The strength of assortative mating for flowering date and its basis in individual variation in flowering schedule

Although it has been widely asserted that plants mate assortatively by flowering time, there is virtually no published information on the strength or causes of phenological assortment in natural populations. When strong, assortative mating can accelerate the evolution of plant reproductive phenology through its inflationary effect on genetic variance. We estimated potential assortative mating for flowering date in 31 old‐field species in Ontario, Canada. For each species, we constructed a matrix of pairwise mating probabilities from the individual flowering schedules, that is the number of flower deployed on successive dates. The matrix was used to estimate the phenotypic correlation between mates, ρ, for flowering date. We also developed a measure of flowering synchrony within species, S, based upon the eigenstructure of the mating matrix. The mean correlation between pollen recipients and potential donors for flowering date was  = 0.31 (range: 0.05–0.63). A strong potential for assortative mating was found among species with high variance in flowering date, flowering schedules of short duration and skew towards early flower deployment. Flowering synchrony, S, was negatively correlated with potential assortment (r = ?0.49), but we go on to show that although low synchrony is a necessary condition for phenological assortative mating, it may not be sufficient to induce assortment for a given phenological trait. The potential correlation between mates showed no seasonal trend; thus, as climate change imposes selection on phenology through longer growing seasons, spring‐flowering species are no more likely to experience an accelerated evolutionary response than summer species.     

Meta-analysis of phenotypic selection on flowering phenology suggests that early flowering plants are favoured

Flowering times of plants are important life-history components and it has previously been hypothesized that flowering phenologies may be currently subject to natural selection or be selectively neutral. In this study we reviewed the evidence for phenotypic selection acting on flowering phenology using ordinary and phylogenetic meta-analysis. Phenotypic selection exists when a phenotypic trait co-varies with fitness; therefore, we looked for studies reporting an association between two components of flowering phenology (flowering time or flowering synchrony) with fitness. Data sets comprising 87 and 18 plant species were then used to assess the incidence and strength of phenotypic selection on flowering time and flowering synchrony, respectively. The influence of dependence on pollinators, the duration of the reproductive event, latitude and plant longevity as moderators of selection were also explored. Our results suggest that selection favours early flowering plants, but the strength of selection is influenced by latitude, with selection being stronger in temperate environments. However, there is no consistent pattern of selection on flowering synchrony. Our study demonstrates that phenotypic selection on flowering time is consistent and relatively strong, in contrast to previous hypotheses of selective neutrality, and has implications for the evolution of temperate floras under global climate change.     

The influence of variation in female fecundity on effective population size

Understanding the relationship between effective population size (N_e) and the number of adults in a population (N) is important for predicting genetic change in small populations. In general, N_e is expected to be close to N/2, i.e. in the range N/4-3N/4, provided that the powerful effect of population bottlenecks on reducing N_e is factored out (using the harmonic mean of N). However, some very low published estimates of N_e/N(< 0.1) raise the possibility that other factors acting to reduce N_e have been underestimated. Here one such factor, variation in female fecundity, is investigated. Its effect on N_e depends on the standardized variance in fecundity (per breeding season), a measure that is generally independent of mean fecundity. Empirical estimates of this standardized variance from 16 animal studies yielded an average value of 0.44, and a maximum value less than 1.5. To investigate the effect of such values, three kinds of fecundity variation were modelled: random (seasonal): individual; and age-related. Fixed individual differences among females reduce N_e the most. However, to reduce N_e to N/10, the resulting standardized variance must usually be 10 or more. Random differences need to be even larger to achieve the same reduction. One possible mechanism, the random loss of whole families, requires very high family mortality (90% or more). The third model, fecundity that increases linearly with age, is ineffective at causing a marked decrease in N_e. Given the finding that very unusual conditions are required to reduce N_e below N_e/10, low estimates of N_e/N need to be examined critically: the lowest published ratio, for a natural population of oysters, was found to be questionable because of possible immigration into the population by cultivated oysters.     

