Evolutionary ecology of plant reproductive strategies download

It is only the rare biologist who studies both plants and animals e. The fact that work so far has looked at either plants or animals exclusively is understandable given the profound differences in ecological and life-history attributes of the two lineages, which diverged at least 1.

Although both plants and animals have diversified tremendously in their reproductive strategies, their fitness is inevitably determined by both quality and quantity of offspring, and is limited by resource availability. Thus, the reproductive strategies of these deeply divergent lineages may have converged due to the underlying similarity in evolutionary pressures and constraints.

Indeed, the cardinal theoretical construct for the offspring size-vs. Following this seminal paper, there has been a substantial body of work dedicated to understanding variation in offspring size and number, but we still do not fully understand why and how this diversity evolves.

Several studies have established a strong negative correlation between seed size and seed number within and across plants species Greene and Johnson, ; Turnbull et al. In recent decades, a few comprehensive reviews have evaluated the differences in seed size in the context of variation a within species Harper et al. The quest to explain the changing shapes of the offspring size-vs. However, strong empirical evidence supporting either argument is scarce. This review revolves around the size-number tradeoff to highlight various unanswered facets of the evolutionary ecology of plant and animal reproductive strategies.

We begin by providing an overview of theoretical models of progeny size and fitness relationships, applicable equally to plants and animals. From that point onwards, we develop the discussion on plants and animals separately. After discussing theory, we summarize important life-history constraints on offspring size and number and then explore evidence for parent-offspring conflict at the genetic level in model organisms Arabidopsis and Drosophila.

Finally, we evaluate how the size-number tradeoff is affected by environmental variables. Wherever feasible, we juxtapose available evidence with the predictions of theoretical models. Reviews focused on animal reproductive strategies already exist e.

The animal section aims to distill information from these reviews and from more recent literature, and synthesize the salient points. On the other hand, detailed reviews on plant reproductive strategies are lacking. Therefore, the plant section endeavors to provide a fairly comprehensive review of the literature related to this theme.

We aim to exploit commonalities between animals and seed plants to identify broad trends in reproductive strategies of the two lineages. The theoretical underpinnings of the offspring size-number tradeoff have their origins in ornithology. Pioneering work by David Lack in the s on the evolution of clutch size in birds ignited a rich series of studies on vertebrate systems, and later also on invertebrates and plants. Initially, clutch size models for vertebrates and invertebrates were developed independently, both inspired by optimal foraging models Charnov, ; Parker and Stuart, ; Stephens et al.

The ground-breaking model by Smith and Fretwell hereafter the SF model assumes that offspring fitness increases with parental investment but experiences diminishing returns Figure 1A.

As a consequence, parental fitness is maximized by an intermediate optimal investment in individual offspring. In the MVT model, energy gain per unit time decreases as a function of time spent in a single patch, and the forager is predicted to leave the patch when the foraging rate the slope of the energy gain curve equals the mean foraging rate in the environment.

The SF model describes parental investment into offspring and profit in terms of parental fitness through offspring survival , whereas the MVT model describes investment in terms of time as well as energy spent foraging, with energy gained as profit.

Figure 1. Optimization models tackling offspring size and fitness A Classical Smith and Fretwell model SF model; Smith and Fretwell, , B SF model in the context of genetic quality of offspring and sibling rivalry see Haig, , C SF model in the context of resource limitation adapted from McGinley and Charnov, , D Changing slope of the SF curve in response to changing environmental condition Einum et al.

Maternal- and offspring-specific optimal sizes, as predicted by the SF model Figure 1A , create a conflict between parent and offspring, which raises the important issue of maternal vs. While competition between animal offspring may be active and even include direct fratricide, competition between sibling seeds in plants is more likely to be passive, via the exhaustion the maternal resource pool but see the section on Endosperms.

