Pollen and Resource Limitation in Veratrum nigrum L. （Liliaceae）, an Andromonoecious Herb

Pollen limitation and resource limitation were invoked to account for the pattern that flowering plants produce more flowers and ovules than fruits and seeds. This study aimed to determine their relative importance in Veratrum nigrum, a self-compatible, perennial, andromonoecious herb. In order to determine whether female production was limited by pollen grains on stigmas or by available resources, we performed supplemental hand pollination in three populations, male-flower-bud removal in three other populations, and emasculation of hermaphroditic flowers in still another population, resulting in a total of seven populations experimentally manipulated. Across the three populations, supplemental hand pollination did not significantly increase fruit set, seed number per fruit, and total seed production per individual, nor did emasculation of hermaphroditic flowers. Taken together, our results suggest that pollen grains deposited on stigmas were abundant enough to fertilize all the ovules. Male-flower-bud removal significantly increased the mean size of hermaphroditic flowers in all three populations. Female reproductive success was increased in one population, but not in the other two populations possibly due to heavy flower/seed predation. We concluded that the female reproductive success of V. nigrum was not limited by pollen grains but by available resources, which is consistent with Bateman＇s principle. Furthermore, the female reproduction increase of male-flower-bud removal individuals might suggest a trade-off between male and female sexual functions.     

The functional ecology of gynodioecy in Eritrichum aretioides (Boraginaceae), the alpine forget-me-not

Eritrichum aretioides is a gynodioecious species with female and hermaphrodite individuals. In populations on Pennsylvania Mountain in central Colorado (USA), the frequency of females ranges from 22 to 41%. Flower number and the number of seeds produced per flower were similar in hermaphrodites and females. However, hermaphrodites produced larger flowers, while females produced larger seeds (P < 0.05 for both). In the field, seed germination was higher for seeds from females than for seeds from hermaphrodites (20 vs. 9% germination; P < 0.05). Unvisited flowers and open-pollinated flowers of hermaphrodites had similar pollen receipt (approx 20 pollen grains per stigma), but seed set following autogamous pollination was significantly lower than seed set following natural pollination. This finding indicates that hermaphrodites have a barrier to selfing and implies that the larger seed size and greater establishment advantage of offspring from females is unlikely to have resulted from female outcrossing advantage. Rather, differences in the quality of seed progeny between morphs probably reflect a trade-off in sexual allocation or pleiotropic effects of the sex-determining genes.     

Pollen viability and limitation of seed production in a population of the circumpolar cushion plant, Silene acaulis (Caryophyllaceae)

Pollen viability among genders and limitation of female seed production in a natural trioecious population of the circumpolar cushion plant Silene acaulis was examined. Pollen viability was estimated by an in vitro pollen germination experiment. Both male and hermaphrodite flowers displayed large variation in pollen viability (0–53% in hermaphrodite and 0–54% in male flowers). There was a significant difference between genders in pollen viability: male plants had on average higher pollen viability than hermaphrodite plants. Resource and pollen limitation of seed production was studied by an experiment consisting of three treatments; (I) hand-pollination and removal of all other flowers on the cushion, (II) hand-pollination without removal of other flowers, and (III) open pollination without removal of flowers. Hand-pollination increased seed production, whereas removal of flowers had no effect on seed production. Abortion of pollinated ovules during seed development and seed mass did not differ among treatments. To control for effect of fruit number on seed production, data from naturally pollinated individuals was used. There was a positive correlation between both total number of seeds and fruit number, mean seed number per fruit and fruit number, respectively. These results indicate that seed production of 5. acaulis is mainly limited by pollen availability whereas resource competition between fruits is not important as a limiting factor. The possible role of male quality differences between genders and pollen limitation of seed production for maintenance of trioecious reproductive systems is discussed.     

