青杨雌雄群体沿海拔梯度的分布特征

以小五台山天然青杨种群为试验对象,通过在其分布海拔范围内(1400-1700 m)设置样方的方式研究青杨雌雄群体的平均胸径、密度、性比、大小级结构和空间分布的差异.结果显示:雌株群体的平均胸径在整个海拔梯度上无显著差异,而海拔1700 m处的群体密度显著低于其它海拔;雄株群体的平均胸径在海拔1700 m最大,显著高于其它海拔,而群体密度在各海拔梯度间无显著差异.在海拔1600 m,雌雄群体的平均胸径最接近,同时密度也最接近.从整个海拔范围来看,青杨雌雄个体的比例(雄/雌)为0.80∶1,性比不偏离1∶1(x2=2.94,P＞0.05).但在不同海拔梯度上性比有所不同,低海拔性比为0.44,显著偏雌(X21400=5.91,P＜0.05),而高海拔性比为2.55,显著偏雄(x21700=6.56,P＜0.05);随着海拔接近1600 m,性比逐渐趋于1:1(x21600=0,P＞0.05).除海拔1700 m为下降型外,青杨雌雄群体的大小级结构在其它海拔主要表现为稳定型.而最稳定群体结构的分布海拔在雌雄群体间不同.在海拔1400 m,雌株群体中I级和Ⅱ级植株占群体比例最大;雄株群体I级和Ⅱ级植株占群体比例最大的海拔是1600 m.群体空间分布以聚集分布为主,彼此间无明显差异;但群体间聚集强度的变化在海拔梯度上各有不同,主要表现为雌株群体的PAI随海拔的升高逐渐增加,而雄株群体的PAI则随海拔的升高呈现出先增加后减少的趋势.研究结果表明了青杨雌雄群体的分布特征沿海拔梯度的变化明显不同,中等海拔区域可能为青杨种群的最适繁衍区.     

小五台山青杨雌雄植株树轮生长特性及其对气候变化的响应差异

近年来逆境导致植物雌雄幼苗的生长出现差异被许多控制实验所证实, 而有关气候变化对雌雄异株植物成树生长的潜在影响尚未引起人们广泛的关注。为进一步揭示气候变化对雌雄植株树木径向和密度生长的不同影响, 该文通过树轮生态学的研究方法, 选择小五台山天然青杨(Populus cathayana)种群为研究对象, 对青杨雌雄植株近30年(1982-2011)的树轮生长特性及其与气候的相关性进行了分析。结果显示: 1)在近30年当地气温不断升高的气候条件下, 雌株的年轮最大密度和晚材平均密度均高于雄株(p < 0.05), 但雌雄植株的径向生长无显著差异; 2)雌雄植株年轮最大密度和宽度差值年表的变化趋势具有一致性, 但在年轮最大密度差值年表的变化上雄株波动幅度大于雌株; 3)青杨雌雄植株年轮密度差值年表对温度响应的月份明显不同。雌株年轮最大密度与当年8月的月平均最高气温显著正相关, 而雄株年轮最大密度与当年1月和4月的气温负相关; 4)生长季前的气候变化对青杨雌雄植株的径向生长均有明显的限制作用。此外, 当年6月的高温对于早材生长的限制作用特别明显。上述结果表明, 雌雄异株植物在树木年轮生长方面对全球气候变暖可能具有不同的响应机制, 雌株比雄株更侧重于密度生长。     

Gender-specific characteristics of tree-ring growth and differential responses to climate change in the dioecious tree Populus cathayana in Xiaowutai Mountains,China

近年来逆境导致植物雌雄幼苗的生长出现差异被许多控制实验所证实, 而有关气候变化对雌雄异株植物成树生长的潜在影响尚未引起人们广泛的关注。为进一步揭示气候变化对雌雄植株树木径向和密度生长的不同影响, 该文通过树轮生态学的研究方法, 选择小五台山天然青杨(Populus cathayana)种群为研究对象, 对青杨雌雄植株近30年(1982-2011)的树轮生长特性及其与气候的相关性进行了分析。结果显示: 1)在近30年当地气温不断升高的气候条件下, 雌株的年轮最大密度和晚材平均密度均高于雄株(p < 0.05), 但雌雄植株的径向生长无显著差异; 2)雌雄植株年轮最大密度和宽度差值年表的变化趋势具有一致性, 但在年轮最大密度差值年表的变化上雄株波动幅度大于雌株; 3)青杨雌雄植株年轮密度差值年表对温度响应的月份明显不同。雌株年轮最大密度与当年8月的月平均最高气温显著正相关, 而雄株年轮最大密度与当年1月和4月的气温负相关; 4)生长季前的气候变化对青杨雌雄植株的径向生长均有明显的限制作用。此外, 当年6月的高温对于早材生长的限制作用特别明显。上述结果表明, 雌雄异株植物在树木年轮生长方面对全球气候变暖可能具有不同的响应机制, 雌株比雄株更侧重于密度生长。     