Reproductive success and effective population size in woodrats (Neotoma macrotis)

Discrepancies between the census size and the genetically effective size of populations (N(e)) can be caused by a number of behavioural and demographic factors operating within populations. Specifically, strong skew in male reproductive success, as would be expected in a polygynous mating system, could cause a substantial decrease in N(e) relative to census size. Because the mating system of Neotoma macrotis had previously been described as one nearing harem polygyny, I examined the distribution of reproductive success and genetic variation within a population of this species. Combining genetic data and three years of field observations, I show that variance in reproductive success does not deviate from poisson expectations within either sex and variance in success is similar between the sexes. Furthermore, both males and females had multiple partners across litters in addition to some evidence of multiple paternity within litters. Despite a lack of strong skew in reproductive success, an estimate of N(e) based on a number of demographic parameters suggests that the ratio of N(e)/N in this population is 0.48. Although the ratio of N(e)/N suggests that the population is experiencing higher rates of genetic drift than would be expected based on census size alone, the population maintains high levels of genetic diversity. Estimates of neighbourhood size and patterns of recruitment to the study site suggest that immigration plays an important role in this population and may contribute to the maintenance of high levels of genetic diversity.     

Testing the 'assortative mating' hypothesis on a variation maintenance mechanism for flowering time within a forest-floor population of Primula sieboldii

Flowering phenology and allozyme variation were studied to test the existence of positive assortative mating for flowering time in a natural population of Primula sieboldii E. Morren, a heterostylous perennial herb, consisting of approximately 180 genets in a deciduous forest. There was significant variation in flowering date among genets, but not between heterostylous morphs. The temporal order of the flowering time of genets was fairly constant for the two years of the study. The spatial heterogeneity of light availability at the study site was small during the flowering season of the species. In order to analyze the extent of genetic differentiation between early- and late-flowering genet groups, allozyme diversities were analyzed with 10 loci. The G_ST between the early- and late-flowering groups was not significantly different from zero. Evidence of positive assortative mating for flowering time was not detected. Prolongation of flowering duration due to pollen limitation may be one important factor preventing the genetic differentiation of early- and late-flowering groups by enhancing the overlap of flowering time among genets.     

Time spent in distinct life history stages has sex‐specific effects on reproductive fitness in wild Atlantic salmon

In species with complex life cycles, life history theory predicts that fitness is affected by conditions encountered in previous life history stages. Here, we use a 4‐year pedigree to investigate if time spent in two distinct life history stages has sex‐specific reproductive fitness consequences in anadromous Atlantic salmon (Salmo salar). We determined the amount of years spent in fresh water as juveniles (freshwater age, FW, measured in years), and years spent in the marine environment as adults (sea age, SW, measured in sea winters) on 264 sexually mature adults collected on a river spawning ground. We then estimated reproductive fitness as the number of offspring (reproductive success) and the number of mates (mating success) using genetic parentage analysis (>5,000 offspring). Sea age is significantly and positively correlated with reproductive and mating success of both sexes whereby older and larger individuals gained the highest reproductive fitness benefits (females: 62.2% increase in offspring/SW and 34.8% increase in mate number/SW; males: 201.9% offspring/SW and 60.3% mates/SW). Younger freshwater age was significantly related to older sea age and thus increased reproductive fitness, but only among females (females: ?33.9% offspring/FW and ?32.4% mates/FW). This result implies that females can obtain higher reproductive fitness by transitioning to the marine environment earlier. In contrast, male mating and reproductive success was unaffected by freshwater age and more males returned at a younger age than females despite the reproductive fitness advantage of later sea age maturation. Our results show that the timing of transitions between juvenile and adult phases has a sex‐specific consequence on female reproductive fitness, demonstrating a life history trade‐off between maturation and reproduction in wild Atlantic salmon.     