ADVANTAGES OF BEING A SPECIALIST FEMALE IN NODIOECIOUS SILENE VULGARIS S.L. (CARYOPHYLLACEAE)

According to sex allocation theory, to maintain a mutant male-sterile plant in a population of hermaphrodites such a plant must compensate its loss of fitness caused by inhibition of pollen production with a higher reproductive success through its female function. In the present study of a gynodioecious population of Silene vulgaris (Caryophyllaceae) I show that hermaphrodites not only benefit from outcrossing, in that progeny from outcrossed flowers are more vigorous than those from selfed flowers within an individual plant, but they also suffer heavily from self-pollination between different flowers of the same individuals, which could be demonstrated in experimentally made male-sterile (emasculated) individuals. Seeds from the emasculation period were heavier and germinated better than when the same individual was an intact hermaphrodite. Naturally male-sterile (female) individuals produced more fruits due to flowers staying open longer for pollen to arrive via some vector. However, the higher seed number alone could not provide the fitness advantage needed for females to be maintained in the population, but females also produced heavier seeds as compared to the hermaphrodites. Differences in seed survival and seedling establishment in the field are expected to add the advantages necessary for female plants to be selectively plausible.     

Bateman’s principle and plant reproduction: The role of pollen limitation in fruit and seed set

Bateman’s principle states that male fitness is usually limited by the number of matings achieved, while female fitness is usually limited by the resources available for reproduction. When applied to flowering plants this principle leads to the expectation that pollen limitation of fruit and seed set will be uncommon. However, if male searching for mates (including pollen dissemination via external agents) is not sufficiently successful, then the reproductive success of both sexes (or both sex functions in hermaphroditic plants) will be limited by number of matings rather than by resources, and Bateman’s principle cannot be expected to apply. Limitation of female success due to inadequate pollen receipt appears to be a common phenomenon in plants. Using published data on 258 species in which fecundity was reported for natural pollination and hand pollination with outcross pollen, I found significant pollen limitation at some times or in some sites in 159 of the 258 species (62%). When experiments were performed multiple times within a growing season, or in multiple sites or years, the statistical significance of pollen limitation commonly varied among times, sites or years, indicating that the pollination environment is not constant. There is some indication that, across species, supplemental pollen leads to increased fruit set more often than increased seed set within fruits, pointing to the importance of gamete packaging strategies in plant reproduction. Species that are highly self-incompatible obtain a greater benefit relative to natural pollination from artificial application of excess outcross pollen than do self-compatible species. This suggests that inadequate pollen receipt is a primary cause of low fecundity rates in perennial plants, which are often self-incompatible. Because flowering plants often allocate considerable resources to pollinator attraction, both export and receipt of pollen could be limited primarily by resource investment in floral advertisement and rewards. But whatever investment is made is attraction, pollinator behavioral stochasticity usually produces wide variation among flowers in reproductive success through both male and female functions. In such circumstances the optimal deployment of resources among megaspores, microspores, and pollinator attraction may often require more flowers or more ovules per flower than will usually be fertilized, in order to benefit from chance fluctuations that bring in large number of pollen grains. Maximizing seed set for the entire plant in a stochastic pollination environment might thus entail a packaging strategy for flower number or ovule number per flower that makes pollen limitation of fruit or seed set likely. Pollen availability may limit female success in individual flowers, entire plants (in a season or over a lifetime), or populations. The appropriate level must be distinguished depending on the nature of the question being addressed.     

Female reproductive success in gynodioecious Thymus vulgaris: pollen versus nutrient limitation and pollinator foraging behaviour

Gynodioecy is a dimorphic breeding system in which female individuals coexist with hermaphroditic individuals in the same population. Females only contribute to the next generation via ovules, and many studies have shown that they are usually less attractive than hermaphrodites to pollinators. Several mechanisms have been proposed to explain how females manage to persist in populations despite these disadvantages. The ‘resource reallocation hypothesis’ (RRH) states that females channel resources not invested in pollen production and floral advertisement towards the production of more and/or larger seeds. We investigated pollination patterns and tested the RRH in a population of Thymus vulgaris. We measured flower display, flower size, nectar production, visitation rates, pollinator constancy and flower lifespan in the two morphs. In addition, we measured experimentally the effects of pollen and resource addition on female reproductive success (fruit set, seed set, seed weight) of the two morphs. Despite lower investment in floral advertisement, female individuals were no less attractive to pollinators than hermaphrodites on a per flower basis. Other measures of pollinator behaviour (number of flowers visited per plant, morph preference and morph constancy) also showed that pollinators did not discriminate against female flowers. In addition, stigma receptivity was longer in female flowers. Accordingly, and contrary to most studies on gynodioecious species, reproductive success of females was not pollen limited. Instead, seed production was pollen limited in hermaphrodites, suggesting low levels of cross‐pollination in hermaphrodites. Seed production was resource limited in hermaphrodites, but not in females, thus providing support for the RRH. © 2014 The Linnean Society of London, Botanical Journal of the Linnean Society, 2014, 175 , 395–408.     

