intrafloral protogyny of the perfect flowers, which were pollinated before their anthers dehisced. Flowering individuals produced a single primary (terminal) umbel, one or two secondary (lateral) umbels, and occasionally a tertiary (sublateral) umbel. Umbels of different orders matured sequentially, producing multiple cycles of protogyny. Weak interfloral protogyny within and among umbels allowed the possibility of geitonogamous selfing. All individuals expressed the same pattern of variation in sex expression among sequentially blooming umbels. Over 90% of flowers in primary umbels were staminate, but the frequency of staminate flowers fell to 65% or less in secondary and tertiary umbels. Primary and secondary umbels differed significantly in sex expression, even though their prepollination reproductive efforts (floral biomass) were the same. We favor the view that this distinctive pattern of within‐plant sex expression, i.e., decreasing proportions of staminate flowers in sequentially blooming umbels, evolved in response to protogyny. In apioid umbellifers, protogyny appears to have been derived from protandry. The selective forces involved in the derivation of protogyny may have included more specialized interactions with pollinators and the ability to geitonogamously self‐pollinate if cross‐pollination failed. Sex allocation theory predicts that protogyny is a sufficient condition for the evolution of increasing relative allocations to female function in sequentially blooming flowers or inflorescences. Plants with higher reproductive effort had higher absolute allocations to both male and female reproductive functions as well as higher relative allocations to female function (higher phenotypic femaleness). The more female‐biased gender of larger plants may result from higher frequencies of geitonogamy in inflorescences with more flowers.

The Role of Geitonogamy in the Gradual Evolution towards Dioecy in Cosexual Plants

We present a model for the gradual evolution towards dioecy in cosexual plants with geitonogamous selfing. We show how geitonogamous selfing (i.e. transfer of pollen between flowers on the same plant) can facilitate the evolution of dioecy (i.e. separate male and female individuals) in cosexual plants (i.e. both sexual functions on the same plant). We study the effect of parameters such as inbreeding depression, the attraction costs per flower, and the total amount of resources available per plant. We also consider different flower architectures (limited versus unlimited potential number of seeds per flower) and pollination biologies (biotic versus abiotic). We find that (1) if there is no maximum to the number of seeds per flower, then cosexuality is evolutionarily stable whenever the inbreeding depression is less than one-half. With abiotic pollination and an inbreeding depression greater than one-half, dioecy evolves via evolutionary branching, that is, through the gradual differentiation towards male and female plants, but only after the population has first evolved to a cosexual strategy with an intermediate sex ratio. The evolution of dioecy requires higher levels of inbreeding depression if pollination is by insects, but is facilitated by increasing the total amount of resources available per plant. (2) If the potential number of seeds per flower is limited, we get basically the same results as with an unlimited seed number per flower, but the outcome now also depends on the attraction costs per flower. With high attraction costs, the population can evolve to gynodioecy (females and cosexuals in the same population). Further increasing inbreeding depression leads to dioecy. Our results give a possible explanation of Darwin's observation that dioecy is more common in plant species with abiotic pollination and in large species with many flowers such as trees.

Implications of geitonogamous pollination for floral longevity in Iris gracilipes

Summary We analysed the longevity of individual flowers with respect to the effects of the floral display size of a plant (number of flowers open at one time) and spatial arrangement of flowers in Iris gracilipes (A. Gray). We found that the actual floral longevity of this species (about 2 days) was much shorter than that predicted by the model of Schoen and Ashman (about 7–10 days). A large display size and an overlap of flowering periods within a shoot facilitated successive pollinator probes within a plant, though the frequency of pollinator visits per flower was independent of display size. Consequently, the cost of geitonogamous selfing increased with an increase in display size and an overlap of flowering periods within a shoot. The short floral longevity of this species would be effective in such situations because shorter floral longevity leads to a smaller display size and a smaller overlap of flowering periods within a shoot. A small display size and small overlap of flowering periods within a shoot could also be realized by the strategy that staggers the flowering period of each flower without floral longevity being shortened (extended blooming). However, a mutant plant with an extended blooming schedule would be unlikely to invade the population because of the stabilizing selection pressure on the flowering date. This may be why such a short floral longevity, rather than extended blooming, has evolved in this species.

