system: [SYSTEM]

user: 4. 花色表型变异的自然选择机制 花色的变异可能会带来传粉者或其访问频率的变异，传粉者与花色的相互关系对花色进化具有重要的意义。长期以来，人们对传粉者在花色亿万年的选择中发挥的作用十分好奇。此外，非传粉者因素(自然环境、病原菌、啃食者、基因多效性等)也会影响花色多态性 [ 132 ] [ 180 ] [ 181 ] 。 4.1. 花色转换方向与传粉者偏好 在陆地植物中发生过多次独立的花色从蓝/紫色到红/粉色或花色从蓝/紫/红色到白/黄色的转换，如曼陀罗属(Iochroma)、番薯属(Ipomoea)、钓钟柳属(Penstemon)、耧斗菜属(Aquilegia) (图3a-d)、闭鞘姜属(Costus) [ 45 ] [ 75 ] - [ 81 ] [ 87 ] [ 92 ] [ 130 ] [ 131 ] [ 132 ] 。在这些属中蓝紫色常为原始色，花色从蓝/紫色到红/粉色和蓝/紫/红色到白/黄色的转换比较常见，而反向转换则较少 [ 92 ] [ 132 ] [ 133 ] 。已记录的反向转换包括在苦苣苔科(Gesneriaceae)的三个属(Sinningia，Paliavana和Vanhouttea)中曾发生过多次花色从红色到蓝色的转换 [ 134 ] ，在黄蕊花属(Dalechampia)和枫属(Acer)中曾分别发生过三次花色从绿色或黄色到红色或紫色的转换 [ 135 ] ，在沟酸浆属(Mimulus)中也发生过花色从黄色到红色或粉色的转换(图3e) [ 52 ] 。花色转换方向的不对称性对花色的进化方向具有很大的限制 [ 132 ] 。目前对于花色着色模式的进化转换研究较少。仅有研究认为金鱼草的脉色可能为祖先性状，在全红的花瓣背景下脉色的作用被红色掩盖进而导致脉色消失 [ 50 ] [ 124 ] 。 通常，花色从蓝/紫色到红/粉色的转换伴随传粉者从蜂类(bees)到蜂鸟(hummingbirds)的转换，从蓝/紫/红色到白/黄色的转换伴随传粉者从蜂类或蜂鸟到蛾类(moths)的转换 [ 45 ] [ 87 ] [ 132 ] [ 136 ] 。花色的质变导致的传粉者类型的改变在许多科属如番薯属 [ 75 ] 、沟酸浆属 [ 137 ] [ 138 ] [ 139 ] 、耧斗菜属 [ 100 ] 、碧冬茄属(Petunia) [ 140 ] 、假番薯属(Ipomopsis) [ 141 ] [ 142 ] 中均观察到。通常为了避免对同种传粉者的竞争，同域分布的两近缘种(异域分布时通常具有相同花色)会分化出不同的花色 [ 53 ] [ 113 ] [ 143 ] [ 159 ] [ 160 ] [ 161 ] [ 162 ] 。传粉者对花色的量变是否具有区分能力还存在许多不确定性 [ 144 ] [ 145 ] [ 146 ] 。着色模式的变化通常会影响传粉者的访问频率，具有较明显斑点或花蜜指示标志的花色类型 [ 147 ] - [ 152 ] 会受到偏爱。 4.2. 传粉者介导的自然选择对花色进化的影响 传粉者对花色表型变异的区别访问是否对花色的进化产生选择性作用，通常要看该区别访问是否造成不同花色类型植物适合度(fitness)的不同 [ 132 ] 。植物适合度组分(fitness components)包括结果率、结籽率、萌发率、存活率、可育率等生活史性状。适合度反映一个个体能够产生的有活力的后代数量。 多数研究缺乏对不同花色变异的适合度的评估(e.g. [ 75 ] [ 100 ] [ 124 ] [ 137 ] [ 138 ] [ 140 ] )。一些研究只测量了部分适合度成分但未检测到传粉者对花色的选择作用(e.g. [ 113 ] [ 143 ] [ 154 ] [ 156 ] )。还有的研究注意到不同花色类型间适合度成分有差异且传粉者有偏爱，但无确切的实验和分析证据表明该差异是由传粉者造成(e.g. [ 72 ] [ 139 ] [ 152 ] [ 164 ] [ 166 ] [ 169 ] [ 175 ] [ 191 ] )。但相当数量的研究观测到传粉者的区别访问会带来不同花色类型植物适合度组分的不同(表5)。不同传粉者对不同花色类型的偏爱可能会使得同一属不同物种或同种物种不同个体间基因流减少，导致花色的歧化选择(disruptive selection) [ 141 ] [ 142 ] [ 157 ] [ 158 ] 。在同域地区为了避免对同种传粉者的竞争，花色也会发生歧化 [ 159 ] 。同种传粉者对不同花色类型访问次数及取食时间的差异导致的花粉输出与沉积量的差异对花色多样性的维持具有重要作用 [ 153 ] [ 167 ] 。传粉者对花色某一类型的偏爱性可导致花色的方向性选择(directional selection) [ 145 ] [ 146 ] [ 151 ] [ 165 ] [ 168 ] [ 170 ] [ 172 ] 或稳定性选择(stabilizing selection) [ 163 ] 。有时，某一花色类型在受到传粉者低访问时自交率的增加会维持其基因频率在一定水平，对花色产生平衡选择(balancing selection) [ 168 ] [ 169 ] [ 170 ] [ 177 ] [ 178 ] [ 179 ] ，从而维持花色多态性。 Table 5 Effects of flower color on pollinator visits and fitness component 物种名 比较的花色 或着色模式 传粉者 适合度成分 传粉者偏爱性 传粉者介导的自然选择 非传粉者 因素介导 的自然选择 参考文献 Penstemon digitalis White， white with purple striping Small-to large-bodied bees Fruit set， seed number - No - [ 154 ] Phlox. drummondii， P. cuspidate Light/dark blue， light/dark red Butterflies， moths， and hawkmoths Fruit set， seed number， survival rate Light blue No - [ 113 ] [ 143 ] Wahlenbergia albomarginata Blue to white Solitary bees Seed number， pollen export No No - [ 155 ] Iris atropurpurea， Purple， dark brown Male bee shelters Seed set Dark brown No - [ 156 ] I. haynei Mimulus aurantiacus Red， yellow Hummingbirds and hawkmoths Survival rate， plant size Hummingbirds for red， hawkmoths for white - Red morphs had higher survival rate and plant size than yellow