Gender variation of sequential inflorescences in a monoecious plant Sagittaria trifolia (Alismataceae)

In protogynous plants, female flowers of early blooming plants are at a reproductive disadvantage because they cannot set fruit due to the lack of available pollen. To study this phenomenon, gender expression of the monoecious herb Sagittaria trifolia was investigated over the entire flowering season in two field and two cultivated populations in Hubei and Hunan Provinces, China. In racemes of S. trifolia, flowers open sequentially from bottom to top, with female flowers opening first followed by male flowers. This creates a temporal separation of sexes in the species. Under field conditions small plants are often male, with production of both male and female flowers increasing with plant size. Femaleness increased among sequential inflorescences since female flower production increased whereas male flower production did not. Seed production was greater in large inflorescences because they contain more female flowers, and the number of ovules increased in female flowers at basal positions within the raceme. A consistent pattern of high seed set was observed in flowers from both field and cultivated populations. About 1 % of unfertilized ovules resulted from no pollination and 2 % of the seeds produced were only partly developed due to resource limitation. In the first inflorescence of the six experimental populations, 6.7-40.0 % of individuals produced only male flowers, and female flowers of 1.9-6.5 % individuals were aborted. The occurrence of male flowers in early blooming inflorescences could be an adaptive strategy to conserve resources and enhance pollination of female flowers in protogynous S. trifolia.     

Temporal patterns of seed set and decelerating fitness returns on female allocation in Zigadenus paniculatus (Liliaceae), an andromonoecious lily

I describe temporal patterns of seed production in the andromonoecious lily Zigadenus particulatus. Fruit set per flower and seed set per fruit declined through time within plants. Hand pollination experiments showed that this was not due to increasing pollen limitation. Nutrient supplementation had little effect on seed output, but leaf clipping reduced seed production, especially in late-blooming flowers, and removal of early-blooming flowers increased seed set by later flowers. Thus, the temporal pattern of seed output was due to declining availability of photosynthates. Plants with larger bulbs produced larger inflorescences, a greater proportion of hermaphrodite flowers, more fruits per hermaphrodite flower, and more seeds per fruit, but lost a greater fraction of their initial bulb mass as a consequence of fruiting. After controlling for the effects of bulb mass, plants with larger inflorescences produced a greater proportion of male flowers, and plants with more hermaphrodite flowers produced fewer fruits per hermaphrodite flower and fewer seeds per fruit. Thus, the female fitness gain curve was decelerating. The temporal decline in seed output provides a partial explanation for the parallel decline in allocation to pistils. However, a complete explanation for the pattern of gamete packaging requires an understanding of factors controlling male, as well as female, fitness.     

Ovule pre-emption and pollen limitation in a self-fertile perennial herb (Blandfordia grandiflora,Liliaceae)

The extent, frequency and causes of pollenlimited seed production were examined in partially selffertile populations of Blandfordia grandiflora for 2 years. Percentage seed set of open-pollinated plants (50–57%) did not differ within or between years, and was about 19% less than experimentally cross-pollinated plants (70–75%). Floral visits by honeybees did not differ through the flowering season and the number of pollen grains deposited on stigmas within 12 h of flowers opening exceeded the number of ovules per flower, indicating that the quality rather than the quantity of pollination limited seed set. Pollen limitation was caused by concurrent self- and cross-pollination and the subsequent abortion of some selfed ovules due to inbreeding depression. Natural seed set (55%) was intermediate between selfed (43%) and crossed (75%) flowers and was not increased when flowers that had been available to pollinators for 24 h were hand cross-pollinated, suggesting that ovules were already fertilized. Similarly, experimental pollination with both cross and self pollen within 24 h of flowers opening did not increase seed set relative to natural seed set, indicating that both cross- and self-fertilizations had occurred. In contrast, when selfing followed crossing by 48 h, or vice versa, seed set did not differ from crossed-only or selfed-only flowers, respectively, indicating that ovules were pre-empted by the first pollination. Collectively, these results indicate that under natural conditions self pollen pre-empts ovules, rendering them unavailable for cross-fertilization. This selfing reduces fecundity by 50%, as estimated from the natural production of cross seeds when selfing was prevented. Consequently, selection should favour floral traits, such as increased stigma-anther separation or protandry, that reduce interference between male and female functions that leads to selfing.     