Fitness-consequences of geitonogamous selfing in a clonal marine angiosperm (Zostera marina).

Plant mating systems have received considerable attention because the proportion of selfed vs. outcrossed progeny is an important evolutionary factor. In clonally reproducing plants, geitonogamous selfing between distant ramets belonging to the same genet is expected to be widespread, yet empirical data are sparse. Nothing is known about between-ramet selfing in aquatic flowering plants with subaqueous pollen transfer, most of which display pronounced clonal reproduction. From two locations in the western Baltic Sea, I present data on the effects of patch isolation and clonal diversity on the outcrossing rate of eelgrass, Zostera marina L., based on the genotypes of maternal plants and recently fertilized ovules scored at eight microsatellite loci. There were no differences in outcrossing rates between vegetation patches and continuous meadow although patches were nearly always composed of single genets. Quantitative effects of clonal diversity were present in the continuous vegetation where a significant positive correlation between genet diversity and the proportion of outcrossed offspring was detected (Kendall's τ=0.82, P=0.0017). On a population-scale as well, the genotypic diversity was positively correlated with outcrossing. The relative fitness of selfed offspring was low (ω ± 95% confidence interval=0.56 ± 0.032 and 0.322 ± 0.15) indicating that geitonogamy incurred substantial fitness costs. Selfing rates in Z. marina may not be in evolutionary equilibrium because of spatial and temporal heterogeneity of clonal size and diversity. The high prevalence of dioecy in seagrasses may have evolved to avoid the fitness costs associated with geitonogamy.

Low outcrossing rates and shift in pollinators in New Zealand pohutukawa (Metrosideros excelsa;Myrtaceae)

New Zealand pohutukawa (Metrosideros excelsa), a member of the Myrtaceae, is a large, mass-flowering tree endemic to northern New Zealand coastlines. Mainland populations have been reduced to fragmented stands, and the original suite of bird pollinators has been largely replaced by introduced species. The native pollinator fauna on several offshore islands is largely intact and includes three species of the New Zealand honeyeaters (Meliphagidae) and native, solitary bees. We estimated multilocus outcrossing rates for three mainland and two island populations and found that they were among the lowest in the Myrtaceae (t(m) = 0.22-0.53). The shift in pollinators had no measurable effect on the mating system. Mass-flowering facilitates geitonogamous selfing, and inbreeding depression in seedling height was detectable at 6 mo of growth. F(s) [Wright's (1965) Fixation Index] was consistently higher than F(m) in all populations, indicating that selection may eliminate selfed offspring from populations prior to achieving reproductive maturity. Results suggest that increased selfing in mainland populations due to pollinator changes is not responsible for current patterns of poor regeneration of this species.

From pollen dynamics to adaptive dynamics

AbstractOn plants with many flowers, bumblebees tend to visit more flowers in sequence. This induces pollen transfer between flowers on the same plant (geitonogamy). Consequently, in self‐compatible plants the selfing rate increases with the number of simultaneously open flowers on the plant, and pollen export to other plants in the population can be a decelerating function of the number of flowers. I argue that geitonogamy is important in relation to two phenomena. First, it may explain the low number of seeds per flower and the size‐dependent sex allocation observed in some species. Applying sex allocation theory to the boraginaceous Cynoglossum officinale and Echium vulgare shows that hermaphroditism is stable in both species. The predicted evolutionarily stable values for seeds per flower are close to observed values in the field. The model generally predicts that seeds per flower increase with plant size. This prediction is fine for C. officinale but E. vulgare defies the theory. Second, geitonogamy is important in the evolution of dioecy, as already suggested by Darwin. With abiotic pollination the export of pollen to other plants is likely to be proportional to the number of flowers produced, while it decelerates in animal‐pollinated plants. Dioecy can then evolve gradually in a species with abiotic pollination, provided that inbreeding depression exceeds 50%, geitonogamous selfing increases with the number of flowers and that genetic variation exists. With complete pollen discounting as in the models of geitonogamous pollination by animals, hermaphroditism is always stable. The model explains Darwin’s observation that dioecy is more common in wind‐pollinated species and in species in which individuals are large, reproducing with many flowers.