河北小五台山青杨种群中雌雄群体的空间分布及其成因

为揭示雌雄异株植物种群的空间分异现象的内在原因, 以河北小五台山天然青杨(Populus cathayana)种群为研究对象, 采用平均最近邻指数、核密度、方差分析等方法分析了青杨雌雄群体的聚集程度、群体密度和分布格局以及其与到河岸的距离(河距)、海拔和土壤理化性质间的内在关系。结果显示: 1)青杨种群总体上沿金河沟呈狭长带状集聚分布, 中海拔区域个体数量最多, 密度最大; 2)随海拔增加, 雌雄个体数与密度均表现“先增后减”的特点, 而平均胸径的变化正好相反; 3)海拔对性比具有显著影响: 低海拔区域(1 300-1 400 m)偏雌, 中低海拔区域(1 400-1 500 m)偏雄, 其他海拔段的性比接近1:1; 4)随河距增加, 雌雄个体数、密度和平均胸径均表现出逐渐减少或减小的特点, 并且雌株优势分布区内的土壤有机质和全氮含量显著高于雄株优势分布区; 5)局部区域由于海拔和河距改变了土壤理化性质和生长环境, 导致了该区域内的性比发生偏倚。上述结果表明, 青杨雌雄群体的空间分布、动态变化以及性比受海拔、河距以及所在区域的土壤理化性质的综合影响, 雌雄植株在资源需求和环境适应力方面存在的差异会导致局部区域内的性比发生偏倚。     

Spatial distribution of male and female Populus cathayana populations and its drivers in Xiaowutai Mountains,Hebei, China

为揭示雌雄异株植物种群的空间分异现象的内在原因, 以河北小五台山天然青杨(Populus cathayana)种群为研究对象, 采用平均最近邻指数、核密度、方差分析等方法分析了青杨雌雄群体的聚集程度、群体密度和分布格局以及其与到河岸的距离(河距)、海拔和土壤理化性质间的内在关系。结果显示: 1)青杨种群总体上沿金河沟呈狭长带状集聚分布, 中海拔区域个体数量最多, 密度最大; 2)随海拔增加, 雌雄个体数与密度均表现“先增后减”的特点, 而平均胸径的变化正好相反; 3)海拔对性比具有显著影响: 低海拔区域(1 300-1 400 m)偏雌, 中低海拔区域(1 400-1 500 m)偏雄, 其他海拔段的性比接近1:1; 4)随河距增加, 雌雄个体数、密度和平均胸径均表现出逐渐减少或减小的特点, 并且雌株优势分布区内的土壤有机质和全氮含量显著高于雄株优势分布区; 5)局部区域由于海拔和河距改变了土壤理化性质和生长环境, 导致了该区域内的性比发生偏倚。上述结果表明, 青杨雌雄群体的空间分布、动态变化以及性比受海拔、河距以及所在区域的土壤理化性质的综合影响, 雌雄植株在资源需求和环境适应力方面存在的差异会导致局部区域内的性比发生偏倚。     

丛枝菌根真菌对不同性别组合模式下青杨雌雄植株根系生长的影响

丛枝菌根真菌(AMF)对雌雄异株植物根系生长, 尤其是对邻近生长的不同雌雄个体的影响还鲜有研究。该研究以泥土:河沙:蛭石体积比为1:1:1的混合物为培养基质, 分别在雄-雄、雌-雌和雄-雌3种组合栽培模式下对青杨(Populus cathayana)雌雄幼苗进行接种和不接种摩西球囊霉(Funneliformis mosseae)处理, 通过比较接种AMF与否雌雄植株根系在侵染率、生物量、形态、碳、氮含量等方面的差异来分析AMF对青杨雌雄幼苗根系生长发育的影响。结果发现: 与对照组相比, 接种AMF对3种栽培模式下青杨雌雄植株的侵染率、根干质量、根系形态(除分枝强度、比表面积)和碳、氮含量影响显著。此外, 不同性别组合模式对青杨雌雄植株的根干质量、根系形态和碳、氮含量影响显著。接种AMF后, 与雌-雌合栽模式下的雌株相比, 雄-雌合栽模式下雌株的根干质量、氮含量都有不同程度的提高, 根系形态发生改变; 而与雄-雄合栽模式下的雄株相比, 雄-雌合栽模式下雄株的相应指标出现降低或轻微增加。该研究表明AMF对不同性别组合模式下青杨植株根系生长具有显著促进作用, 尤其是雄-雌合栽模式下AMF接种最有利于雌株根系的生长发育。     