Genetic consequences of specialization: yucca moth behavior and self-pollination in yuccas

Reciprocal specialization in interspecific interactions, such as plant-pollinator mutualisms, increases the probability that either party can have detrimental effects on the other without the interaction being dissolved. This should be particularly apparent in obligate mutualisms, such as those that exist between yucca and yucca moths. Female moths collect pollen from yucca flowers, oviposit into floral ovaries, and then pollinate those flowers. Yucca moths, which are the sole pollinators of yuccas, impose a cost in the form of seed consumption by the moth larvae. Here we ask whether there also is a genetic cost through selfish moth behavior that may lead to high levels of self fertilization in the yuccas. Historically, it has been assumed that females leave a plant immediately after collecting pollen, but few data are available. Observations of a member of the Tegeticula yuccasella complex on Yucca filamentosa revealed that females remained on the plant and oviposited in 66% of all instances after observed pollen collections, and 51% of all moths were observed to pollinate the same plant as well. Manual cross and self pollinations showed equal development and retention of fruits. Subsequent trials to assess inbreeding depression by measuring seed weight, germination date, growth rate, and plant mass at 5 months revealed significant negative effects on seed weight and germination frequency in selfed progeny arrays. Cumulative inbreeding depression was 0.475, i.e., fitness of selfed seeds was expected to be less than half that of outcrossed seeds. Single and multilocus estimates of outcrossing rates based on allozyme analyses of open-pollinated progeny arrays did not differ from 1.0. The discrepancy between high levels of behavioral self-pollination by the moths and nearly complete outcrossing in mature seeds can be explained through selective foreign pollen use by the females, or, more likely, pollen competition or selective abortion of self-pollinated flowers during early stages of fruit development. Thus, whenever the proportion of pollinated flowers exceeds the proportion that can be matured to ripe fruit based on resource availability, the potential detrimental genetic effects imposed through geitonogamous pollinations can be avoided in the plants. Because self-pollinated flowers have a lower probability of retention, selection should act on female moths to move among plants whenever moth density is high enough to trigger abortion. Received: 18 March 1996 \Accepted: 30 July 1996     

Differences in female reproductive success between female and hermaphrodite individuals in the subdioecious shrub Eurya japonica (Theaceae)

Subdioecy is thought to occupy a transitional position in the gynodioecy–dioecy pathway, explaining one of the evolutionary routes from hermaphroditism to dioecy. Quantifying any female reproductive advantage of females versus hermaphrodites is fundamental to examining the spectrum between subdioecy and dioecy; however, this is challenging, as multiple interacting factors, such as pollen limitation and resource availability, affect plant reproduction. We compared the female reproductive success of females and hermaphrodites via a field experiment in which we hand‐pollinated individuals of the subdioecious shrub Eurya japonica of similar size growing under similar light conditions. Effects of pollen limitation and seed quality were also evaluated through comparing the results of hand‐ and natural‐pollination treatments and performing additional laboratory and greenhouse experiments. Overall, females had higher fruit set and produced heavier fruit and more seeds than hermaphrodites, and these results were more pronounced for hand‐pollinated than for natural‐pollinated plants of both sexes. We also found that seeds naturally produced by females had a higher mean germination rate. These results indicate that females had a pronounced advantage in female reproductive success under conditions of no pollen limitation. The sexual difference in the degree of pollen limitation suggests a pollinator‐mediated interaction, whereas the higher female reproductive success of females even under natural conditions implies that E. japonica is a good model species for elucidating the later stages of the gynodioecy–dioecy pathway.     

