Pollen Tube Growth Rate Research Articles

Can selection on a male mating character result in evolutionary change? A selection experiment on California wild radish, Raphanus sativus.

Whenever more pollen grains arrive on stigmas than necessary to fertilize ovules, sexual selection is possible. However, the role of sexual selection remains controversial, in part because of lack of evidence on genetic bases of traits and the response of relevant characters to selection. In an experiment with Raphanus sativus, we selected on tendency to sire seeds in the stylar or basal regions of fruits. This character is likely related to pollen tube growth rate, and seed position affects rates of abortion and seed predation. We measured differences among families in seed siring and related characters and evaluated responses to selection. All replicates showed strong effects of pollen donor family on proportion of seeds sired per fruit in mixed pollinations. Most also showed effects of pollen donor family on number of pollen grains per flower and pollen diameter. Two of four replicates showed a response to selection on position of seeds sired. In responding replicates, we found trade-offs in pollen grain size and number; plants with larger pollen grains sired more seeds in the basal region. Our data suggest a genetic basis for pollen donor ability to sire seeds in competition. The significant response to selection in two replicates shows that position of seeds sired can respond to selection. Thus, all components for sexual selection to occur and affect traits are present. Variation in results among replicates might be due to changes in greenhouse conditions. Environmental effects may contribute to the maintenance of variation in these fitness-related characters.

Paternal-specific S-allele transmission in sweet cherry (Prunus avium L.): the potential for sexual selection.

Homomorphic self-incompatibility is a well-studied example of a physiological process that is thought to increase population diversity and reduce the expression of inbreeding depression. Whereas theoretical models predict the presence of a large number of S-haplotypes with equal frequencies at equilibrium, unequal allele frequencies have been repeatedly reported and attributed to sampling effects, population structure, demographic perturbation, sheltered deleterious mutations or selection pressure on linked genes. However, it is unclear to what extent unequal segregations are the results of gametophytic or sexual selection. Although these two forces are difficult to disentangle, testing S-alleles in the offspring of controlled crosses provides an opportunity to separate these two phenomena. In this work, segregation and transmission of S-alleles have been characterized in progenies of mixed donors and fully compatible pollinations under field conditions in Prunus avium. Seed set patterns and pollen performance have also been characterized. The results reveal paternal-specific distorted transmission of S-alleles in most of the crosses. Interestingly, S-allele segregation within any given paternal or maternal S-locus was random. Observations on pollen germination, pollen tube growth rate, pollen tube cohort size, seed set dynamics and transmission patterns strongly suggest post-pollination, prezygotic sexual selection, with male-male competition as the most likely mechanism. According to these results, post-pollination sexual selection takes precedence over frequency-dependent selection in explaining unequal S-haplotype frequencies.

Class XI Myosins Move Specific Organelles in Pollen Tubes and Are Required for Normal Fertility and Pollen Tube Growth in Arabidopsis.

Pollen tube growth is an essential aspect of plant reproduction because it is the mechanism through which nonmotile sperm cells are delivered to ovules, thus allowing fertilization to occur. A pollen tube is a single cell that only grows at the tip, and this tip growth has been shown to depend on actin filaments. It is generally assumed that myosin-driven movements along these actin filaments are required to sustain the high growth rates of pollen tubes. We tested this conjecture by examining seed set, pollen fitness, and pollen tube growth for knockout mutants of five of the six myosin XI genes expressed in pollen of Arabidopsis (Arabidopsis thaliana). Single mutants had little or no reduction in overall fertility, whereas double mutants of highly similar pollen myosins had greater defects in pollen tube growth. In particular, myo11c1 myo11c2 pollen tubes grew more slowly than wild-type pollen tubes, which resulted in reduced fitness compared with the wild type and a drastic reduction in seed set. Golgi stack and peroxisome movements were also significantly reduced, and actin filaments were less organized in myo11c1 myo11c2 pollen tubes. Interestingly, the movement of yellow fluorescent protein-RabA4d-labeled vesicles and their accumulation at pollen tube tips were not affected in the myo11c1 myo11c2 double mutant, demonstrating functional specialization among myosin isoforms. We conclude that class XI myosins are required for organelle motility, actin organization, and optimal growth of pollen tubes.