丛枝菌根真菌对不同性别组合模式下青杨雌雄植株根系生长的影响

丛枝菌根真菌(AMF)对雌雄异株植物根系生长,尤其是对邻近生长的不同雌雄个体的影响还鲜有研究。该研究以泥土:河沙:蛭石体积比为1:1:1的混合物为培养基质,分别在雄-雄、雌-雌和雄-雌3种组合栽培模式下对青杨(Populus cathayana)雌雄幼苗进行接种和不接种摩西球囊霉(Funneliformis mosseae)处理,通过比较接种AMF与否雌雄植株根系在侵染率、生物量、形态、碳、氮含量等方面的差异来分析AMF对青杨雌雄幼苗根系生长发育的影响。结果发现:与对照组相比,接种AMF对3种栽培模式下青杨雌雄植株的侵染率、根干质量、根系形态(除分枝强度、比表面积)和碳、氮含量影响显著。此外,不同性别组合模式对青杨雌雄植株的根干质量、根系形态和碳、氮含量影响显著。接种AMF后,与雌-雌合栽模式下的雌株相比,雄-雌合栽模式下雌株的根干质量、氮含量都有不同程度的提高,根系形态发生改变;而与雄-雄合栽模式下的雄株相比,雄-雌合栽模式下雄株的相应指标出现降低或轻微增加。该研究表明AMF对不同性别组合模式下青杨植株根系生长具有显著促进作用,尤其是雄-雌合栽模式下AMF接种最有利于雌株根系的生长发育。     

胡杨茎叶化学计量和异速生长关系特征随发育阶段变化的性别差异

胡杨(Populus euphratica Oliv.)是荒漠河岸林防风固沙和水土保持的雌雄异株树种。该研究以不同径阶(8、12、16、20 cm)的胡杨雌雄株为研究对象,通过当年生茎、叶化学计量元素(C、N、P、K)含量及生长关系分析,探讨不同器官化学计量随发育阶段的变化及异速生长关系的性别差异。结果表明：(1)胡杨雌雄株叶片C含量表现为大径阶(20 cm)显著高于小径阶(8 cm),而其叶片N含量随着径阶的增加显著增加;雌雄株茎、叶化学计量随着径阶的增加总体上呈增加趋势,且C、N含量均与径阶呈显著正相关关系;随着径阶的增加,雌株叶片P含量呈下降趋势,与径阶呈显著负相关关系,而雌雄株茎的P、K含量呈上升趋势,且与径阶呈显著正相关关系。(2)雌株各径阶叶片的N含量及8、12、20 cm径阶的叶片P含量均显著大于相应雄株,8、16、20 cm径阶当年生茎C含量以及20 cm径阶茎N、P含量均显著高于相应雄株。(3)雌株叶片C与N在20 cm径阶的斜率指数最大,而雄株在12 cm径阶斜率指数最大,雌雄株在各发育阶段N与P的变化较稳定;在相同C含量时,雄株茎能获得更多的N含量,雌株茎在相同N的情况下能获得更多P元素。研究发现,胡杨雌雄株间茎、叶化学计量元素含量和异速生长关系特征在不同发育阶段存在着明显性别差异,成熟雌株叶片需要更多的化学计量特征含量来满足生殖需求,总体反映了自身生长及环境适应的养分分配策略。     