The roles of female and hermaphroditic flowers in the gynodioecious–gynomonoecious Silene littorea: insights into the phenology of sex expression

Some gynodioecious species have intermediate individuals that bear both female and hermaphroditic flowers. This phenomenon is known as a gynodioecious–gynomonoecious sexual system. Gender expression in such species has received little attention in the past, and the phenologies of male and female functions have also yet to be explored. In this study, we examined variations in gender patterns, their effects on female reproductive success and sex expression in depth throughout the flowering period in two populations. The studied populations of Silene littorea contained mostly gynomonoecious plants and the number of pure females was very low. The gynomonoecious plants showed high variability in the total proportion of female flowers. In addition, the proportion of female flowers in each plant varied widely across the flowering season. Although there was a trend towards maleness, our measures of functional gender suggested that most plants transmit their genes via both pollen and ovules. Fruit set and seed set were not significantly different among populations; in contrast, flower production significantly varied between the two populations – and among plants – with consequent variation in total seed production. Conversely, gender and sex expression were similar in both populations. Plants with higher phenotypic femaleness did not have higher fruit set, seed set or total female fecundity. The mating environment fluctuated little across the flowering period, but fluctuations were higher in the population with low flower production. We therefore conclude that the high proportion of gynomonoecious individuals in our studied populations of S. littorea may be advantageous for the species, providing the benefits of both hermaphroditic and female flowers.     

Lifetime fitness components in female colour morphs of a damselfly: density‐ or frequency‐dependent selection?

In many damselfly species mature females exhibit colour polymorphism: one female morph resembles the conspecific male (androchrome) while the others do not (gynochromes). Hypotheses for the maintenance of such polymorphisms differ mainly as to whether they are based on density- and/or frequency-dependent selection and on the nature of the frequency dependence. We collected lifetime fitness data (individual lifespan, number of copulations and number of ovipositions) for female morphs of the damselfly Ischnura elegans from 15 insectaries differing in population parameters (density, sex ratio and ratio of andro- to gynochromes). Both density and frequency affected a specific set of the studied fitness components. While morph frequency influenced lifespan, sex ratio influenced the number of copulations, and density affected lifespan and the number of ovipositions. Clearly, discrepancies among studies may be generated if the studied fitness components differ. Our final fitness estimate, the number of ovipositions, was only influenced by density, thereby not supporting frequency-based hypotheses. Contrary to expectation under the current density-based hypothesis, androchromes compared to gynochromes had a lower number of ovipositions at high density. We discuss our findings in the light of mechanisms maintaining the female polymorphism.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 86 , 515–523.     

Rainfall and flowering synchrony in a tropical shrub: Variable selection on the flowering time ofErythroxylum havanense

Summary We tested the adaptive significance of flowering synchrony by means of a quantitative analysis of selection and by flowering induction experiments with the deciduous shrubErythroxylum havanense. Temporal schedules of flower and fruit production were determined for a local population (in three sites) in a Mexican seasonal forest for 2 years (1987–1988). The consequences of natural variation in flowering time (flowering initiation day) on maternal reproductive success (fecundity) were evaluated. We observed high levels of inter- and intraindividual flowering synchrony in 1987, but not in 1988 and this contrast was related to differences in rainfall patterns between the two years. A significant proportion (15.4%) of the phenotypic variation in flowering initiation day was accounted for by environmental variance. The expression of phenotypic variance of flowering time and, consequently, the opportunity for selection to act, are controlled by annual variation in rainfall. Despite the between-year difference in flowering synchrony, we detected a relatively intense directional selection on flowering initiation day in both years, but selection coefficients were of opposite sign (standardized directional gradients were –0.326 and 0.333 for 1987 and 1988, respectively). For both years there was a significant relationship between individual relative fitness and the number of neighbouring flowering plants in a given day, suggesting positive frequency-dependent selection.     