Differential selection on pollen and pistil traits in relation to pollen competition in the context of a sexual conflict over timing of stigma receptivity.

Sexual conflict and its evolutionary consequences are understudied in plants, but the theory of sexual conflict may help explain how selection generates and maintains variability. Here, we investigated selection on pollen and pistil traits when pollen arrives sequentially to partially receptive pistils in relation to pollen competition and a sexual conflict over timing of stigma receptivity in the mixed-mating annual Collinsia heterophylla (Plantaginaceae). In this species the conflict is generated by early fertilizing pollen that reduces seed production, which is counteracted by delaying receptivity in the recipient. We performed sequential two-donor pollinations at early floral developmental stages involving two pollen deposition schedules (with or without a time lag of 1 day), using only outcross or self and outcross pollen. We investigated pollen and pistil traits in relation to siring success (male fitness) and seed production (female fitness). In contrast to previous findings in receptive pistils in C. heterophylla and in other species, last arriving pollen donors showed highest siring success in partially receptive pistils. The last male advantage was weaker when self pollen was the first arriving donor. Two measures of germination rate (early and late) and pollen tube growth rate of first arriving donors were important for siring success in crosses with a time lag, while only late germination rate had an effect in contemporary crosses. Curiously, late stigma receptivity was negatively related to seed production in our contemporary crosses, which was opposite to expectation. Our results in combination with previous studies suggest that pollen and pistil traits in C. heterophylla are differentially advantageous depending on stage of floral development and varying pollen deposition schedules. Variation in success of these traits over floral development time may result from sexually antagonistic selection.

Arabidopsis ACT11 modifies actin turnover to promote pollen germination and maintain the normal rate of tube growth.

Actin is an ancient conserved protein that is encoded by multiple isovariants in multicellular organisms. There are eight functional actin genes in the Arabidopsis genome, and the precise function and mechanism of action of each isovariant remain poorly understood. Here, we report the characterization of ACT11, a reproductive actin isovariant. Our studies reveal that loss of function of ACT11 causes a delay in pollen germination, but enhances pollen tube growth. Cytological analysis revealed that the amount of filamentous actin decreased, and the rate of actin turnover increased in act11 pollen. Convergence of actin filaments upon the germination aperture was impaired in act11 pollen, consistent with the observed delay of germination. Reduction of actin dynamics with jasplakinolide suppressed the germination and tube growth phenotypes in act11 pollen, suggesting that the underlying mechanisms involve an increase in actin dynamics. Thus, we demonstrate that ACT11 is required to maintain the rate of actin turnover in order to promote pollen germination and maintain the normal rate of pollen tube growth.

A preliminary study on the crossability in Robinia pseudoacacia L.

To explore the feasibility of intra specific hybridisation between Robinia pseudoacacia (common diploid) and other Robinia varieties, tetraploid R. pseudoacacia, R. pseudoacacia var. decaisneana (Carr.) Voss., R. pseudoacacia Frisia, and R. pseudoacacia “Idaho” were collected as male parents, and hybridisation trials were conducted over a period of three consecutive years. The average seedling emergence rates of the five hybridised combinations were ~2.3,2.0, 3.3, 1.3, and 0 % per year, respectively. To ensure maximum seedling emergence rates, we found that the best pollinating times were the blooming days. To investigate the causes of low crossability, we also examined pollen-tube growth and fruit setting rates. The results indicated that the causes of low crossability were abnormal pollen-tube growth, failed development of fertilised ovule, and poor seed germination. Low fruit-setting rates caused by emasculation may also lead to low crossability.

Managed bumblebees outperform honeybees in increasing peach fruit set in China: different limiting processes with different pollinators.