morphs at both coast and inland regions [ 139 ] Malva moschata Red， white Bumblebees and honeybees Fruit set， seed number Bumblebees preferred for red， honeybees had no preferences - Honeybee abundance may maintain flower color polymorphism [ 166 ] Bixa orellana White， amaranth rose， petunia purple， cobalt violet Honeybees， carpenter bees， ants， and small flies Fruit set， mature fruit number， seed number， seed mass Amaranth rose - Increased selfing rate of morphs under lower visits may maintain flower color polymorphism [ 169 ] Clarkia gracilis ssp. sonomensis Spotted/unspotted pink flowers Evylaeus pullilabris Sired seed number， seed number Weak preference for spotted flowers - Small postpollination siring advantages for spotted plants [ 72 ] [ 164 ] [ 191 ] Ursinia calenduliflora spotted， unspotted yellow flowers Male Megapalpus capensis Pollen export， number of fertilized ovules Spotted flowers - Herbivores (preference for spotted morph); Low latitude and high altitude were beneficial to spotted morph [ 152 ] Aquilegia coerulea White to blue Bumblebees and hawkmoths Seed set Bumblebees for blue， hawkmoths for white Whiter flowers were associated with the annual presence of hawkmoths. Higher altitude was correlated with bluer flowers [ 157 ] [ 158 ] Ipomopsis. aggregata， I. tenuituba Red， pink (hybrids)， white Hummingbirds and hawkmoths Fruit set， seed set， sired seed number Hummingbirds for red， hawkmoths for white Disruptive selection - [ 141 ] [ 142 ] M. bicolor All yellow， white and yellow bicolor Small-bodied bees Pollen deposition， seed number No Avoidance of pollinator competition with M. guttatus in sympatric area， disruptive selection Latitude explained 52% of the variation in morph frequency across sites [ 159 ] Antirrhinum majus Red， yellow， white Bumblebees Pollen receipt and export， fruit set Yellow or white Maintenance of flower color polymorphism - [ 153 ] [ 167 ] Claytonia virginica White to red Andrena erigeniae Fruit production， fruit set - Directional selection for redness (marginal significant) Intermediately colored flowers set more fruit than white or red-flowered plants， balancing selection; Herbivores (preferring red morph) showed selection antagonistic to pollinators; Pathogen (preferring infection of white morph) showed selection opposing to herbivores. [ 165 ] Lapeirousia oreogena With or without white arrows-markings long-proboscid nemestrinid fly Prosoeca sp. nov. Pollen export， fruit set， seed set Flowers with white arrows-markings Directional selection - [ 151 ] Centaurea cyanus Violet-blue (dark， violet and strong UV reflective) Bumblebees Fertilization rate， seed number - Directional selection - [ 145 ] Lysimachia arvensis Blue， red Solitary bees and Bombus terrestris Germination， seedling survival， seedling mass， ovule and pollen production， pollen export， seeds per fruit， seed set Blue Directional selection for blue in blue biased population Dry and hot were beneficial to blue morphs; Delayed selfing may exert balancing selection on red morph in blue biased population [ 168 ] [ 170 ] Gentiana