去叶对青杨雌雄植株生长和非结构性碳水化合物的影响

近年来,森林食叶害虫在全世界呈爆发趋势。树木的非结构性碳水化合物(NSC)如何响应叶片损失对其生长和生存至关重要。雌雄异株植物在维持森林生态系统稳定性方面扮演着重要角色。然而,目前对该类植物性别之间如何响应去叶的研究还比较少。本文以我国重要的经济和生态恢复树种青杨(Populus cathayana)为研究材料,比较了雌雄青杨幼苗的生长、NSC含量和储量对去叶(0,50%和100%叶片去除)的响应差异。结果表明:随去叶强度的增加,植物的生物量和植株NSC呈降低趋势,且根系(尤其是粗根)的生物量和NSC比地上部分受去叶的影响更大;雌株叶、粗根、细根和植株NSC储量总是高于雄株;随去叶强度增加,雄株的生物量积累和NSC含量和储量降低得比雌株更多。这些结果表明,青杨雌雄植株生长和NSC对不同去叶强度的响应存在性别差异,且去叶对青杨雄株的影响更大。这暗示了雌性青杨对去叶的耐性比雄性强。这些结果有助于理解雌雄异株植株性别水平上的碳平衡机理,也可为杨树人工林的选育提供支撑。     

铅污染对青杨雌雄幼苗铅富集和营养特征的影响

该研究以青杨雌雄幼苗为实验材料,采用盆栽控制实验的方式,设置20.5(CK)、50(T1)和100mg·kg-1(T2)3个铅浓度水平,分析面源水平铅污染条件下青杨性别间铅的积累和营养状况的差异。结果表明:(1)与CK相比,面源水平铅污染条件下,青杨雄株幼苗的总生物量未受到显著影响,雌株总生物量显著降低,但相同处理条件下雌株生物量均显著高于雄株。(2)2个梯度的铅污染均未显著影响两种性别青杨各器官中N的浓度,且雌雄间的性别差异不显著,但T2处理一定程度降低了两种性别各器官中P的浓度;与CK相比,雄株在受铅胁迫处理时N/P逐渐增加,说明此时雄株的生长可能会逐渐受到P素营养缺乏的影响。(3)随着铅处理浓度的增加,青杨雌株幼苗对铅的积累总量逐步增加,但雄株的积累总量的增加却不显著。可见,青杨雄株在面源水平铅污染条件下的抗性和耐受性优于雌株;雌雄青杨对土壤铅污染有不同的适应策略,雌株对铅的积累策略是其生物量下降以及营养平衡受到影响的主要原因,雄株对铅低吸收的策略使其更适应在铅污染区域生长。     

不同性别邻体和土壤灭菌对青杨幼苗生物量的影响

尽管植物邻体关系受到广泛关注,但关于雌雄异株植物性别间邻体关系的研究还较少。本研究比较了雌、雄青杨幼苗生物量的积累对其邻株性别和土壤灭菌的响应差异,并分析了性别间的邻体效应关系及不同性别混栽下生物量的差异。结果表明: 同性邻株模式降低了青杨雌株或雄株生物量的积累,而异性邻株模式在未灭菌条件下促进了雌株的生物量积累;土壤灭菌抑制了雌雄株生物量的积累,尤其是雌株;灭菌对同性邻体效应无显著影响,但异性邻株对雌株的效应由正相关关系变为中性相关,对雄株的效应由中性关系变为负相关;在所有组合中,未灭菌下的雌雄组合生物量最高,而灭菌下的雄雄组合生物量最低。雌雄青杨的生长对邻株性别的响应不同,且邻体效应受土壤微生物的影响。研究结果可为高产杨树人工林的栽培提供支撑。     

淹水胁迫对青杨雌雄幼苗生理特性和生长的影响

为揭示青杨(Populus cathayana)雌雄幼苗对淹水胁迫的适应性, 在实验地内通过土培盆栽淹水方式从植株生理生态和生长发育方面探讨淹水胁迫对青杨扦插苗的影响。试验分为对照和淹水2个处理, 处理时间为40天。结果显示: (1)淹水胁迫导致青杨幼苗叶片中的丙二醛(MDA)含量和茎部淹水区的不定根数显著升高, 植株的净光合速率(P_n)、叶绿素含量、超氧化物歧化酶(SOD)活性、株高、基径、总叶面积、比叶面积(SLA)、根生物量、叶生物量、茎生物量、总生物量干重和根冠比(R/S)显著降低。(2)与雄株相比, 淹水胁迫显著增加了雌株幼苗的MDA含量, 降低了SOD活性、P_n、类胡萝卜素(Caro)含量、叶绿素a/b、SLA、根生物量和R/S, 并导致雄株在淹水胁迫下具有比雌株更高的气孔导度(G_s)、胞间CO₂浓度(C_i)、蒸腾速率(T_r)、不定根数和株高。可见, 淹水胁迫对青杨雌雄幼苗的形态生长和生理过程均有严重的抑制作用, 但表现出显著的性别间差异。雄株可以通过维持更高的光合作用能力和增加不定根数量来维持植株的生长, 从而表现出比雌株更强的抗逆性。     