Peach Prunus persica (L.) Batsch is self-compatible and largely self-fertile, but under greenhouse conditions pollinators must be introduced to achieve good fruit set and quality. Because little work has been done to assess the effectiveness of different pollinators on peach trees under greenhouse conditions, we studied ‘Okubo’ peach in greenhouse tunnels near Beijing between 2012 and 2014. We measured pollen deposition, pollen-tube growth rates, ovary development, and initial fruit set after the flowers were visited by either of two managed pollinators: bumblebees, Bombus patagiatus Nylander, and honeybees, Apis mellifera L. The results show that B. patagiatus is more effective than A. mellifera as a pollinator of peach in greenhouses because of differences in two processes. First, B. patagiatus deposits more pollen grains on peach stigmas than A. mellifera, both during a single visit and during a whole day of open pollination. Second, there are differences in the fertilization performance of the pollen deposited. Half of the flowers visited by B. patagiatus are fertilized 9–11 days after bee visits, while for flowers visited by A. mellifera, half are fertilized 13–15 days after bee visits. Consequently, fruit development is also accelerated by bumblebees, showing that the different pollinators have not only different pollination efficiency, but also influence the subsequent time course of fertilization and fruit set. Flowers visited by B. patagiatus show faster ovary growth and ultimately these flowers produce more fruit. Our work shows that pollinators may influence fruit production beyond the amount of pollen delivered. We show that managed indigenous bumblebees significantly outperform introduced honeybees in increasing peach initial fruit set under greenhouse conditions.

Distyly and cryptic heteromorphic self‐incompatibility inMussaenda macrophylla(Rubiaceae)

AbstractTypical distylous species display both reciprocal herkogamy and heteromorphic incompatibility, which exclude self‐ and intramorph pollination and accordingly promote intermorph pollination. Here we explore an unusual distylous flower associated with self‐ and intramorph compatibility inMussaendamacrophyllaWall. The species did not exhibit a precise reciprocal herkogamy, and the populations studied had a slight bias in favor of the short‐styled morph (S‐morph). In addition, pollen tube growth in intermorph pollination was faster than either in intramorph or self‐pollination, indicating that cryptic heteromorphic self‐incompatibility occurred inM. macrophylla. Specifically, pollen tube growth rates after legitimate and illegitimate pollination were much more highly differentiated in long‐styled morph (L‐morph) than in S‐morph, which might be attributed to the increased de‐esterified pectin in the tip of L‐morph pollen tube wall in comparison with S‐morph. Transmission electron microscopic observation revealed illegitimate self‐pollinated pollen tubes of two morphs were seriously degraded with the large autolysosome‐like vacuoles in cytoplasm of L‐morph pollen tube but accumulating small vacuoles in that of S‐morph pollen tube, suggesting that incompatible pollen tubes might undergo autophagic programmed cell death except self‐incompatible S‐morph pollen tubes. Our results indicate thatM. macrophyllais morphologically distylous with a cryptic heteromorphic self‐incompatibility breeding system, which functions differently in S‐morph with self‐pollination compared with other illegitimate pollination.

Organelle trafficking, the cytoskeleton, and pollen tube growth.

The pollen tube is fundamental for the reproduction of seed plants. Characteristically, it grows relatively quickly and uni-directionally ("polarized growth") to extend the male gametophyte to reach the female gametophyte. The pollen tube forms a channel through which the sperm cells move so that they can reach their targets in the ovule. To grow quickly and directionally, the pollen tube requires an intense movement of organelles and vesicles that allows the cell's contents to be distributed to sustain the growth rate. While the various organelles distribute more or less uniformly within the pollen tube, Golgi-released secretory vesicles accumulate massively at the pollen tube apex, that is, the growing region. This intense movement of organelles and vesicles is dependent on the dynamics of the cytoskeleton, which reorganizes differentially in response to external signals and coordinates membrane trafficking with the growth rate of pollen tubes.

Brassinosteroids promote Arabidopsis pollen germination and growth.