Effects of pollen quantity and quality on reproduction and offspring vigor in the rare plant Scorzonera humilis (Asteraceae)

We studied the effects of pollinator exclusion, interparental distance, and supplementary hand pollination on reproduction and progeny vigor in Scorzonera humilis (Asteraceae), a rare plant of fragmented, nutrient-poor grasslands. Caged flowers produced no seeds and selfed flowers only very rarely, indicating that S. humilis is mainly self-incompatible. Seed production, seed mass, and seed germination following between-population crosses were consistently, but not significantly, higher than after within-population crosses. Seed set increased with local density of conspecifics, indicating that the reduced plant density in fragmented populations may reduce plant reproductive success. Seed set was pollen limited in all four populations studied. Supplementary hand-pollination strongly increased the survival of offspring, indicating that either pollinators transferred pollen from related individuals resulting in inbreeding depression in spite of the incompatibility system or that higher pollen loads increased pollen competition and the selectivity among gametes. In one of the populations, adding pollen from a different population strongly increased progeny fitness compared with both natural pollination and pollen supplementation from the same population. The results indicate that S. humilis is sensitive to inbreeding and that pollen limitation can reduce both the number and quality of offspring.     

Effects of pollen quality and quantity on pollen limitation in Crataegus monogyna (Rosaceae) in NW Spain

Pollen limitation occurs when plants produce less fruits and/or seeds than they would with adequate pollen receipt. If the addition of cross-pollen to stigmas increases fruit/seed production, it is interpreted as an evidence of pollen limitation. Much of the limitation may be associated with the quality rather than quantity of pollen; however, most studies do not discriminate between the two, which may lead to misinterpretation of the results. We studied the effects of quality and quantity of pollen on the reproduction of a northern Spanish population of Crataegus monogyna. The treatments included self- and cross-pollination, and supplementation to open and bagged flowers. The response variables considered were number of pollen grains per stigma, pollen tubes per style, and initial and final fruit set. In the Cantabrian range, C. monogyna requires insect pollinators to set fruit and is partially self-incompatible. We found that the number of pollen tubes did not differ between cross- and self-pollination treatments; however, self-pollinated flowers set less fruits than flowers that received pure cross-pollen or were supplemented with both cross- and self-pollen. The experimental design allowed us to infer qualitative rather than quantitative pollen limitation. Comparison of the number of pollen grains and tubes, and initial and final fruit set among pollination treatments suggested post-zygotic embryo selection against selfed progeny.     

Nectar replenishment and pollen receipt interact in their effects on seed production of Penstemon roseus

Resource supply and pollen delivery are often thought to equally limit seed production in animal-pollinated plants. At equilibrium, plants should show no response to experimental pollen supplementation because resources limit seed set above the current level of pollen attraction, while experimental reduction in pollen deposition below the equilibrium level would reduce seed set. The predicted equilibrium may be disrupted, however, if plants expend additional energy to replenish removed nectar. We investigated the combined effects of nectar removal and pollen delivery on female reproductive success of Penstemon roseus (Plantaginaceae), a hummingbird-pollinated plant that replenishes removed nectar. We first documented that the frequency of experimental nectar removal was correlated with total nectar secretion; and increased frequency of nectar removal resulted in increased female reproductive costs to the plant. Trade-offs between investing resources in nectar and investing resources in seeds were then investigated in two contrasting natural populations by removing nectar from flowers at increasing frequencies while simultaneously hand-pollinating flowers with increasing amounts of pollen. Seed set was lowest at low levels of pollen deposition, highest at medium-sized pollen loads, and intermediate when pollen loads were highest. At both sites, the frequency of nectar removal and pollen deposition had an interactive effect on seed production, in that intermediate levels of nectar removal result in the absolute highest seed set, but only at intermediate pollen loads. At high pollen loads, seed set was higher following little to no nectar removal, and at low pollen loads, all rates of nectar removal affected fecundity equally. Seed mass responded to nectar removal and pollination differently than did seed set. High levels of nectar removal and pollen delivery both lowered seed mass, with little interaction between main effects. Our findings are among the first to demonstrate that nectar replenishment costs and pollination intensity jointly affect seed production. This conflict between nectar replenishment costs and pollen-limiting factors results in trade-offs between pollinator attraction and seed provisioning. Thus, resource allocation towards nectar production should more often be considered in future studies of pollen limitation.     

