Pollen and resource limitations to lifetime seed production in a wild population of the endangered plant Disanthus cercidifolius var. longipes H. T. Chang (Hamamelidaceae)

Disanthus cercidifolius Maxim. var. longipes H. T. Chang, a plant species that only occurs in a few counties in Hunan, Jiangxi and Zhejiang Provinces and with a relatively small number of individuals, is ranked as a second Class endangered species for conservation in China. We have studied the effect of pollen and resources available to female reproduction, and the reproductive mechanism of “excess flowers with low fruit set” in Disanthus cercidifolius Maxim. var. longipes H. T. Chang was discussed. Results are as follows Pollen from different sources has significant effects on fruit set and seed set of Disanthus cercidifolius Maxim. var. longipes H. T. Chang. The pollen source rather than pollen numbers significantly affected reproduction of this species. In wild populations, producing one fruit needs about 54.8 flowers, and one satiation seed needs about 6.60 flowers or 83.19 ovules. After fertilizing, which was propitious to flower development, the abortion rate of flower buds was decreasing, but the flowering rate was increasing. The fruit set and seed set was also significantly increasing, while abortion rate of fruit was significantly decreasing. With the increasing percentages of cutting leaves, the fruit set decreased, but the abortion rate of fruit shows no significant differentiation among treatments. After cutting branches that were puny, broken and insectin-fested branches, the flower number seemed to be decreasing, but the fruit set and seed set all increased significantly. After removing some flowers, the fruit set was calculated with respect to the number of flowers remaining after the treatment increased with increasing of percentages of flower removal, whereas fruit set calculated with respect to the initial number of flowers remained constant, and the mean weights of per fruit and per seed all decreased significantly. Sufficient spatial or temporal heterogeneities in nutrient levels might allow limitation of seed set by resources and pollen in a natural population, while supplying resources may indirectly affect pollination by increasing attraction of the flowers to pollinators. There were very low fruit and seed sets in natural populations of Disanthus cercidifolius Maxim. var. longipes H. T. Chang. Different factors may have interacted to effect a low fruit set. A joint adoption of the “selection abortion hypothesis”, “ovary reserve hypothesis” and “male function hypothesis” seems to be the most likely explanation for the reproductive strategy of “excess flowers with few fruit sets” in Disanthus cercidifolius Maxim. var. longipes H. T. Chang. __________ Translated from Acta Ecologica Sinica, 2006, 26(2): 496–502 [译自: 生态学报]     

Pollen and resource limitations to lifetime seed production in a wild population of the endangered plant Disanthus cercidifolius var. longipes H. T. Chang (Hamamelidaceae)

Disanthus cercidifolius Maxim. var. longipes H. T. Chang, a plant species that only occurs in a few counties in Hunan, Jiangxi and Zhejiang Provinces and with a relatively small number of individuals, is ranked as a second Class endangered species for conservation in China. We have studied the effect of pollen and resources available to female reproduction, and the reproductive mechanism of “excess flowers with low fruit set” in Disanthus cercidifolius Maxim. var. longipes H. T. Chang was discussed. Results are as follows Pollen from different sources has significant effects on fruit set and seed set of Disanthus cercidifolius Maxim. var. longipes H. T. Chang. The pollen source rather than pollen numbers significantly affected reproduction of this species. In wild populations, producing one fruit needs about 54.8 flowers, and one satiation seed needs about 6.60 flowers or 83.19 ovules. After fertilizing, which was propitious to flower development, the abortion rate of flower buds was decreasing, but the flowering rate was increasing. The fruit set and seed set was also significantly increasing, while abortion rate of fruit was significantly decreasing. With the increasing percentages of cutting leaves, the fruit set decreased, but the abortion rate of fruit shows no significant differentiation among treatments. After cutting branches that were puny, broken and insectin-fested branches, the flower number seemed to be decreasing, but the fruit set and seed set all increased significantly. After removing some flowers, the fruit set was calculated with respect to the number of flowers remaining after the treatment increased with increasing of percentages of flower removal, whereas fruit set calculated with respect to the initial number of flowers remained constant, and the mean weights of per fruit and per seed all decreased significantly. Sufficient spatial or temporal heterogeneities in nutrient levels might allow limitation of seed set by resources and pollen in a natural population, while supplying resources may indirectly affect pollination by increasing attraction of the flowers to pollinators. There were very low fruit and seed sets in natural populations of Disanthus cercidifolius Maxim. var. longipes H. T. Chang. Different factors may have interacted to effect a low fruit set. A joint adoption of the “selection abortion hypothesis”, “ovary reserve hypothesis” and “male function hypothesis” seems to be the most likely explanation for the reproductive strategy of “excess flowers with few fruit sets” in Disanthus cercidifolius Maxim. var. longipes H. T. Chang.     