云南铁杉开花结实特性及其与环境因子的关系

以进入生殖阶段的8个天然云南铁杉(Tsuga dumosa)种群为对象, 对开花、结实植株(24个样株)的生长量进行测定, 并采用标准枝法对不同树冠层次与方位的开花及结实数量进行调查, 研究了开花结实规律和对其有主要影响的环境因子。结果表明: 云南铁杉雌、雄球花比为1:2, 雌球花转化为果实的数量仅占28%, 胸径为7 cm (树龄约25年)的植株最早进入开花结实阶段, 胸径为25-30 cm的植株开花结实量最高, 胸径为89 cm (树龄约200年)的植株开花结实最晚; 树冠不同层次与方位上开花结实数量有着明显的差异; 相关性分析得出植株胸径与开花结实量极显著相关(p < 0.01)、与冠幅显著相关, 生境中林分郁闭度与开花结实量、株高、胸径之间均呈负相关关系, 其中与结实量的相关性达显著水平; 主成分分析得出植株开花及结实主要受到温度因子、水分因子、光照因子协同作用的影响。云南铁杉雌雄球花的比例偏雄性、结实周期长、结实方式不经济、灾害性天气的影响是造成该种群逐渐走向濒危的主要原因。因此, 采用人工促进天然更新、建立母树林和种子园以提高种子的质量, 对云南铁杉种群的恢复和保护具有重要意义。     

Comparative proteomics analysis of salt response reveals sex-related photosynthetic inhibition by salinity in Populus cathayana cuttings

Male and female poplar ( Populus cathayana Rehd.) cuttings respond differently to salinity stress. To understand these differences better, comparative morphological, physiological, and proteomics analyses were performed. Treatments with different concentrations of NaCl applied to male and female poplar cuttings for 4 weeks showed that females reacted more negatively at the morphological and physiological levels than did males, visible as shriveled leaves, decreased growth, lowered photosynthetic capacities, and greater Na(+) accumulation. The proteome analysis identified 73 proteins from 82 sexually related salt-responsive spots. They were involved in photosynthesis, protein folding and assembly, synthesis and degradation, carbon, energy and steroid metabolism, plant stress and defense, redox homeostasis, signal transduction, and so forth. The sex-related changes of these proteins were consistent with the different morphological and physiological responses in males and females. In conclusion, the higher salt resistance of male P. cathayana cuttings is related to higher expression and lower degradation of proteins in the photosynthetic apparatus, more effective metabolic mechanism and protective system, and greater capacity of hydrogen peroxide scavenging. This research allows us to further understand the possible different management strategies of cellular activities in male and female Populus when confronted by salt stress.     

Production of male flowers does not decrease with plant size in insect‐pollinated Sagittaria trifolia,contrary to predictions of size‐dependent sex allocation

Abstract It has been proposed that relative allocation to female function increases with plant size in animal‐pollinated species. Previous investigations in several monoecious Sagittaria species seem to run contrary to the prediction of size‐dependent sex allocation (SDS), throwing doubt on the generalization of SDS. Plant size, phenotypic gender, and flower production were measured in experimental populations of an aquatic, insect‐pollinated herb Sagittaria trifolia (Alismataceae) under highly different densities. The comparison of ramets produced clonally can reduce confounding effects from genetic and environmental factors. In the high‐density population, 48% of ramets were male without female flowers, but in the low‐density population all ramets were monoecious. We observed allometric growth in reproductive allocation with ramet size, as evident in biomass of reproductive structures and number of flowers. However, within both populations female and male flower production were isometric with ramet size, in contrast to an allometric growth in femaleness as predicted by SDS. Phenotypic gender was not related to ramet size in either population. The results indicated that large plants may increase both female and male function even in animal‐pollinated plants, pointing towards further studies to test the hypothesis of size‐dependent sex allocation using different allocation currencies.     