Floral sex ratios, disease and seed set in dioecious Silene dioica

1 In the dioecious, perennial herb Silene dioica , the density of pollen donors in a population is determined by overall plant density, the sex ratio and the proportion of plants infected with the anther-smut fungus Microbotryum violaceum , which results in permanent sterility of both male and female plants.
2 Pollinators ( Bombus spp.) were found to prefer male flowers and to avoid diseased flowers. This may result in an overall lower visitation frequency and increased risk for pollen limitation in populations with a low density of males or a high incidence of disease.
3 Compared with open-pollinated flowers, hand pollination resulted in a significant increase in the number of seeds produced per fruit in populations with an experimentally reduced proportion of males (25% and 50% male flowers) but not in a naturally male-dominated population (75% male flowers). Seed production per plant was increased by hand pollination only in the most female-dominated population. Because the floral sex ratio is often male-biased, resources rather than pollen availability are likely to set the upper limit for total seed production per individual in most healthy populations of S. dioica.
4 There was a negative relationship between seed set and incidence of disease across 22 populations in both years of a field study. However, there was no consistent difference between the responses of highly diseased populations (incidence 30–56%) and populations with a low disease incidence (incidence 0–8%) to hand pollination.
5 In a greenhouse experiment with cloned hand-pollinated females, the presence of spores on healthy flowers was found to reduce seed set significantly. In highly diseased populations, therefore, the frequent deposition of spores by flower visitors onto remaining healthy plants may decrease seed production below the potential level determined by resources or pollen availability.     

毛翠雀花花序内的性分配和繁殖成功

两性花植物花序内不同位置的性分配和繁殖成功一般存在差异,通常认为资源竞争、结构效应和交配环境是形成这种差异的主要原因。为了研究雄性和雌性繁殖资源在花序内不同位置间的最优分配问题,该文以青藏高原高寒草甸典型高山植物毛翠雀花为材料,通过比较花序内不同位置的花部特征和种子性状,对其花序内的性分配模式和雌性繁殖成功进行研究,并通过观察传粉者运动特点以及人工去花和补授花粉实验,探讨花序内资源竞争和交配环境对繁殖资源分配的影响。结果表明:(1)不同位置间的雄蕊数、雄蕊鲜重/雌蕊鲜重、花粉数及花粉胚珠比从花序基部到上部显著增加,而雌蕊鲜重和胚珠数逐渐减少,表现出上部花偏雄的性分配;上部花的结籽率显著低于基部花和中部花,不同位置间的发育种子数/果实和发育种子重/果实随着花位置的升高而显著降低,说明基部花具有更佳的雌性繁殖成效。(2)去花处理后,剩余果实的单个种子重/果实显著增加,但发育种子数/果实没有显著增加;而给上部花人工补授异花花粉后,位置间结籽率的差异消失,说明传粉限制而非资源竞争导致了花序内位置依赖的种子生产模式。(3)毛翠雀花雄性先熟的开花特征,以及传粉者苏氏熊蜂从花序基部到上部的定向访花行为,导致了花序内交配环境的变化。综上结果表明,毛翠雀花花序内的性分配和繁殖成功差异是对交配环境适应的结果,对其在高山环境中实现雌雄适合度最优化具有重要意义。     

Maintenance of gynodioecy in Wurmbea biglandulosa (Colchicaceae): gender differences in seed production and progeny success

 In gynodioecious species, females contribute genes to future generations only through ovules, and to persist in populations they must have a compensatory advantage compared with hermaphrodites that reproduce via ovules and pollen. This compensation can result from greater fecundity and/or superior success of progeny from females. We examined differences in seed production and progeny success between females and hermaphrodites in the geophyte Wurmbea biglandulosa to explain the maintenance of females. Females produced more ovuliferous flowers and had more ovules per flower than did hermaphrodites but this did not necessarily result in greater fecundity, in part because seed production of females was pollen-limited. Over four years in one population, open-pollinated females produced 1.32 more seeds than open-pollinated hermaphrodites (range 1.09–1.63). In two other populations examined for one year only females produced 1.07 and 0.79 as many seeds as hermaphrodites. Seed production of open-pollinated females and hermaphrodites was only 55% and 73% that of cross-pollinated plants, respectively, indicating that both genders were pollen-limited but females more so than hermaphrodites. Open-pollinated seeds from females were 1.18–1.27 times more likely to germinate than seeds from hermaphrodites. No gender differences existed in seedling growth or survival. Hermaphrodites were self-compatible, but selfed seed set was only 80% that of crossed seed set. Crossed seed set of females and hermaphrodites did not differ. Assuming nuclear control of male sterility, relative female fitness is insufficient to maintain females at their current frequencies of 17%, and substantial female fitness advantages at later life-cycle stages are required. Received May 4, 2001 Accepted February 25, 2002     