Importantly, the nutrient drawing ability Uma Shaanker et al. Similarly, some animals selectively starve the weakest offspring when resources become limited after oviposition or birth Lack, ; Ricklefs, ; Klopfer and Klopfer, By selectively aborting or starving inferior offspring by exerting direct control over offspring number , the parent will also sacrifice some of its control over the size of the remaining offspring.

This is because the surviving offspring will receive more resources, grow bigger, and thereby in the case of seeds increase their nutrient drawing ability, meaning that zygotic control becomes stronger. On the other hand, shifting resources from inferior to superior offspring is also in the parent's own best interest.

This argument requires some mechanism of post-zygotic recognition of genetic quality by parents. Little is known about such mechanisms, but offspring size is a very reasonable proxy for genetic quality, and indeed, all known instances of selective abortion in plants and starvation in animals involve elimination of the smallest among competing siblings e.

Furthermore, there is evidence that seeds generated from cross-fertilization presumably of higher quality tend to be larger than those from self-fertilization e. There is also some evidence to suggest that maternal genetic quality determines the extent of selective abortion in plants e. Yet, we do know what role paternal genetic quality plays in determining offspring fitness.

More specifically, we do not know how the maternal genome discriminates between offspring that are superior due to better quality eggs and those that are superior due to better quality pollen. The SF model depends on two critical assumptions- a an optimum fraction of available total resources should be invested in reproduction such that it maximizes lifetime reproduction of a parent and b the resource pool is a homogenous entity, which parents invest to determine offspring size.

While point a is intuitively true, resources are often heterogeneous. For example, plants require both carbon and nitrogen, and the availability of these two components may vary over the course of a season.

Both seed size and numbers generally increase after CO 2 fertilization e. Increases in seed size are attributed to greater carbon fixation through photosynthesis under elevated CO 2 , while increases in seed number are attributed to increased mobilization of nitrogen from leaves to seeds. This is surprising given that nitrogen is often a limiting factor for plant growth, and this limitation is expected to be enhanced under elevated CO 2. But, the results are not that surprising when we take into account greater water use efficiency and greater photosynthetic rates in plants under elevated CO 2 , and this is despite decreases in leaf nitrogen content Jablonski et al.

The second and third crop of flowers and seeds may become small since carbon fixed through photosynthesis declines with plant age because of senescence , but seed numbers may be maintained since nitrogen supply can be ensured through continued remobilization of nitrogen from senescing leaves to seeds. It needs to be emphasized that resource or nutrient limitation is one of many different types of sub-optimal harsh environments experienced by plants, and plant response may be different for a different type of harsh condition.

A simple example is shown in Figure 1D after Einum et al. In the classical SF model, the offspring size-vs. However, it seems unrealistic that diminishing returns should set in immediately after the size viability threshold, especially since increasing seed size increases nutrient drawing ability which suggests that the curve should have an early exponential—or at least concave—part.

Thus, the SF curve is more appropriately represented as a sigmoid Figure 1E. Therefore, the offspring size-vs. Like the classical SF function, a sigmoid predicts an intermediate maternal optimum situated in the convex part of the function. Yet, exponential decrease or increase in fitness of seeds belonging to size classes close to a maternal optimum as seen in a sigmoid suggests strong selection pressures on seed size optimization.

The sigmoid accommodates the idea that maternal control increases above a certain offspring size for animal examples see Levitan, ; Reeve et al. However, the assumption that big offspring always show greater fitness compared to small offspring may not hold in some special cases where developmental and habitat constraints play a role. Hendry et al. Theoretically, larger hatchlings from bigger eggs are expected to show higher fitness in favorable environments.

Nonetheless, big eggs may not survive to hatchling stage either due to space constraints for incubation lack of parental care or suffer from asphyxia in sub-optimal aquatic habitats.

In such cases, optimum offspring size for maximizing maternal and offspring fitness are likely identical contrast Figures 1D,F. We will reassess shifts in maternal optima in greater detail under the empirical section dealing with environmental responses of offspring size and number tradeoff.