冬季水淹对秋华柳的开花物候及繁殖分配的影响

开花物候及繁殖分配是植物适应环境的重要因素,为了解长期冬季水淹对三峡库区耐淹物种秋华柳(Salix variegata Franch.)繁殖的影响,研究了长期冬季水淹条件下秋华柳的开花物候和繁殖分配情况。实验在2006年11月份设置了如下处理:对照,完全水淹(植株置于水中,顶部距水面2m)30,60,90,120d和150d。结果表明:(1)对照及各水淹处理的秋华柳花期都较长,在7-11月份持续开花,个体开花进程(开花振幅曲线)呈单峰曲线。(2)冬季水淹对秋华柳群体及个体的开花物候有显著影响。水淹时间越长,始花期越晚,花期持续时间越短(P0.05)。(3)长期冬季水淹下,秋华柳显著降低了繁殖分配比例和全株生物量及单株花序数(P0.05)。(4)开花物候指数与繁殖分配的相关分析表明:始花时间越晚的个体,花期持续时间越短。花期持续时间越短的个体花序数越少,致使繁殖分配越小。总的来说,冬季水淹下,秋华柳通过推迟开花日期、缩短花期持续时间使繁殖分配比例降低,将更多的资源分配到生存力上,是秋华柳对长期冬季水淹的一种适应。同时,在长期冬季水淹后,秋华柳仍保持一定的开花繁殖能力,是其在应用于三峡水库消落区植被构建后产生后代延续种群的前提条件。     

开花时间与伴生种对鹤首马先蒿传粉和生殖成功的影响

开花物候是物种间相互作用的重要生活史特征和适合度因子,在全球气候变化的背景下而备受关注.为探讨开花时间如何存种内和种间水平上影响植物的传粉和生殖成功,我们连续3年(2003-2005)对不同花期和伴生种存在情况下的鹤首马先蒿(Pedicularis gruina)的传粉者访花忠实度、受粉率、坐果率、单果种子产量和果实被啃食频率进行了比较研究.结果表明鹤首马先蒿的坐果率主要受其传粉环境的影响:在没有伴生种时,不同时期鹤首马先蒿的坐果率没有显著差异,34-38％的花可以坐果;在有伴生种存在时既可以显著提高其坐果率,也可显著降低其坐果率,这取决于传粉者类型以及伴生种密穗马先蒿(P.densispica)花期的差异.密穗马先蒿具有花蜜和花粉双重报酬,在群落中可以作为主体物种吸引传粉者并间接促进与其伴生的鹤首马先蒿的传粉和生殖成功.同样无蜜的管花马先蒿(P.siphonantha)和鹤首马先蒿伴生,则是通过提高群落水平对传粉者的吸引力进而直接促进鹤首马先蒿的传粉和生殖成功.此外,研究结果也表明开花时间对坐果率没有显著影响,但是显著影响单果种子产量和果实被啃食的频率.在相同的传粉条件下早花期植株单果种子产量显著高于晚花期的种子产量,同时早花期的果实被啃食的频率显著增加.     

Multiple mating and its effect on the reproductive success of female Epiphyas postvittana (Lepidoptera: Tortricidae)

Abstract.

• 1 Multiple mating and its effect on reproductive performance of female Epiphyas postvittana (Walker) moths were studied under controlled conditions.
• 2 The age at which the moths mated for the first time ranged from the first to the tenth day after emergence, but 71% of first matings were during the first 3 days.
• 3 The majority (63%) of females had one or two spermatophores in the bursa copulatrix. Some (24%) were found with three to five spermatophores, whereas no successful mating occurred among 13% of individuals. The number of matings was partly dependent on the number of mates available to the female. Between the range of sex ratios of one male to one female and four males to one female maximal mating success occurred at the ratio of three males to one female.
• 4 Virgin females were capable of egg-laying, but mating stimulated and accelerated oviposition. Mated individuals laid twice as many eggs as unmated ones.
• 5 The level of copulatory activity did not influence the longevity of females irrespective of the number of males available to them.
• 6 Sex ratios with greater than one male to a female improved the reproductive success by marginally increasing fecundity and fertility.
• 7 It is concluded that multiple mating would enhance population growth, and is of particular benefit to populations with a preponderance of females, as is known to occur naturally in this species.