Evolution of the Selfing Rate and Resource Allocation Models

Abstract First, evolutionary theories of selfing of terrestrial plants are reviewed briefly. The evolution of the selfing rate is controlled mainly by (1) the benefit of enhanced genetic relatedness to seeds and (2) the cost of lowered fitness of selfed offspring (inbreeding depression), being modified by (3) fertility assurance under pollen limitation, (4) reduced performance as pollen donor, (5) reduced expenditure to male function, and (6) lowered genetic recombination. Models of the joint evolution of selfing and inbreeding depression predict either strong outcrossing or predominant selfing. Although wind-pollinated plants fit the prediction, some animal-pollinated species have intermediate selfing rates, refuting the theory.
Second, three resource allocation models are analyzed, in which an individual plant optimally allocates limited resources to outcrossed seeds, selfed seeds, and to energy reserves for the next year. The first model explains how the number of outcrossed and selfed offspring change with plant size when they differ in dispersal distance. The second model predicts that, in a disturbed habitat, the plant is likely to be annual and to produce both selfed and outcrossed seeds; in contrast, in a stable habitat, the plant tends to be perennial and to abort selfed seeds selectively. Hand pollination may increase seed production for perennials but not for annuals. The third model explains the observed difference between animal and wind pollinated plants in the out-crossing rate pattern by the difference in the way pollen acquisition increases with investment.     

Pollen source and resource limitation to fruit production in the rare species Eremosparton songoricum (Fabaceae)

Eremosparton songoricum (Litv.) Vass. is a rare, central Asian desert species which shows lower fruit set and seed set (<16%) than most hermaphroditic species. We hypothesized that fruit production was limited by pollen and resources. To evaluate potential fruit abortion due to pollen limitation, supplemental hand‐pollination was undertaken, the mating system was investigated and the foraging behavior of pollinators was recorded. To investigate possibile resource limitation, flowers and young pods were artificially removed and fertilization were manipulated. The results showed that under natural pollination, the number of pollen deposited on the stigma greatly exceed the number of ovules per ovary. Mating system experiments showed that the species is self‐compatible, but depended on pollinators to set seeds. Supplemental outcross pollination increased fruit set significantly. The most frequent effective pollinator Megachile terminate Morawitz, was observed pollinating many flowers of the same individual plant (74.5±1.3%). These results suggested that fruit production is affected by insufficient outcross pollen rather than by pollen quantity. Removal of 2/3 of the flowers and young pods led to significantly higher fruit set, as did addition of fertilizers (N–P–K: 0.025–0.05–0.013 g, N–P–K: 0.05–0.1–0.025 g) showing that reducing resource acceptors and increasing inorganic resources both helps to improve fruit set. We therefore conclude that reproductive success of E. songoricum is limited by both outcross pollen and available nutrients.     

Resource limitation and pollen source (self and outcross) affecting seed production in two louseworts, Pedicularis siphonantha and P. longiflora (Orobanchaceae)

Two animal-pollinated hermaphrodite plants, Pedicularis siphonantha and P. longiflora , have been used to investigate factors limiting seed production in natural populations. To evaluate the potential seed abortion due to resources limitation, seed development has been observed and seed count conducted twice. Seed production per capsule has been compared when flowers have been removed and in a control group. Open pollination has been investigated and pollen supplementation undertaken to estimate the possibility of pollen limitation. Results show that seed abortion is frequent. Stigmatic pollen load is significantly higher than ovule number per ovary under open pollination for both species. Additional self and outcross pollen did not affect seed production. Flower removal significantly increases seed production per capsule, which indicates that seed production of the studied species is limited by available resources. To detect differences in seed production between flowers pollinated by self and outcross pollen, hand pollination of bagged flowers has also been conducted in natural populations of the two Pedicularis species. Compared with open pollination, hand-pollinating self-pollen decreases, while outcross pollen increases seed production per capsule. Such results suggest that inbreeding depression in the two self-compatible species may also result in partial seed abortion under open pollination if mixed pollen is deposited on the stigma. Our results also suggest that pollen interference plays an important role in low female fertility in the two Pedicularis species.  © 2005 The Linnean Society of London, Botanical Journal of the Linnean Society , 2005, 147 , 83–89.