濒危植物长柄双花木(Disanthus cercidifolius var. longipes)叶功能性状随生长发育阶段的变化

对珍稀濒危植物长柄双花木(Disanthus cercidifolius var. longipes H. T. Chang)不同生长发育阶段的叶厚度(LT)、叶面积(LA)、叶含水量(LWC)、比叶面积(SLA)、叶片全氮含量(LNC)和叶片全磷含量(LPC)等叶功能性状及性状症候群的变化进行了研究。结果显示,不同发育阶段长柄双花木的叶功能性状存在显著差异,其中LT、LA随植株发育呈递增趋势,而LWC、SLA、LNC和LPC则呈递减趋势。主成分分析显示,随着长柄双花木的不断发育,性状症候群朝着LT、LA增大而LWC、SLA、LNC和LPC减小的方向变化。表明该物种的经济策略随着发育进程由快收益向慢收益转变。     

濒危植物长柄双花木的花部综合特征与繁育系统

 通过野外观察,运用杂交指数、花粉-胚珠比、人工授粉和套袋实验及等位酶电泳等方法,对长柄双花木(Disanthus cercidifolius Maxim. var. longipes H. T. Chang)自然种群和人工种群的开花状态、繁育系统进行了研究。结果如下：该种单花花期一般6～7 d,开花过程中柱头始终高于花药的位置,花药散粉具有先后顺序,一般2花药先散粉,其余3花药滞后1～2 d。单花花期依其形态和散粉时间可分为5个时期：即散粉前期、散粉初期、散粉盛期、凋谢期。种群开花一般历时49～55 d,但不同地点、年份之间有所差异,推测开花持续时间不同主要是环境条件所致。由于其杂交指数大于4,花粉-胚珠比等于1 250,结合人工套袋和授粉实验以及等位酶电泳结果可以确定该种的繁育系统属于异交为主,部分自交亲和,传粉过程需要传粉者。花粉竞争可能是导致该种濒危的主要的生殖生物学原因。     

长柄双花木愈伤组织诱导及植株再生

以长柄双花木当年生嫩梢上的叶柄、嫩茎、嫩叶为外植体,对影响长柄双花木愈伤组织诱导和继代、分化主要因素进行研究。结果表明:在培养基MS+NAA0.5mg/L+2,4-D2.0mg/L上,三种外植体均可诱导出愈伤组织,其中叶片愈伤组织诱导率最高。该培养基还可作为愈伤组织继代培养基,但继代培养周期不超过2周。愈伤组织接种在MS+BA2mg/L上分化不定芽,根的诱导在1/2MS+IBA0.5mg/L培养基上进行。     

Isolation and characterization of nine polymorphic microsatellite loci in the endangered shrub Disanthus cercidifolius var. longipes (Hamamelidaceae)

Nine polymorphic microsatellite loci were isolated and characterized from an AC-enriched genomic library of Disanthus cercidifolius var. longipes. Microsatellite polymorphism was investigated using 24 individuals from one natural population. The observed number of alleles per locus ranged from two to four. Observed and expected heterozygosities ranged from 0.17 to 0.92 and from 0.16 to 0.72, respectively. These polymorphic microsatellite loci provide useful tools for the ongoing population genetic studies of D. cercidifolius var. longipes.     