     

Fire effects on plant reproductive fitness vary among individuals,reflecting pollination-dependent mechanisms

Premise

Fire induces flowering in many plant species worldwide, potentially improving reproductive fitness via greater availability of resources, as evident by flowering effort, and improved pollination outcomes, as evident by seed set. Postfire increases in flowering synchrony, and thus mating opportunities, may improve pollination. However, few studies evaluate fire effects on multiple components of fitness. Consequently, the magnitude and mechanism of fire effects on reproductive fitness remain unclear.

Methods

Over multiple years and prescribed burns in a prairie preserve, we counted flowering stems, flowers, fruits, and seeds of three prairie perennials, Echinacea angustifolia, Liatris aspera, and Solidago speciosa. We used aster life-history models to assess how fire and mating opportunities influenced annual maternal fitness and its components in individual plants.

Results

In Echinacea and Liatris, but not in Solidago, fire increased head counts, and both fire and mating opportunities increased maternal fitness. Burned Echinacea and Liatris plants with many flower heads produced many seeds despite low seed set (fertilization rates). In contrast, plants with an average number of flower heads had high seed set and produced many seeds only when mating opportunities were abundant.

Conclusions

Fire increased annual reproductive fitness via resource- and pollination-dependent mechanisms in Echinacea and Liatris but did not affect Solidago fitness. The consistent relationship between synchrony and seed set implies that temporal mating opportunities play an important role in pollination. While fire promotes flowering in many plant species, our results reveal that even closely related species exhibit differential responses to fire, which could impact the broader plant community.     

Effects of multiple matings on reproductive fitness of male and female Diaeretiella rapae

Mating frequency and the amount of sperm transferred during mating have important consequences on progeny sex ratio and fitness of haplodiploid insects. Production of female offspring may be limited by the availability of sperm for fertilizing eggs. This study examined multiple mating and its effect on fitness of the cabbage aphid parasitoid Diaeretiella rapae McIntosh (Hymenoptera: Aphidiidae). Female D. rapae mated once, whereas males mated with on average more than three females in a single day. The minimum time lag between two consecutive matings by a male was 3 min, and the maximum number of matings a male achieved in a day was eight. Sperm depletion occurred as a consequence of multiple mating in D. rapae. The number of daughters produced by females that mated with multiple‐mated males was negatively correlated with the number of matings achieved by these males. Similarly, the proportion of female progeny decreased in females that mated with males that had already mated three times. Although the proportion of female progeny resulting from multiple mating decreased, the decrease was quicker when the mating occurred on the same day than when the matings occurred once per day over several days. Mating success of males initially increased after the first mating, but then males became ‘exhausted’ in later matings; their mating success decreased with the number of prior matings. The fertility of females was affected by mating with multiple‐mated males. The study suggests that male mating history affects the fitness of male and female D. rapae.     

Index of intensity of sexual selection: A comment on Nishida's article

An index of intensity of sexual selection proposed by Nishida (1992: Res. Popul. Ecol. 34: 373–382) was examined. Two examples were presented to show that Nishida's index was not free from confounding effect of mortality schedule. Importance of removing the phylogenetic effects in comparative analyses was also discussed.     

Sexual conflict in twins: male co-twins reduce fitness of female Soay sheep

Males and females often have different requirements during early development, leading to sex-specific interactions between developing offspring. In polytocous mammals, competition for limited resources in utero may be asymmetrical between the sexes, and androgens produced by male foetuses could have adverse effects on the development of females, with potentially long-lasting consequences. We show here, in an unmanaged population of Soay sheep, that female lambs with a male co-twin have reduced birth weight relative to those with a female co-twin, while there was no such effect in male twins. In addition, females with a male co-twin had lower lifetime breeding success, which appeared to be mainly driven by differences in first-year survival. These results show that sex-specific sibling interactions can have long-term consequences for survival and reproduction, with potentially important implications for optimal sex allocation.     