Selfing rates in natural populations of Echium vulgare: a combined empirical and model approach

1. We quantified geitonogamous selfing in Echium vulgare, a self‐compatible, bumble‐bee pollinated plant. A maximum estimate of selfing was determined using a paternity analysis with RAPDs. In the first experiment, bumble‐bees visited a sequence of virgin flowers. The percentage selfing increased rapidly from 12% in the first flower visited, up to 50% in the 15th flower visited in the sequence. In the second experiment, when bees visited plants in a natural population, the average selfing of plants increased with the number of open flowers from 0% to maximally 33%. 2. The results obtained in both experiments are consistently lower than predicted by our model on pollen dynamics (Rademaker, de Jong &amp; Klinkhamer 1997). We modified the model on pollen dynamics to link it more to the field situation with observations on flower stage, flower opening and bumble‐bee preference, so that the bumble‐bees encounter a variable number of pollen grains per flower. We also adjusted the parameters. If less pollen adheres to the bee (25% instead of 50%) after removal from the anthers, or if bees arrive at a plant with more pollen grains (6000 instead of 4448), the predictions of the model in regard to selfing could be improved but were still high compared with the observed selfing rates measured with RAPDs. 3. We suggest that the model is consistent with pollen dynamics in the field. However, post‐pollination processes like selective abortion could play a role in E. vulgare.

How geitonogamous selfing affects sex allocation in hermaphrodite plants

Abstract Does the mode of self-pollination affect the evolutionarily stable allocation to male vs. female function? We distinguish the following scenarios. (1) An ‘autogamous’ species, in which selfing occurs within the flower prior to opening. The pollen used in selfing is a constant fraction of all pollen grains produced. (2) A species with ‘abiotic pollination’, in which selfing occurs when pollen dispersed in one flower lands on the stigma of a nearby flower on the same plant (geitonogamy). The selfing rate increases with male allocation but a higher selfing rate does not mean a reduced export of pollen. (3) An ‘animal-pollinated’ species with geitonogamous selfing. Here the selfing rate also increases with male allocation, but pollen export to other plants in the population is a decelerating function of the number of simultaneously open flowers. In all three models selfing selects for increased female allocation. For model 3 this contradicts the general opinion that geitonogamous selfing does not affect evolutionarily stable allocations. In all models, the parent benefits more from a female-biased allocation than any other individual in the population. In addition, in models 2 and 3, greater male allocation results in more local mate competition. In model 3 and in model 2 with low levels of inbreeding depression, hermaphroditism is evolutionarily stable. In model 2 with high inbreeding depression, the population converges to a fitness minimum for the relative allocation to male function. In this case the fitness set is bowed inwards, corresponding with accelerating fitness gain curves. If the selfing rate increases with plant size, this is a sufficient condition for size-dependent sex allocation (more allocation towards seeds in large plants) to evolve. We discuss our results in relation to size-dependent sex allocation in plants and in relation to the evolution of dioecy.

Reproductive biology and population genetic structure ofKandelia candel(Rhizophoraceae), a viviparous mangrove species