濒危植物长柄双花木自然种群年龄结构及其生态对策

通过对濒危植物长柄双花木自然种群的年龄结构、图解生命表以及生殖价分析等途径,研究了其种群结构动态及生态对策,结果表明:长柄双花木种群为衰退型种群,虽然种群中也存在一定数量的幼龄个体,但个体死亡率较高。长柄双花木种群表现出r～K对策的混合生态适应策略。该种群数量在下降过程中还存在波动,仍具有一定的实现生殖和恢复的可能,但依靠有性生殖途径实现恢复的能力有限。     

南岭大东山长柄双花木群落物种组成与种群结构特征分析

采用样方法以及用基径结构代替年龄结构的方法,对位于南岭大东山不同海拔的3个长柄双花木(Disanthus cercidifolius var.longipes Chang)样带(包括小洞、大洞和鸡子场样带,海拔分别为1 010、1 170和1 260 m)进行了调查,并编制了种群静态生命表和生存曲线,对其群落物种组成、种群年龄结构和生殖力现状进行了分析。结果显示:3个样带群落乔木层优势种数量及种类组成差异明显,共包含26个种类,但共有种仅3种,除长柄双花木外还有多花杜鹃(Rhododendron cavaleriei Lévl.)和两广杨桐(Adinandra glischroloma Hand.-Mazz.)2种;小洞样带长柄双花木种群的重要值(46.116%)明显高于另2个样带。根据基径可将长柄双花木种群分为68个龄级,3个样带的种群年龄结构基本一致,但小洞样带个体数量更多且基径31.0 mm以下的幼体所占比例最高;总体上长柄双花木种群中5 a以下的幼苗数量偏少,其他龄级的个体数量呈现随株龄增大波动但整体下降的趋势。种群的个体死亡率在5~10 a和10~15 a龄级较高(分别为20.30%和20.50%),并随年龄增长总体呈逐渐下降的趋势;且不同龄级个体生命期望值为4.49~0.50,总体偏低。种群生存曲线为Ⅰ型,种群净增殖率、内禀增长率、周限增长率和世代平均周期分别为0.504、-0.018、0.982和38.351 a,显示该种群在当前状态下自我更新不良,属缓慢的负增长型种群。根据研究结果,对该区域长柄双花木种群提出了相应的保护对策和建议。     

Testing hypotheses for excess flower production and low fruit-to-flower ratios in a pollinating seed-consuming mutualism

Pollinator attraction, pollen limitation, resource limitation, pollen donation and selective fruit abortion have all been proposed as processes explaining why hermaphroditic plants commonly produce many more flowers than mature fruit. We conducted a series of experiments in Arizona to investigate low fruit-to-flower ratios in senita cacti, which rely exclusively on pollinating seed-consumers. Selective abortion of fruit based on seed predators is of particular interest in this case because plants relying on pollinating seed-consumers are predicted to have such a mechanism to minimize seed loss. Pollinator attraction and pollen dispersal increased with flower number, but fruit set did not, refuting the hypothesis that excess flowers increase fruit set by attracting more pollinators. Fruit set of natural- and hand-pollinated flowers were not different, supporting the resource, rather than pollen, limitation hypothesis. Senita did abort fruit, but not selectively based on pollen quantity, pollen donors, or seed predators. Collectively, these results are consistent with sex allocation theory in that resource allocation to excess flower production can increase pollen dispersal and the male fitness function of flowers, but consequently results in reduced resources available for fruit set. Inconsistent with sex allocation theory, however, fruit production and the female fitness function of flowers may actually increase with flower production. This is because excess flower production lowers pollinator-to-flower ratios and results in fruit abortion, both of which limit the abundance and hence oviposition rates, of pre-dispersal seed predators.     