Genetic consequences of polygyny and social structure in an Indian fruit bat, Cynopterus sphinx. II. Variance in male mating success and effective population size

Variance in reproductive success is a primary determinant of genetically effective population size (Ne), and thus has important implications for the role of genetic drift in the evolutionary dynamics of animal taxa characterized by polygynous mating systems. Here we report the results of a study designed to test the hypothesis that polygynous mating results in significantly reduced Ne in an age-structured population. This hypothesis was tested in a natural population of a harem-forming fruit bat, Cynopterus sphinx (Chiroptera: Pteropodidae), in western India. The influence of the mating system on the ratio of variance Ne to adult census number (N) was assessed using a mathematical model designed for age-structured populations that incorporated demographic and genetic data. Male mating success was assessed by means of direct and indirect paternity analysis using 10-locus microsatellite genotypes of adults and progeny from two consecutive breeding periods (n = 431 individually marked bats). Combined results from both analyses were used to infer the effective number of male parents in each breeding period. The relative proportion of successfully reproducing males and the size distribution of paternal sibships comprising each offspring cohort revealed an extremely high within-season variance in male mating success (up to 9.2 times higher than Poisson expectation). The resultant estimate of Ne/N for the C. sphinx study population was 0.42. As a result of polygynous mating, the predicted rate of drift (1/2Ne per generation) was 17.6% higher than expected from a Poisson distribution of male mating success. However, the estimated Ne/N was well within the 0.25-0.75 range expected for age-structured populations under normal demographic conditions. The life-history schedule of C. sphinx is characterized by a disproportionately short sexual maturation period scaled to adult life span. Consequently, the influence of polygynous mating on Ne/N is mitigated by the extensive overlap of generations. In C. sphinx, turnover of breeding males between seasons ensures a broader sampling of the adult male gamete pool than expected from the variance in mating success within a single breeding period.     

青藏高原高寒草甸植物花寿命与花吸引特征的关系及其对雌性繁殖成功的影响

花寿命指花保持开放且具有功能的时间长度, 是开花植物繁殖成功的一个重要功能性状。可塑性延长花寿命是植物在不利的传粉环境中保障繁殖的一种策略, 但延长花寿命也会增加繁殖成本。花寿命的可塑性变异不仅受传粉环境的影响, 而且还受资源分配权衡的影响。花寿命的理论模型指出, 植物的花寿命与花吸引特征之间存在资源分配权衡。为了研究在花粉限制环境中, 植物花寿命与花吸引特征之间的资源权衡及其对雌性适合度的相对重要性。该研究以青藏高原高寒草甸不同海拔(2 900和3 600 m)的11种开花植物为研究对象, 分析了不同植物群落中, 物种水平上: (1)花寿命与花吸引特征(花大小以及开花数目)之间的相关关系; (2)花寿命与花吸引特征对植物雌性适合度的相对贡献。结果表明, 无论是低海拔还是高海拔植物群落, 植物的花寿命与开花数目之间均存在权衡关系, 且长的花寿命增加了植物的雌性适合度。但在高海拔环境中, 植物的雌性适合度只与花寿命有关。这说明相对于低海拔植物, 花寿命对高海拔植物的雌性繁殖成功更为重要。     

Male–female evolutionary game on mate-locating behaviour and evolution of mating systems in insects

We present a model of a male–female evolutionary game on mate-locating behaviour. Two major mating systems are considered: "lek polygyny" (in which males aggregate to wait for females searching for males) and "searching polygyny" (in which males search for females emerging or waiting for males). The model predicts that lek polygyny is favoured (i) when male survivorship during lekking is sufficiently higher than that during mate searching, (ii) when female survivorship while visiting a lek is sufficiently higher than at the emergence site, or (iii) when searching efficiency is higher at a lek than at an emergence site. Furthermore, the model shows that a reduction in the reproductive value of females later in the day, which prevents males from performing riskier mate-locating behaviour, can result in a change of mating system. In addition, mixed mating systems can be realized as transient states during this shift.