The pollination biology, mating system, and population genetic structure of Kandelia candel were investigated. Field observations on its pollination and reproductive biology suggested that this species is pollinator dependent for fruit set, and bee activities can lead to substantial geitonogamous selfing. Quantitative analysis of the mating system parameters was performed using progeny arrays assayed for allozyme markers. Multilocus outcrossing rates (t(m)) were estimated to be 0.697 ± 0.091 and 0.797 ± 0.062 in two populations. In comparison to other plant species with mixed-mating system, the level of allozyme variation was very low in the 13 populations sampled along the coastlines of Hong Kong. At the species level, the proportion of polymorphic loci was 20%, number of alleles per locus was 1.2, and heterozygosity was 0.0362. The total gene diversity was primarily distributed within populations (H(S )= 0.0339), and the coefficient of genetic differentiation among populations was low (G(ST )= 0.064). This pattern of population genetic structure suggests that gene flow, primarily in the form of water-dispersed seedlings in viviparous mangrove species, is not as limited as previously thought. However, microgeographic pattern in allele frequency at the marker loci could still be detected between the western and eastern coastal populations.

Cryptic self‐incompatibility in tristylous Decodon verticillatus (Lythraceae)

There is growing evidence that many self-compatible plants control the level of self-fertilization with postpollination processes that give a siring advantage to cross pollen over self pollen through "cryptic self-incompatibility" (CSI). Previous marker-gene experiments with self-compatible, tristylous Decodon verticillatus (Lythraceae) have demonstrated a siring advantage to cross pollen, though the extent to which this advantage results from prezygotic discrimination vs. early acting inbreeding depression is not clear. Here, we provide evidence that prezygotic mechanisms are involved in this siring advantage by comparing pollen tube numbers at various times following cross- and self-pollination conducted in a natural population. In the 24 h following pollination, cross pollen yielded almost twice as many pollen tubes at various positions in the style compared to self pollen. After 36 and 48 h, the difference between pollen types had disappeared, suggesting that the advantage to cross pollen results from differences in the rate of pollen germination and;clor tube growth rather than pollen tube attrition. Comparison of tube numbers after legitimate vs. illegitimate cross-pollination did not reveal any difference, suggesting that D. verticillatus possesses CSI unrelated to heteromorphic self- and intramorph-incompatibility found in other heterostylous members of the Lythraceae. CSI resulting from differential pollen tube growth may minimize geitonogamous selfing when cross pollen is abundant, while maximizing fecundity when cross pollen is scarce due to local clonal spread.

Sexual Reproduction in Saxifraga oppositifolia L. and Saxifraga biflora All. (Saxifragaceae) in the Alps

Plant life in high alpine or arctic areas is strongly limited by abiotic rather than biotic environmental factors. In climate warming, invading plants from lower altitudes may outcompete indigenous alpine species, which might only survive by colonizing new habitats. Because diaspores are most effective for long-distance dispersal, colonizing species mostly depend on reproduction by seeds. The breeding systems of Saxifraga oppositifolia and Saxifraga biflora were investigated in the field. Saxifraga oppositifolia was found to be mostly cross-pollinated, and S. biflora showed a mixed mating system. Relative seed set (seed: ovule ratio) was higher in S. biflora than in S. oppositifolia. Saxifraga biflora also showed a higher relative reproductive success (RRS) in naturally cross-pollinated plants, because of high fruit set, and in geitonogamous selfing compared with S. oppositifolia. Germination percentage was lower in S. biflora than in S. oppositifolia, but only a few seeds germinated in both species. Variation of germinability could not be attributed to the pollen source of the various pollination treatments. Saxifraga oppositifolia is among the earliest plants to flower in spring, at a time when low temperatures limit pollinator activities. Since pollen transfer is essential in this preferentially outbreeding species, S. oppositifolia is a typical "pollen risker." Late-flowering species such as S. biflora have numerous pollinators at their disposition and usually display a high selfing rate. Early snowfall, however, can destroy the whole seed crop. In this context, S. biflora resembles a "seed risker." Because of a high outcrossing rate, "pollen riskers" are expected to have a high genetic variability, which is regarded as preadaptation to stochasticity in tundra habitats and to climate fluctuations. A possible future climate warming will reduce the chances that "seed riskers" lose their whole seed yield, which could be advantageous for colonizing new habitats. This demonstrates the need of learning about species-specific reproductive strategies in order to make predictions on the vulnerability of tundra plants in a changing climate.