Pollen tubes are among the fastest tip-growing plant cells and represent an excellent experimental system for studying the dynamics and spatiotemporal control of polarized cell growth. However, investigating pollen tube tip growth in the model plant Arabidopsis remains difficult because in vitro pollen germination and pollen tube growth rates are highly variable and largely different from those observed in pistils, most likely due to growth-promoting properties of the female reproductive tract. We found that in vitro grown Arabidopsis pollen respond to brassinosteroid (BR) in a dose-dependent manner. Pollen germination and pollen tube growth increased nine- and fivefold, respectively, when media were supplemented with 10µM epibrassinolide (epiBL), resulting in growth kinetics more similar to growth in vivo. Expression analyses show that the promoter of one of the key enzymes in BR biosynthesis, CYP90A1/CPD, is highly active in the cells of the reproductive tract that form the pathway for pollen tubes from the stigma to the ovules. Pollen tubes grew significantly shorter through the reproductive tract of a cyp90a1 mutant compared to the wild type, or to a BR perception mutant. Our results show that epiBL promotes pollen germination and tube growth in vitro and suggest that the cells of the reproductive tract provide BR compounds to stimulate pollen tube growth.

Long-chain base phosphates modulate pollen tube growth via channel-mediated influx of calcium.

Long-chain base phosphates (LCBPs) have been correlated with amounts of crucial biological processes ranging from cell proliferation to apoptosis in animals. However, their functions in plants remain largely unknown. Here, we report that LCBPs, sphingosine-1-phosphate (S1P) and phytosphingosine-1-phosphate (Phyto-S1P), modulate pollen tube growth in a concentration-dependent bi-phasic manner. The pollen tube growth in the stylar transmitting tissue was promoted by SPHK1 overexpression (SPHK1-OE) but dampened by SPHK1 knockdown (SPHK1-KD) compared with wild-type of Arabidopsis; however, there was no detectable effect on in vitro pollen tube growth caused by misexpression of SPHK1. Interestingly, exogenous S1P or Phyto-S1P applications could increase the pollen tube growth rate in SPHK1-OE, SPHK1-KD and wild-type of Arabidopsis. Calcium ion (Ca(2+) )-imaging analysis showed that S1P triggered a remarkable increase in cytosolic Ca(2+) concentration in pollen. Extracellular S1P induced hyperpolarization-activated Ca(2+) currents in the pollen plasma membrane, and the Ca(2+) current activation was mediated by heterotrimeric G proteins. Moreover, the S1P-induced increase of cytosolic free Ca(2+) inhibited the influx of potassium ions in pollen tubes. Our findings suggest that LCBPs functions in a signaling cascade that facilitates Ca(2+) influx and modulates pollen tube growth.

Patterns of pollen quantity and quality limitation of pre‐zygotic reproduction in Mimulus guttatus vary with co‐flowering community context

Pollen quantity limitation has been widely recognized as one of the main causes of plant reproductive failure in nature. However, its negative effects on fruit and seed production have been often confounded with those of low pollen quality (i.e. low conspecific pollen viability and/or slow pollen tube growth rate). The lack of differentiation between these two aspects of pollen limitation has resulted not only in a potential overestimation of the incidence of pollen quantity limitation but has led to a poor understanding of the factors and mechanisms that affect pollen quality limitation. Knowledge of the relative importance and underlying causes of both aspects of pollen limitation (quantity and quality) is required to fully understand the ecological and evolutionary consequences of pollen limitation in natural populations. Co‐flowering community context (e.g. species richness and conspecific density), in particular, can be an important driver of overall pollen limitation. However, how pollen quantity and quality limitation vary separately with the community context has not previously been examined, even though they may vary differently as they arise from different mechanisms, e.g. insect visitation rate versus inbreeding depression. Here we evaluate the effect of co‐flowering diversity and conspecific density on the relative importance of pollen quantity and quality limitation for Mimulus guttatus pre‐zygotic reproduction (i.e. pollen tube success) at serpentine seeps in California over two years. We found overall pollen limitation of pre‐zygotic reproductive success at all seeps regardless of the co‐flowering context. However, plants in high‐diversity/low‐ conspecific density communities were mostly limited by pollen quantity, whereas plants in low‐diversity/high‐conspecific density ones experienced stronger quality limitation – a pattern that was consistent across years. These results not only highlight the importance of conducting comprehensive studies on pollen limitation that evaluate both, its quantity and quality aspects, but are the first to show how their relative contribution to overall pollen limitation can vary with extrinsic factors such as the co‐flowering community context.