华北驼绒藜自然种群结实的花粉和资源限制

运用人工授粉、补充无机营养、去叶处理以及疏花处理研究了华北驼绒藜自然种群花粉和资源有效性对结实的影响,并进一步探讨了该植物的选择性败育现象和繁殖对策。结果表明:花粉来源而不是花粉数量对华北驼绒藜结实存在显著影响;同枝授粉结籽率和结实率均低于自然授粉,异株授粉则显著高于同枝授粉和自然授粉;异株授粉种子千粒重显著高于自然授粉和同株异枝授粉,极显著高于同枝授粉。华北驼绒藜结实存在资源限制,补充施肥提高了单枝开花数和结籽率。随摘除叶片数量的增加,结籽率明显降低,摘除叶片处理与对照之间均形成显著差异,摘除1/2叶、3/4叶以及去全叶处理使种子千粒重显著低于对照。人工疏花实验表明华北驼绒藜存在选择性败育现象。华北驼绒藜可根据花粉和资源的可利用性来调整性分配、授粉方式和结实以获得最大适合度,具有复杂的繁殖对策,表现出较好的环境适应性。     

Variation in Floral Sex Allocation and Reproductive Success in Sequentially Flowering Inflorescence of Corydalis remota var. lineariloba (Fumariaceae)

In hermaphroditic plants, female reproductive success often varies among different positions within an inflorescence.However, few studies have evaluated the relative importance of underlying causes such as pollen limitation, resource limitation or architectural effect, and few have compared male allocation. During a 2-year investigation, we found that female reproductive success of an acropetally flowering species, Corydalis remota Fisch. ex Maxim. var. Iineariloba Maxim. was significantly lower in the upper late developing flowers when compared with the lower early flowers. Supplementation with outcross pollen did not improve female reproductive success of the upper flowers, while removal of the lower developing fruits significantly increased female reproductive success of the upper flowers in both years, evidencing resource limitation of the upper flowers. Female production in upper flowers was greatly improved by simultaneous pollen supplementation of the upper flowers and removal of the lower fruits, suggesting that, when resources are abundant, pollen may limit the female reproductive success of the upper flowers. The less seed mass in the upper flowers didn't increase in all treatments due to architecture. In the upper flowers, ovule production was significantly lower and the pollen : ovule ratio was significantly higher. These results suggest that male-biased sex allocation in the upper flowers may lead to increased male reproductive success, whereas the lower flowers have higher female reproductive success.     

Causes of Fruit Set Variation in Polygonatum odoratum (Liliaceae)

Abstract: In Polygonatum odoratum (Liliaceae), flowers located distally within the flowering shoot typically exhibit lower fruit set than flowers located proximally. In this study, experiments were performed to investigate whether the lower fruit set of distal flowers is due to pollen limitation, resource competition or "architectural" effects. We found that (1) fruit set and mean number of ovules per flower decline within the flowering shoot from proximal to distal position; (2) variations in the amount of pollen received do not explain the observed differences in fruit set; (3) the fruit set of the most distal flowers was significantly increased by removal of other flower buds in the flowering shoot just before flower opening, indicating that fruit set in these flowers is resource-limited; and (4) removal of proximal flowers increased the fruit set of later flowers to levels similar to those of early flowers of control shoots, arguing against the architectural effects hypothesis.     

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.     

A Top-Down Hierarchy in Fruit Set on Inflorescences in Iris fulva (Iridaceae)

Abstract: In Iris fulva, the apical flower on an inflorescence opens first, and the flowering sequence then proceeds from the most basal flower upward to the apex (acropetally). Flowering order and flower position are thus partially decoupled, and this provides an opportunity to separately investigate the effects of these two factors upon fruit formation. We recorded natural patterns of fruit set and seed production, and found that fruit set patterns were determined by what appears to be a form of apical dominance. The first, apical flower had the highest fruit set and seed production. Fruit set decreased towards the base of the inflorescence, with later-opening flowers having a higher fruit set. This is contrary to the pattern usually observed in other acropetally flowering plants, which is a higher fruit set for basal, early flowers. By performing additional hand pollinations and counting pollen grains on naturally pollinated flowers, we found that pollen deposition was not a major factor limiting fruit set, and that it could not explain the large difference in fruit set between the apical and basal flowers. Removing the first flower after it had wilted increased fruit set in the remaining flowers, but mainly in the more apical flowers. Only by removing the two topmost flowers could we obtain an increase in fruit set for the basal flower. The fact that the basal flower rarely sets fruit, despite being closer to resources, suggests that the apical meristem is either a strong sink for resources or produces hormones that form a gradient along the inflorescence, which is comparable with apical dominance.     