The origin s , evolution and diversity of seeds themselves are thought to be critically important in the domination of seed plants in terrestrial ecosystems.

Seeds ensured the safety of the stationary female gametophyte ovule , and led to novel mechanisms of long-distance dispersal Nathan et al. In conjunction with breakthrough adaptations such as efficient xylem transport and stomatal mechanisms e.

Seed diversity is dramatic across the plant kingdom, and principal aspects of this variation are size and number. Plant form, habitat, ecophysiology, and reproductive modes are some of the higher-order life- history determinants of seed size and number, and these factors have both restricted variation and led to common trends within and between plant species and families. While these constraints are not directly related to the theoretical models presented earlier, they have a central place in our discussion, since it may turn out that explanations for observed trends in seed plants may lie in these deep life history constraints on reproductive strategies.

Ovules are the progenitors of seeds in higher plants, and seed size is influenced by the thickness of their four anatomical layers, viz. Variation in these four components manifests in the structural and morphological diversity of seeds, and each component likely has a different impact on the relationship between seed size and fitness, although these have not been assessed individually until now.

The reduced size of angiosperm seeds typically between 0. Ovule size in angiosperms is also strongly constrained by flower size, since large flowers are expensive to build and maintain. In cases where angiosperm flowers did become large, natural selection appears to have favored a higher number of ovules per flower rather than increased ovule size Endress, ; Greenway and Harder, Increase in ovule number per flower has also been attributed to the higher probability of receiving a high amount of pollen grains on a single large flower rather than an even spread of pollen grains on a greater number of flowers Burd et al.

The origin, function and diversity of endosperms has a complex relationship with seed size and function in angiosperms Li and Berger, The endosperm is a specialized embryo-nourishing tissue and is one of the products of double fertilization, a unique feature of angiosperm reproduction Baroux et al. As a general trend, the endosperm is larger in primitive angiosperm clades and small or absent in younger clades. Consequently, the embryo-to-seed ratio is significantly higher in younger clades than in primitive clades Forbis et al.

In such plants, the size, capacity for dormancy, viability, durability and fitness of seeds is determined by the size of the endosperm and not that of the embryo Martin, ; Bremner et al.

In contrast, in younger dicot families, such as cucurbits and orchids, the endosperm is reduced to a couple of layers of cells. It is argued that the endosperm in early angiosperms was a competing embryo that gradually evolved into an aborting, altruistic nourishing tissue Friedman, There also is some evidence to suggest that the endosperm competes with the embryo for maternal resources until it is consumed by the developing embryo Zhou et al. While the presence of an endosperm can be seen as a maternal strategy of offering parental care in absentia , an important factor often overlooked in simple seed size-vs.

Sugars can accumulate in seeds from the breakdown of lipids via the glyoxylate cycle or hydrolysis of starch, and both can support seed germination and seedling growth. Soluble oligosaccharides in seeds increase seed tolerance to oxidative stress, desiccation, chilling, and salinity stresses Obendorf, ; Nishizawa et al. It is logical that large seeds can store larger amounts of all nutrients starch, soluble sugars, and lipids , and hence be more resilient during periods of stress.

Nutrition quality is also associated with seed dispersal mode. Wind- and animal-dispersed species produce lighter seeds with more fat than carbohydrate and protein content Lokesha et al. A global phylogenetic analysis of land plants shows that growth form whether a plant is a herb, shrub, or a tree is the most significant determinant of seed size Moles et al. Herbaceous plants many are annuals bear small seeds and fruits unless they are crawlers or climbers Levin, ; Westoby et al.

To bear large, heavy seeds and fruits a plant needs mechanical strength lignification and secondary growth. Even within a given growth-form, seed size generally increases with body size vegetative biomass , following allometric principles Thompson and Pellmyr, ; Shipley and Dion, At another level, the production of large seeds needs more resources and more time longer generations. Allometric principles are also shown to underpin the correspondence between leaf size and the size of reproductive appendages in trees.