lutea Yellow to orange Bumblebees Seed number Yellowness in Torrestı´o population Directional selection in Torrestı´o population; Variation in pollinator communities among populations may maintain color polymorphism Seed predators avoided yellow flowers in Torrestío population， same directional selection with pollinators; Orangeness increased westward [ 171 ] [ 172 ] Anacamptis morio Violet-blue (lighter， strong contrast) Bumblebees Fruit number， fruit mass - Directional selection - [ 146 ] Delphinium nelsonii Blue， white Hummingbirds and bumblebee queens Seed set Blue Stabilizing selection - [ 163 ] I. purpurea Dark purple (MYB1MYB1)， light purple (MYB1myb1)， white (myb1) Bumblebees Flower number， fruit number， seed mass， seed number， survival rate When white phenotypes constitute 25% of the population， pollinators were against visiting white flowers; no preference for dark and light flowers Negative frequency dependent selection Increased selfing rate of white morph under lower visits and overdominance of heterozygote exerted balancing selection [ 173 ] [ 174 ] [ 177 ] [ 178 ] [ 179 ] Cosmos bipinnatus Purple， pink， white Bumblebees and honeybees Fruit set and seed number per capitulum Pink - Overdominance of heterozygote exerted balancing selection [ 175 ] Sisyrinchium sp. Purple (dd)， white (DD， Dd) Bees， hover flies， and thrips Fruit set， size of mature fruit， seed number - - Overdominance of heterozygote exerted balancing selection [ 176 ] P. drummondii Red， white Butterflies， moths， and hawkmoths Flower number， fruit set， seed number， germination， survival rate， fecundity， relative fitness No No Pleiotropy， purifying selection for white individual [ 188 ] I. purpurea Purple (CHS-D-)， white (chs-d) Bumblebees Flower number， survival rate， fecundity， germination No No Pleiotropy， purifying selection for white individual [ 93 ] [ 94 ] [ 189 ] Linanthus parryae Blue， white Beetles Seed number No No High spring precipitation was beneficial to white morph， fluctuating selection [ 190 ] 表5. 花色对传粉者和适合度组成分的影响 “比较的花色或着色模式”中粗体部分表示该花色类型适合度较高；“适合度成分”中粗体部分表示有显著差异的适合度成分；“-”表示无实验证据。 图3. 一些属植物花色转换系统树 在进化过程中，植物为适应不同的传粉者会使花部特征特异化。如被蜂鸟传粉的花通常为红色、具有较长的花筒、较窄的花檐、外突的雄蕊和柱头、分泌大量但糖浓度较低的花蜜，被蜂类传粉的花大多为蓝紫色、具较宽的花筒和花檐、内嵌的雄蕊和柱头、分泌少量但高浓度的花蜜，而被蛾类传粉的花则多为白色、花筒较长、有芳香气味且经常在夜间开放，具低浓度高量花蜜 [ 132 ] [ 136 ] [ 182 ] [ 183 ] 。花色与其他花性状之间具有高度的相关性(e.g. [ 184 ] [ 185 ] [ 186 ] )，对其他花性状的选择也可能会影响花色的变异。性状之间的相互关联和权衡是限制花色方向性进化的一个因素 [ 187 ] 。 4.3. 非传粉者因素介导的自然选择对花色进化的影响 非传粉者因素对不同花色类型的适合度也具有一定的影响(表5)。因为不同的花色类型对光照、降水量、温度、海拔、经纬度等环境的适应能力可能不同，所以环境的异质性可导致不同花色类型在不同地区分布的差异 [ 152 ] [ 157 ] [ 158 ] [ 159 ] [ 168 ] [ 170 ] [ 171 ] [ 190 ] ，可能会介导花色的歧化或动态选择(fluctuating selection)。啃食者与传粉者偏爱的相同可能带来与传粉者作用相反的拮抗选择(antagonistic selection) [ 152 ] [ 165 ] 。环境因子、啃食者或病原菌在花色的多态性维持方面起着非常重要的作用，其通常还会限制花色的方向性进化 [ 165 ] [ 168 ] [ 170 ] [ 171 ] [ 172 ] 。 基因的多效性也会影响花色。有时，杂合体花色表型会表现出超显性(overdominance) [ 165 ] [ 173 ] [ 174 ] [ 175 ] [ 176 ] 而维持等位基因频率的平衡，对花色产生平衡选择。由于突变基因的有害多效性，一些白花个体与有色花相比，通常具有较低的萌发率、存活率、可育率或抵御啃食者和病原菌的能力较差，因而具有较低的适合度而受到纯化选择(purifying selection)，结果在群体中占有较低的频率 [ 93 ] [ 94 ] [ 165 ] [ 188 ] [ 189 ] 。