Pollen tube NAD(P)H oxidases act as a speed control to dampen growth rate oscillations during polarized cell growth

Reactive oxygen species (ROS) produced by NAD(P)H oxidases play a central role in plant stress responses and development. To better understand the function of NAD(P)H oxidases in plant development, we characterized the Arabidopsis thaliana NAD(P)H oxidases RBOHH and RBOHJ. Both proteins were specifically expressed in pollen and dynamically targeted to distinct and overlapping plasma membrane domains at the pollen tube tip. Functional loss of RBOHH and RBOHJ in homozygous double mutants resulted in reduced fertility. Analyses of pollen tube growth revealed remarkable differences in growth dynamics between Col-0 and rbohh-1 rbohj-2 pollen tubes. Growth rate oscillations of rbohh-1 rbohj-2 pollen tubes showed strong fluctuations in amplitude and frequency, ultimately leading to pollen tube collapse. Prior to disintegration, rbohh-1 rbohj-2 pollen tubes exhibit high-frequency growth oscillations, with significantly elevated growth rates, suggesting that an increase in the rate of cell-wall exocytosis precedes pollen tube collapse. Time-lapse imaging of exocytic dynamics revealed that NAD(P)H oxidases slow down pollen tube growth to coordinate the rate of cell expansion with the rate of exocytosis, thereby dampening the amplitude of intrinsic growth oscillations. Using the Ca(2+) reporter Yellow Cameleon3.6, we demonstrate that high-amplitude growth rate oscillations in rbohh-1 rbohj-2 pollen tubes are correlated with growth-dependent Ca(2+) bursts. Electrophysiological experiments involving double mutant pollen tubes and pharmacological treatments also showed that ROS influence K(+) homeostasis. Our results indicate that, by limiting pollen tube growth, ROS produced by NAD(P)H oxidases modulate the amplitude and frequency of pollen tube growth rate oscillations.

不同理化因子对枇杷(Eriobotrya japonica (Thunb.) L.)花粉体外萌发和花粉管生长的影响

本实验以枇杷花粉为材料，采用花粉离体培养法研究了蔗糖浓度、pH值以及γ-氨基丁酸(GABA)对枇杷花粉萌发和花粉管生长的影响。试验以31 mg/L H3BO3、196.8 mg/L MgSO4o7H2O、7.45 mg/L KCl + 56 mg/L CaCl2为基本培养基组分，在25℃暗条件下培养5 h后统计枇杷花粉萌发率及花粉管的生长速率。结果表明，蔗糖浓度为10%时，花粉萌发率达到最高值60%，同时花粉管生长速率达到最大值70.4 um/h；pH 8.0时花粉萌发率最高，达63%；此外，添加不同浓度的GABA对花粉萌发的影响不显著，但对花粉管生长的影响有显著作用。In this study, loquat fresh pollens were used as materials to explore the effect of sucrose concen-tration, pH and gamma-amino butyric acid (GABA) on pollen germination and pollen tube growth in vitro. The basic culture medium consists with 31 mg/L boric acid + 196.8 mg/L MgSOIn this study, loquat fresh pollens were used as materials to explore the effect of sucrose concen-tration, pH and gamma-amino butyric acid (GABA) on pollen germination and pollen tube growth in vitro. The basic culture medium consists of 31 mg/L boric acid + 196.8 mg/L MgSO4o7H2O + 7.45 mg/L KCl + 56 mg/L CaCl2. Pollens were cultured in dark at 25°C for 5 h. The results showed that pollen germination rate reached to 60% and pollen tube growth rate was 70.4 um/h. Given 10% sucrose and pH value 8.0, pollen germination reached the highest ratio of 63%. Moreover, the ad-dition of GABA to the medium can effectively promote the growth rate of pollen tubes.for 5 h. The results showed that pollen germination rate reached to 60% and pollen tube growth rate was 70.4 um/h. Given 10% sucrose and pH value 8.0, pollen germination reached the highest ratio of 63%. Moreover, the addition of GABA to the medium can effectively promote the growth rate of pollen tubes.

