POLLEN DISCOUNTING AND THE SPREAD OF A SELFING VARIANT IN TRISTYLOUS EICHHORNIA PANICULATA: EVIDENCE FROM EXPERIMENTAL POPULATIONS

Floral traits that increase self-fertilization are expected to spread unless countered by the effects of inbreeding depression, pollen discounting (reduced outcross pollen success by individuals with increased rates of self-fertilization), or both. Few studies have attempted to measure pollen discounting because to do so requires estimating the male outcrossing success of plants that differ in selfing rate. In natural populations of tristylous Eichhornia paniculata, selfing variants of the mid-styled morph are usually absent from populations containing all three style morphs but often predominate in nontrimorphic populations. We used experimental garden populations of genetically marked plants to investigate whether the effects of population morph structure on relative gamete transmission by unmodified (M) and selfing variants (M‘) of the mid-styled morph could explain their observed distribution. Transmission through ovules and self and outcross pollen by plants of the M and M’ morphs were compared under trimorphic, dimorphic (S morph absent), and monomorphic (L and S morphs absent) population structures. Neither population structure nor floral morphology affected female reproductive success, but both had strong effects on the relative transmission of male gametes. The frequency of self-fertilization in the M' morph was consistently higher than that of the M morph under all morph structures, and the frequency of self-fertilization by both morphs increased as morph diversity of experimental populations declined. In trimorphic populations, total transmission by the M and M' morphs did not differ. The small, nonsignificant increase in selfing by the M' relative to the M morph was balanced by decreased outcross siring success, particularly on the S morph. In populations lacking the S morph, male gamete transmission by the M' morph was approximately 1.5 times greater than that by the M morph because of both increased selfing and increased success through outcross pollen donation. Therefore, gamete transmission strongly favored the M' morph only in the absence of the S morph, a result consistent with the distribution of the M' morph in nature. This study indicates that floral traits that alter the selfing rate can have large and context-dependent influences on outcross pollen donation.     

PATERNAL EFFECTS IN INHERITANCE OF A PATHOGEN RESISTANCE TRAIT IN IPOMOEA PURPUREA

For continuously variable, polygenic characters, the response to selection depends upon the proportion of phenotypic variance that is caused by additive genetic variance, or narrow-sense heritability. Thus, a major goal of quantitative genetics is to partition phenotypic variance for a trait in a way that isolates additive genetic variance from other causes. The variance among paternal half-sib families, which is frequently used to estimate additive variance, is commonly recognized to include additive epistatic effects. However, this variance component can also include non-Mendelian paternal effects. We report here the results from a diallel crossing design used to isolate nonnuclear effects from the paternal nuclear contribution to disease resistance in the common morning glory, Ipomoea purpurea. In particular, we found that genetic variance for resistance to anthracnose, a disease caused by the fungal pathogen Colletotrichum dematium, was determined largely by a nonnuclear, additive paternal effect. We discuss potential mechanisms for this effect as well as some of their evolutionary implications.     

THE ROLE OF INBREEDING DEPRESSION IN MAINTAINING THE MIXED MATING SYSTEM OF THE COMMON MORNING GLORY,IPOMOEA PURPUREA

Theoretical studies show that, although inbreeding depression (ID) will counterbalance the transmission advantage of selfing, it can only maintain a mixed mating system in plants when at least one of the following two conditions is met: (1) there is a positive association between selfing rates and the level of ID; and (2) ID is greater than 0.5 for the female component of fitness, while the average ID for male and female fitness is less than 0.5. This study tests whether these two conditions hold in the common morning glory, Ipomoea purpurea, which has a mixed mating system with 30% self-fertilization. Inbreeding depression was found in all but one fitness component measured in two groups of plants with distinct anther-stigma distances (ASD), a character that influences selfing rates. However, when examined separately, a negative association was found between selfing rates and ID; plants with large ASD (low-selfing-rate genotypes) tended to have higher ID than ones with small ASD (high-selfing-rate genotypes). Furthermore, the overall lifetime ID for male (12.5%) and female (24%) components of fitness, averaged across two ASD groups, were lower than what is necessary for ID to maintain an evolutionarily stable mixed mating system. Therefore, although inbreeding depression contributes to balancing the transmission advantage of selfing, it is not likely to be the primary mechanism maintaining the mixed mating system of I. purpurea. The contribution of other mechanisms is discussed.     

SELECTION ON A FLORAL COLOR POLYMORPHISM IN THE TALL MORNING GLORY (IPOMOEA PURPUREA): TRANSMISSION SUCCESS OF THE ALLELES THROUGH POLLEN

The W locus, a codominant locus influencing floral pigment intensity in the tall morning glory, Ipomoea purpurea, is polymorphic throughout the southeastern United States. Previous studies suggest that this polymorphism is actively maintained by balancing selection, and that increased selfing accompanied by lack of pollen discounting (“Fisher effect”) may act to protect the white allele when it is rare. Processes that act to protect the dark allele and thus stabilize the polymorphism in conjunction with the Fisher effect have not been previously detected. The goal of this study was to determine whether any of three such processes might operate in I. purpurea. Estimates of breeding system parameters in a large experimental population in which the white allele was in higher than normal frequency (0.5) provided little evidence that either dark- or light-flowered plants were more successful as pollen parents than white-flowered plants. In addition, no evidence was found for a transmission bias favoring the dark allele in the ovules produced by light heterozygotes. In contrast, a strong transmission bias favoring the dark allele in pollen of heterozygotes was observed. A simple model using parameter estimates derived from this and previous studies indicates that a balance between the Fisher effect and biased transmission in heterozygote pollen could account for many properties of the polymorphism.     