FLOWER,FRUIT AND SEED ABORTION IN TROPICAL FOREST TREES: IMPLICATIONS FOR THE EVOLUTION OF PATERNAL AND MATERNAL REPRODUCTIVE PATTERNS

Flower, fruit and immature seed abortion was studied in seven self-incompatible species of trees in a tropical lowland semideciduous forest. The species showed considerable variability in fruit and seed set and the rate at which flowers and fruits were aborted. The amount of flower and fruit abortion also varied over time within species. Small samples of open-pollinated flowers in three species showed adequate amounts of pollen on the stigma, but it could not be determined whether the pollen was compatible or incompatible. In a species with multiseeded fruits, the aborted fruits contained significantly fewer seeds than those retained on the plant. Position of fruit within the inflorescence and of seed within the ovary also had a marked effect on abortion: fruits and seeds at certain positions had a higher probability of abortion than those at other positions. Experiments to test the effect of pollen source on abortion were inconclusive. The factors underlying abortions were evaluated in the context of three mutually non-exclusive hypotheses. It is concluded that selection for increased pollen dispersal and uncertainty in paternity of the zygotes are major factors underlying abortions.     

Floral sex allocation and reproductive success within inflorescences of Hosta ventricosa,a pseudogamous apomict

Aims Within inflorescences of sexually reproducing hermaphrodites, the production of ovules, fruits and seeds commonly declines from basal (early-opening) to distal (late-opening) flowers, while pollen production remains constant or only changes slightly, with the result that distal (late-opening) flowers become functionally male. However, few empirical studies have specifically examined whether or not changes in allocation to pollen production actually lead to changes in the number of seeds sired, a more direct measure of male fitness. In pseudogamous apomicts, fitness depends on the number of seeds produced; thus, a contrasting pattern of variation in the pollen-to-ovule (P/O) ratio within inflorescences might be expected.Methods We investigated floral sex allocation and reproductive success within racemes of Hosta ventricosa, a pseudogamous apomictic hermaphrodite possessing flowers that open acropetally. We quantified variations in pollen number, ovule number, the P/O ratio and fruit and seed production, from 2007 to 2011, among flowers within racemes of four populations of H. ventricosa in southwest China. Ecological causes for fruit and seed production were evaluated by observing patterns of pollen deposition, flower removal and supplemental pollination.Important findings Pollen number, ovule number and the P/O ratio declined from basal-to-distal positions in all sampled populations (years). Fruit and seed production decreased distally in most populations (years). Low fruit and seed set of distal flowers was not due to pollen limitation because pollen deposition never declined distally and the low fruit and seed set of distal flowers remained even after supplementary pollination was provided. The flower-removal experiment indicated that inter-fruit competition for resources among flowers was common. The low P/O ratio of distal flowers in H. ventricosa might be favored because they were unable to obtain fitness by donating pollen and siring seeds on other plants. Our study may help to understand the adaptive significance of sex allocation among flowers within inflorescences of sexually reproducing hermaphrodites.     

Pollen tube Growth and Self-Incompatibility in Heuchera micrantha var. diversifolia (Saxifragaceae)

A crossing study and an analysis of pollen tube growth were conducted in diploid and autotetraploid Heuchera micrantha var. diversifolia to distinguish between the possible mechanisms that could promote the high outcrossing rates observed and that could also result in the absence of fruit set following self-fertilization. The crossing study indicated that no fruit set occurred after self-pollination, whereas fruit set occurred in all of the hand-pollinated outcrosses. After 4 d, the self-pollinated flowers shriveled and abscised. Pollen tube growth following hand pollination was assessed in selfed and outcrossed flowers using fluorescence microscopy. The self-pollinated flowers exhibited far fewer pollen tubes than did the outcrossed flowers. Furthermore, in self-pollinated flowers, some of the pollen tubes extended into the style; fewer than one-half of the pollen tubes reached the base of the style and still fewer reached the ovules. The variable length of pollen tube growth, the uniform timing of floral abortion after self-pollination, and the absence of variability among individuals in the level of fruit set following self-pollination are all consistent with a system that lies somewhere between classic gametophytic self-incompatibility and late-acting self-incompatibility as the mechanism that is most likely operating in H. micrantha var. diversifolia. A similar "nonstandard" system may be present in other Saxifragaceae, such as Tolmiea and Lithophragma, as well as in Ribes, the sister group of Saxifragaceae. Our data also indicate that ploidal level (diploid vs. autotetraploid) has no influence on the extent or mechanism of self-incompatibiltiy in autopolyploid H. micrantha var. diversifolia.     