Perennial plants may overcome allometric limitations on seed size by increasing reproductive output over the entire lifespan of the parent Venable and Rees, , while herbaceous plants under severe competition for space and resources may evolve short generation times and production of a large number of small seeds Aarssen and Jordan, It has been suggested that the diversity in seed size across plant lineages follows a positively skewed fractal distribution Hegde et al.

Hence, the frequency distribution of habitats space may explain the frequency distribution of plant body size, and in consequence, seed size Valen, ; Hegde et al. Moreover, larger seeds, despite their low initial RGR, have higher growth potential and attain larger overall body sizes at full maturity Turnbull et al. Dispersal of offspring away from the parent plant is a fundamental means of reducing competition between parent and offspring, as well as among offspring.

Dispersal may also aid offspring escape from a stressful resource-limited habitat. The earliest vascular plants, despite having succeeded in colonizing terrestrial habitats, continued to depend on water for the dispersal of haploid male and female spores, since at least one phase of their reproductive cycle, often the gametophyte, was aquatic.

For various reasons see Bateman and DiMichele, for a review , the female spore of multiple early vascular plant lineages became large heterospory , and the dispersal of such megaspores became restricted. This later led to the evolution of seeds and the diversification of dispersal modes. Seed dispersal syndromes in angiosperms are associated with the phylogeny of fruit types, and they influence seed size Moles et al.

Wind and animal dispersed species that produce seeds with specialized appendages such as wings and fleshy coats tend to have smaller seeds than those dispersed through passive or explosive mechanisms with limited dispersal range Lokesha et al. Costs of dispersal increase with increasing seed size. Interestingly, some plants have evolved a strategy to not invest in dispersal altogether, since the probability of seedling survival is very low. The best examples are seen in viviparous mangrove plants.

Vivipary is a rare form of parental care in plants occurring in only 20 genera and 13 families, Elmqvist and Cox, , where the parent plant bears a few large seeds that mature and germinate while still attached to it. Free from the constraint of optimization for dispersal, the mother plant can invest in large seeds. Indeed, large mangrove seeds have some of the longest maturation times from pollination to seedling establishment known in the plant kingdom up to several years.

Thus, the cost of maintaining non-dormant seeds and seedlings is likely to be huge for the mother plant, leading to a strategy of optimizing seed numbers. Mangrove plants achieve offspring number optimization either by premature abscission of damaged seedlings or selective abortion of small seeds Farnsworth and Ellison, ; Saenger, At a macrogenomic level, a positive correlation between genome size and seed size is observed both within and between phylogenetic clades, especially in angiosperm lineages with small genomes and within the pine clade in gymnosperms Thompson, ; Beaulieu et al.

Existing arguments to explain co-divergence of genome size and seed size are inadequate and inconclusive Beaulieu et al.

While it is important to understand genome-phenome associations across plant families, answering such questions may not address the diversity in seed size and numbers at the level of species and ecotypes, which are subtler and likely regulated by microgenomic interactions.

We do not yet have enough information, especially in non-model plant systems, for an overarching genetic paradigm of seed size, shape and number regulation Orsi and Tanksley, ; Gegas et al.

However, some insight is offered by model systems such as Arabidopsis thaliana hereafter simply Arabidopsis. Arabidopsis has a fast generation time, and possesses a vastly under-appreciated natural genetic and phenotypic variation Koornneef et al. However, the microscopic size of Arabidopsis seeds poses methodological difficulties in characterizing seed phenotype, which prevented it from being a model to investigate evolutionary ecology of plant reproductive strategies.

The advent of digital techniques to precisely quantify seed characteristics has led to closer examinations of seeds in Arabidopsis mutants e. In their study of 64 different mutant lines, Van Daele et al. A re-examination of the same data shows that the mean seed size in Arabidopsis has limited variation 0. Total seed number per plant showed a fold range —4,, including outliers while total seed weight per plant showed a 5-fold range 50— mg.