TRANSMISSION ADVANTAGE FAVORS SELFING ALLELE IN EXPERIMENTAL POPULATIONS OF SELF‐INCOMPATIBLE WITHERINGIA SOLANACEA (SOLANACEAE)

The evolution of self‐fertilization is one of the most commonly traversed transitions in flowering plants, with profound implications for population genetic structure and evolutionary potential. We investigated factors influencing this transition using Witheringia solanacea, a predominantly self‐incompatible (SI) species within which self‐compatible (SC) genotypes have been identified. We showed that self‐compatibility in this species segregates with variation at the S‐locus as inherited by plants in F1 and F2 generations. To examine reproductive assurance and the transmission advantage of selfing, we placed SC and SI genotypes in genetically replicated gardens and monitored male and female reproductive success, as well as selfing rates of SC plants. Self‐compatibility did not lead to increased fruit or seed set, even under conditions of pollinator scarcity, and the realized selfing rate of SC plants was less than 10%. SC plants had higher fruit abortion rates, consistent with previous evidence showing strong inbreeding depression at the embryonic stage. Although the selfing allele did not provide reproductive assurance under observed conditions, it also did not cause pollen discounting, so the transmission advantage of selfing should promote its spread. Given observed numbers of S‐alleles and selfing rates, self‐compatibility should spread even under conditions of exceedingly high initial inbreeding depression.     

EVALUATING A SIMPLE APPROXIMATION TO MODELING THE JOINT EVOLUTION OF SELF‐FERTILIZATION AND INBREEDING DEPRESSION

A comprehensive understanding of plant mating system evolution requires detailed genetic models for both the mating system and inbreeding depression, which are often intractable. A simple approximation assuming that the mating system evolves by small infrequent mutational steps has been proposed. We examine its accuracy by comparing the evolutionarily stable selfing rates it predicts to those obtained from an explicit genetic model of the selfing rate, when inbreeding depression is caused by partly recessive deleterious mutations at many loci. Both models also include pollen limitation and pollen discounting. The approximation produces reasonably accurate predictions with a low or moderate genomic mutation rate to deleterious alleles, on the order of U = 0.02–0.2. However, for high mutation rates, the predictions of the full genetic model differ substantially from those of the approximation, especially with nearly recessive lethal alleles. This occurs because when a modifier allele affecting the selfing rate is rare, homozygous modifiers are produced mainly by selfing, which enhances the opportunity for purging nearly recessive lethals and increases the marginal fitness of the allele modifying the selfing rate. Our results confirm that explicit genetic models of selfing rate and inbreeding depression are required to understand mating system evolution.     

MECHANISM OF SELECTION FAVORING A WIDE TUBULAR COROLLA IN CAMPANULA PUNCTATA

A previous study of Campanula punctata pollinated by Bombus diversus showed greater male reproductive success in plants with wider corollas when male reproductive success was measured by paternity analysis. Pollen removal alone was not a good measure of male reproductive success. The aim of the current study is to elucidate the mechanism underlying the selection on corolla width in C. punctata. Multiple regression analysis of male reproductive success in experimental populations revealed that the advantage of a wide corolla to the male is independent of the corolla width of pollen recipients, indicating that the position of pollen deposited on Bombus may not be an important factor for selection. However, the number of pollen grains that fell on the petal or off the flower and were lost at the first visit of Bombus (pollen fall) decreased with increasing corolla width, so that most pollen grains removed from a wide flower were deposited onto the pollinator. In a narrow flower, a greater proportion of the pollen removed was wasted through pollen fall. Pollen removal does not reflect male reproductive success, at least in part because of the loss of pollen in some flowers through pollen fall. The positive relationship between wider corollas and reduced loss of pollen through pollen fall is likely to lead to greater male reproductive success of wider flowers under repeated visits of pollinators. Decreased pollen fall, probably due to more gentle contact between pollen and bees in flowers with wider corollas, may be one of the mechanisms underlying the selection on corolla width of C. punctata.     