Effect of pistil age on pollen tube growth, fruit development and seed set in Cucurbita pepo L.

The effects of pistil age on pollen tube growth, fruit development and seed set were studied in <em>Cucurbita pepo</em> L., the flower of which opens for only six hours. Stigma receptivity lasts four days, from one day before until two days after anthesis. Style receptivity lasts three days, from the day before to the day after anthesis. Ovule receptivity lasts two days: the day of anthesis and the day before. The rate of pollen tube growth varies in different parts of the pistil and in relation to pistil age. In the stigmatic and stylar region, the tubes grow faster if pollination occurs the day before anthesis; in the ovary they grow faster when pollination occurs at anthesis. In the receptacle region, where the transmitting tissue is reduced, the growth rate decreases independently of the time of pollination. The fruits are larger and heavier with more seeds when pollination occurs at anthesis. There is a positive correlation between seed number and fruit weight when pollination occurred at anthesis and the day before.

Pollen tube performance in assessment of compatibility in olive (Olea europaea L.) cultivars

Most olive cultivars are partially self-incompatible. However, the mechanisms of self-incompatibility are still unclear and the choice of pollinizers for the main olive cultivars becomes difficult especially after the constant introduction of foreign cultivars in new environmental conditions. In order to determine compatibility relationships and to select suitable pollinizers for the main Croatian cultivars, pollen germination, pollen tube growth and fertilization percentage have been assessed in ‘Lastovka’ and ‘Oblica’, as well in the Italian cultivar ‘Leccino’ following self- and cross-pollination treatments in 2009 and 2010 seasons. Massive pollen tube growth, higher pollen tube growth rates, earlier and greater amount of fertilization were found for cross-pollination compared to self-pollination. The flowering periods of the studied cultivars overlapped in a high degree in both years. In ‘Lastovka’, cross-pollination was more efficient than self-pollination regarding pollen germination, tube growth and fertilization. Index of self-incompatibility (ISI), calculated as the ratio of fertilization following self-pollination to that following cross-pollination, was 0.4 and 0.5 in 2009 and 2010, respectively, which categorize this cultivar as partially self-incompatible. Higher ISI was found for ‘Leccino’ (0.7 in 2009; 1.0 in 2010) and ‘Oblica’ (0.5 in 2009; 0.7 in 2010). However, most cross-pollination treatments provided higher fertilization levels than self-pollination, proving that cross-pollination was also beneficial for ‘Leccino’ and ‘Oblica’. Inter-incompatibility reactions between cultivars were not observed.

2, 6-Dichlorobenzonitrile causes multiple effects on pollen tube growth beyond altering cellulose synthesis in Pinus bungeana Zucc.

Cellulose is an important component of cell wall, yet its location and function in pollen tubes remain speculative. In this paper, we studied the role of cellulose synthesis in pollen tube elongation in Pinus bungeana Zucc. by using the specific inhibitor, 2, 6-dichlorobenzonitrile (DCB). In the presence of DCB, the growth rate and morphology of pollen tubes were distinctly changed. The organization of cytoskeleton and vesicle trafficking were also disturbed. Ultrastructure of pollen tubes treated with DCB was characterized by the loose tube wall and damaged organelles. DCB treatment induced distinct changes in tube wall components. Fluorescence labeling results showed that callose, and acidic pectin accumulated in the tip regions, whereas there was less cellulose when treated with DCB. These results were confirmed by FTIR microspectroscopic analysis. In summary, our findings showed that inhibition of cellulose synthesis by DCB affected the organization of cytoskeleton and vesicle trafficking in pollen tubes, and induced changes in the tube wall chemical composition in a dose-dependent manner. These results confirm that cellulose is involved in the establishment of growth direction of pollen tubes, and plays important role in the cell wall construction during pollen tube development despite its lower quantity.