敦煌‘李光杏’花芽发育过程中雌蕊败育观察及生物学特性调查

为克服杏生产中雌蕊败育现象、提高坐果率,该研究以甘肃省敦煌市的‘李光杏’盛果期不同树势(强势树、中庸树和弱势树)和不同类型结果枝(花束状果枝、短果枝、中果枝、长果枝)的花芽为试材,对其开花物候期及生物学特性进行调查,采用石蜡切片观察易发生败育时期(花芽分化初期、雌蕊分化期、花粉细胞期和花蕾膨大期)的花芽内部组织结构形态,同时测定各主要败育时期叶片可溶性糖、淀粉含量及矿质元素的变化。结果表明:(1)弱树花芽的平均败育率最高(91.26%),中庸树最低(71.08%);不同类型结果枝中,花束状果枝花芽的败育率最低,长果枝的败育率最高。(2)不同结果枝类型的花粉生活力表现为花束状果枝>短果枝>中果枝>长果枝,花粉发芽率表现为短果枝>花束状果枝>中果枝>长果枝。各花型花粉生活力表现为雌蕊高于雄蕊>雌雄等长>雌蕊低于雄蕊>无雌蕊,发芽率的表现为雌雄等长>雌蕊高于雄蕊>雌蕊低于雄蕊>无雌蕊。(3)各树势花芽雌蕊分化期叶片可溶性糖与淀粉含量显著低于其他时期;叶片P、K、Ca含量在分化前期较高,但是雌蕊、雄蕊分化期显著下降。(4)与正常花相比,败育花主要表现为子房发育异常,多数生长点不均匀,子房萎缩,胚珠原基发育停滞;雌蕊的花柱低于花丝,且花粉粒急剧减少。研究发现,‘李光杏’树势的强弱会造成败育花比例和坐果率的差异,并以中庸树果实坐果率最高,且花败育率最低;雌蕊发育是否正常直接关系到‘李光杏’正常开花,雌蕊的发育受阻,最终成为整个花芽退化的主要形式;叶片碳水化合物和矿质元素等营养物质参与并保证了花器官的正常发育。     

井冈山长柄双花木种群形态分化的数量分析

利用多种数量分析方法研究了长柄双花木种群形态分化状况,结果表明：长柄双花木5个种群的53个个体的20项形态特征中,只有7项性状的变异达显著水平,其中2个性状达极显著水平,方差分析及多种聚类分析结果都表明,总体形态分化在个体和种群水平上均未达到显著,形态分化与环境因子的灰色关联分析结果表明,在所引用的环境因子中,无一可起绝对主导作用,不同的环境因子对同一性状所起的作用不同,且同一环境因子对不同性状的影响效应也有差异,影响其生长的潜在主导因子是海拔高度,土壤pH值和乔木层透光率。     

Plant size,geitonogamy and seed set in Ipomopsis aggregata

Summary We used powdered fluorescent dyes to estimate receipt of self vs. outcross pollen in the self-incompatible species Ipomopsis aggregata (Polemoniaceae). Flowers on small and large plants received equal amounts of outcross pollen, whereas flowers on large plants received more self pollen, so the proportion of self pollen delivered through geitonogamy increased with plant size. In natural populations emasculation of all flowers on a plant raised average seed set per flower from 5.19 to 6.99 and also raised fruit set, though not significantly. From these results one expects a negative correlation between plant size and seeds per flower. The opposite trend was observed in a sample of plants in the field, suggesting that deleterious effects of geitonogamy on female fecundity in large plants can be overruled by other factors such as size-related fruit or seed abortion. Results are discussed in relation to the evolution of gynodioecy.