西葫芦(Cucurbita pepo L.)花粉壁和非壁蛋白中苹果酸脱氢酶的研究

西葫芦(Cucurbita pepo L.)花粉的壁蛋白和非壁蛋白中都存在苹果酸脱氢酶(EC 1.1-1.37)。壁蛋白苹果酸脱氢酶对OAA的K_m值为30.3μM,最适pH值为8.0。非壁蛋白苹果酸脱氢酶对OAA的K_m值为83.3μM,最适pH值为8.5。在0.05～0.3 M氯化钠浓度范围内,壁蛋白苹果酸脱氢酶和非壁蛋白苹果酸脱氢酶的活性都受到抑制。这种抑制作用随氯化钠浓度提高而增强。0.2 M氯化钾对酶活性的抑制作用与0.2 M氯化钠的抑制作用完全相同。0.1～0.8 M的蔗糖对壁蛋白苹果酸脱氢酶和非壁蛋白苹果酸脱氢酶活性也具有抑制作用。     

苏北盆地晚更新世以来的孢粉记录与气候地层学的初步研究

苏北盆地是我国西部高原至东部海洋之间的过渡地区,该地区的气候演变史对揭示东亚季风的时空变化有着重要的意义。根据气候地层学的原理,用孢粉分析的方法,结合^14C测年数据,再造了兴化地区自倒数第二次冰期以来的古植被与古环境,其演变史能与深海氧同位素记录进行良好的对比,孢粉带Ⅲ、Ⅳ、Ⅴ、Ⅵ、Ⅶ分别对应深海氧同位素阶段5、4、3、2、1,且孢粉带Ⅲ细分的5个亚带与MIS 5a-5e也有良好的对应。此外,从该地区末次冰期孢粉记录中检索出6次冷事件,通过与格陵兰冰芯及会宁黄土记录对比,推测这些冷事件可能对应于末次冰期气候变化中的Heinrich事件。     

末次冰期以来湛江湖光岩玛珥湖孢粉记录及古环境变化

通过对湛江湖光岩玛珥湖钻孔孢粉记录的研究认为,末次冰期以来,该区植被类型从早期的南亚热带季风常绿阔叶林依次演替为,中亚热带常绿、落叶阔叶林→南亚热带季风常绿阔叶林→中亚热带常绿—落叶阔叶—针叶混交林(湖边草地)→热带季雨林→半常绿季雨林。气候最寒冷的时期温度比目前至少降低了4—6℃以上。全新世早期相对于旱。     

Ca^2＋载体A23187对烟草生殖核分裂的调节

用Ｃａ＾２＋载体Ａ２３１８７研究了高钙水平对烟草离体花粉管内生殖核分裂的影响。结果表明,Ａ２３１８７抑制花粉管生长,影响其正常形态,调节生殖核分裂,较高浓度Ａ２３１８７可阻断生殖核分裂,而较代浓度的效应因处理时间而异;萌发６ｈ处理显著抑制生殖核分裂。     

八倍体小偃麦与硬粒小麦及其杂种F_1的小孢子发生和花粉发育的细胞学研究

用压片法对八倍体小偃麦与硬粒小麦及其杂种F_1小孢子发生和花粉发育进行了详细的细胞学观察。结果表明,两亲本八倍体小偃麦和硬粒小麦的小孢子发生和花粉发育是基本正常的,二者结实率也基本正常。[八倍体小偃麦×硬粒小麦]F_1小孢子发生过程非常紊乱,花粉母细胞减数分裂中期Ⅰ出现较高频率的单价体和多价体,其相对紊乱系数高达0.67;减数分裂晚后期,每个细胞落后染色体平均3.56条;四分体期,每个四分体微核数平均2.82个。在杂种F_1花粉发育的每个时期都可产生不同频率的败育花粉。在三细胞成熟花粉期,杂种F_1可育花粉百分率为67.36%,基本能够满足传粉受精的需要,但其结实率仅为2.79%,远远低于两亲本结实率。因此杂种F_1花粉的败育不是导致其结实率低的主导因素。     

云南丽江-大理地区现代表土花粉垂直分布特征

文章主要研究云南丽江-大理地区不同海拔高度表土花粉。虽然松属花粉在各个植被带都占有优势,但是建群种花粉含量相对较高。降趋对应分析法（detrended correspondenc eanalysis,简称DCA）分析显示,表土花粉的分布和海拔之间具有非常密切的关系,DCA第一轴的值和海拔高度之间的线性相关系数达0．82（r^2=0．82）。此次研究表明,在云南山地,可以通过含量相对较高的花粉来重建古植被。     

TESTING MODELS OF SEX RATIO EVOLUTION IN A GYNODIOECIOUS PLANT: FEMALE FREQUENCY COVARIES WITH THE COST OF MALE FERTILITY RESTORATION

In many gynodioecious species, cytoplasmic male sterility genes (CMS) and nuclear male fertility restorers (Rf) jointly determine whether a plant is female or hermaphrodite. Equilibrium models of cytonuclear gynodioecy, which describe the effect of natural selection within populations on the sex ratio, predict that the frequency of females in a population will primarily depend on the cost of male fertility restoration, a negative pleiotropic effect of Rf alleles on hermaphrodite fitness. Specifically, when the cost of restoration is higher, the frequency of females at equilibrium is predicted to be higher. To test this prediction, we estimated variation in the cost of restoration across 26 populations of Lobelia siphilitica, a species in which Rf alleles can have negative pleiotropic effects on pollen viability. We found that L. siphilitica populations with many females were more likely to contain hermaphrodites with low pollen viability. This is consistent with the prediction that the cost of restoration is a key determinant of variation in female frequency. Our results suggest that equilibrium models can explain variation in sex ratio among natural populations of gynodioecious species.