Female mating success and risk of pre-reproductive death in a protandrous grasshopper

Numerous studies have assessed the adaptive value of protandry for males in several insect species, considering that male emergence is determined by female availability. However, the possible advantage of the time of emergence for females on their mating success in protandrous insect species has only been explored theoretically. By studying the grasshopper Sphenarium purpurascens we evaluated the hypothesis that late emergence could be adaptive for females. If female maturation occurs when the population density is higher and the sex ratio (males/females) is biased to males, their probability of mating increases. Thus, in this study we estimated (1) the opportunity for mating in females as a function of their sexual maturation time, population density, and sex ratio at the moment they reached sexual maturity. In addition, (2) an analysis incorporating female body size and the total number of female matings was performed. Both analyses support the occurrence of protandry in the studied population. Under the first approach, females with intermediate maturation time had a higher probability of being mated than earlier and late matured females. Thus, it suggests that stabilising selection is acting on female maturation time and this may affect selection on male maturation time. Furthermore, the proportion of mated females increased when the sex ratio was biased to males, and stabilising selection on maturation time was detected also. However, the number of matings of a female depended on her body size. Females with larger body size had more matings than smaller ones at the beginning of the reproductive season. Because selection acts differently on maturation time in males and females of S. purpurascens this result is consistent with a condition for the maintenance of protandry in the population. The present results are discussed in the light of the models for the evolution of protandry.     

Effects of social environment on sexual maturation in female cotton rats (Sigmodon hispidus)

Age at sexual maturation among female cotton rats was measured in a variety of intraspecific social environments. Two experiments were conducted. In Experiment I, female cotton rats attained vaginal perforation and first estrus at younger ages and lighter body masses when paired from weaning with a conspecific juvenile male than when caged alone. In Experiment II, these findings were replicated and extended. Females housed with juvenile males matured at the youngest ages, while those housed alone matured at the oldest ages. Females housed with adult males matured at intermediate ages. Presence of a second juvenile female during maturation was significantly associated with early vaginal opening but not with early first estrus. The results of this study are discussed in context of similar social environmental effects on female sexual maturation that have been identified in other rodent species.     

Variation in floral sex allocation in Polygonatum odoratum (Liliaceae)

BACKGROUND AND AIMS: It is well known that resource allocation to male and female functions can be highly variable in hermaphroditic plants. The purpose of this study was to investigate variations in sexual investment at different levels (flower, plant and population) in Polygonatum odoratum, a plant with sequentially opening flowers. METHODS: Pollen and ovule production in base, middle and top flowers of P. odoratum flowering shoots from two natural populations were quantified. Plant measurements of phenotypic and functional gender were calculated in both populations. Total leaf number was used to investigate the relationship between gender assessments and plant size. KEY RESULTS: Pollen and ovule production varied depending on flower position, although the precise pattern differed between both studied populations; only investment in female floral function decreased markedly from base to top flowers in both populations. The frequency distribution of phenotypic gender and their relationship with plant size differed between populations. Phenotypic and functional gender were correlated in both populations. CONCLUSIONS: Sexual investment in P. odoratum has shown a marked variability within plants, among plants, and between populations, which confirms the importance of analysing sex expression in plants of this type. Differences in relative investment in male and female components (phenotypic gender) are reflected in the functional gender and it would be expected that the evolution of sexual specialization in Polygonatum odoratum would be promoted.     

Plant Parenthood in Milkweeds: A Direct Test of the Pollen Donation Hypothesis

Abstract For hermaphroditic plant species whose fruit production is limited by maternal resources, the "pollen donation hypothesis" views large floral displays as an adaptation to enhance the probability of fathering seeds on other plants. This hypothesis has frequently been used to describe the evolution of large floral displays in milkweeds ( Asclepias ). Most tests of the pollen donation hypothesis, however, have used indirect measures, such as flower production or pollen removal, to estimate male reproductive success. To test the pollen donation hypothesis directly, we performed a paternity analysis and determined the number of seeds sired by individual genotypes in a natural population of poke milkweed, A. exaltata , in southwestern Virginia. Seeds sired (male success) and seeds produced (female success) were significantly correlated with flower number per plant (for male success: r = 0.32, P < 0.05; for female success: r = 0.66, P < 0.0001). Functional gender of plants that reproduced both as males and females (N = 17) was not correlated with flower number per plant ( r = 0.35, P>0.05), indicating that plants with large floral displays did not reproduce primarily as males. Percent fruit-set and seed number per fruit were higher in 1986, when levels of pollinarium removal also were higher. Furthermore, several umbels that experienced high pollinator activity selectively matured fruits that contained many seeds. We argue that the evolution of large floral displays in milkweeds is the result of selection to increase overall reproductive success rather than male reproductive success alone.