Self‐pollination in buds and homostyly in Mussaenda shikokiana (Rubiaceae), a monomorphic species in a distylous clade

AbstractDistyly is a genetically controlled flower dimorphism. Most species in Mussaenda are distylous or functionally dioecious. Mussaenda shikokiana Makino consisted of one floral morph only in the populations studied in southern China. The present study aimed at illuminating the characteristics of floral monomorphism in M. shikokiana, and elucidating the mechanism behind the evolution of monomorphism from distyly. We examined floral morphology in five natural populations across the species' distribution range. Experiments and observations on the floral biology, pollinating fauna, and breeding system were carried out in two to four natural populations. We also screened allozyme markers to estimate the degree of outcrossing. The results indicated that M. shikokiana was a self‐compatible, partially autogamous species and pollinators were required to maximize seed set. Anthers dehisced and pollen showed the highest viability 8–12 h before flower opening, and stigmas were receptive in bud, resulting in bud‐selfing. Pollen viability decreased sharply after flower opening, whereas stigma receptivity lasted to the third day. Pollinators including bees, butterflies, and hawkmoths visited the flowers of M. shikokiana frequently at the study sites. There was no significant difference among pollen tube growth rates in selfing and outcrossing; pollen tubes reached ovules within 16 h in all treatments, indicating lack of typical self‐incompatibility or cryptic self‐incompatibility. Our results suggested that M. shikokiana is a homostylous species, and that homostyly promoting self‐pollination was selected in its evolutionary history.

Pollen Performance in Clarkia Taxa with Contrasting Mating Systems: Implications for Male Gametophytic Evolution in Selfers and Outcrossers.

We tested three predictions regarding the joint evolution of pollen performance and mating system. First, due to the potential for intense intrasexual competition in outcrossing populations, we predicted that outcrossers would produce faster-growing pollen than their selfing relatives. Second, if elevated competition promotes stronger selection on traits that improve pollen performance, then, among-plant variation in pollen performance would be lower in outcrossers than in selfers. Third, given successive generations of adaptation to the same maternal genotype in selfers, we predicted that, in selfing populations (but not in outcrossing ones), pollen would perform better following self- than cross-pollinations. We tested these predictions in field populations of two pairs of Clarkia (Onagraceae) sister taxa. Consistent with our predictions, one outcrosser (C. unguiculata) exhibited faster pollen germination and less variation in pollen tube growth rate (PTGR) among pollen donors than its selfing sister species, C. exilis. Contrary to our predictions, the selfing C. xantiana ssp. parviflora exhibited faster PTGR than the outcrossing ssp. xantiana, and these taxa showed similar levels of variation in this trait. Pollen performance following self- vs. cross-pollinations did not differ within either selfing or outcrossing taxa. While these findings suggest that mating system and pollen performance may jointly evolve in Clarkia, other factors clearly contribute to pollen performance in natural populations.

Pollen Tubes With More Viscous Cell Walls Oscillate at Lower Frequencies

Pollen tubes are tip growing plant cells that display oscillatory growth behavior. It has been demonstrated experimentally that the reduction of the average pollen tube growth rate through elevated extracellular calcium or borate concentrations coincides with a greater amplitude of the growth rate oscillation and a lower oscillation frequency. We present a simple numerical model of pollen tube growth that reproduces these results, as well as analytical calculations that suggest an underlying mechanism. These data show that the pollen tube oscillator is non-isochronous, and is different from harmonic oscillation.