However, there was no correlation between total seed weight and total seed number or mean seed size Figures 2B,C : plants with high total seed weight had heavier denser , not more or larger, seeds than those with low total seed weight probably due to differences in oil content, see section on nutrient quality and quantity above.

We also found that there was zero correlation between total rosette leaf area and total seed weight and similarly there was no relationship between rosette area and total seed number per plant Figures 2A,D. While photosynthesis and general metabolism in leaves may have been compromised in some Arabidopsis mutants leading to differential seed production, the complete lack of any relationship between leaf mass and seed mass or number remains to be explained.

Figure 2. Relationships between seed traits in Arabidopsis mutants data from Van Daele et al. Total seed weight vs. A Total seed number per plant, B mean seed size, C total leaf tissue area. D Total seed number vs. The orange dots correspond to a minority of outliers mutants with extreme trait values in each plot, whose inclusion into the dataset alters these relationships significantly.

It is not a surprise that some of these orange data points include mutants and mutations in genes directly associated with seed size regulation. Regression lines and coefficients of determination are calculated without these outliers. Research on genetic controls on seed size has been reviewed extensively in recent years Sundaresan, ; Linkies et al. Instead of providing a descriptive account of genes regulating seed size, we present an illustrative summary of seed size variation in Arabidopsis mutants Figure 3 ; Table 1.

The data suggest that genes acting in maternal tissues have less impact on seed size than genes operating directly in zygotes embryos , which can be partly due to the bias in the data set, since we know more about genes acting in maternal tissues. It is equally significant to note that the percent increase in seed size due to loss-of-function mutations in genes whose normal function is to restrict seed size irrespective of whether they act in maternal or zygotic tissue is significantly greater than the percent decrease in seed size due to loss-of-function in genes that are actively promoting larger seeds.

Figure 3. Genetic controls on seed size in Arabidopsis thaliana. The table summarizes mutations in specific genes known to be directly involved in regulating seed size. Table 1. Maternal and zygotic genetic controls on seed size: Insights from Arabidopsis thaliana. Maternal genes involved in restricting the size of inferior small seeds will certainly be different from those involved in diverting more resources toward superior large seeds.

Furthermore, it is well established that during plant development also in animals , maternal genetic controls wean and are successively replaced by zygotic controls e. What will be important to establish, however, is whether increases in the size of superior seeds are due to a weakening control of maternal genes restricting seed size or due to overexpression of zygotic genes promoting seed size Figure 3.

Arabidopsis mutants have provided valuable insights into specific genes and gene-networks that control seed size North et al. In this context, it is important to distinguish between genes that exclusively control seed size and those that indirectly influence seed development through their effects on global resource allocation and organ size within a plant Mizukami and Fischer, ; Adamski et al.

Analysis of genes directly involved in seed-size control in Arabidopsis mutants invariably show that variation in seed size is not linked to seed number Zhou et al. QTL analysis of natural variation also suggests that genetic factors controlling seed size and seed numbers are situated in different parts of the Arabidopsis genome on different chromosomes and can evolve independently Gnan et al.

We know a lot less about the genes involved in regulating seed number than about genes regulating seed size. Some of the genes regulating seed numbers are associated with regulation of sizes of floral meristems and inflorescences. Maize mutants producing more seeds kernels per row do not produce seeds that are smaller than those from wild, control groups Bommert et al. It is also possible that genes involved in initial floral differentiation predetermine the number of ovules per ovary and number of flowers per inflorescence, thus impacting seed number through pleiotropic effects Huang et al.

Some other genes such as APETALA2 and AHKs cytokinin signal receptors have the most dramatic impact on not only seed size but also seed number through their global impact on resource reallocation within a plant Ohto et al. Overall, our findings in Arabidopsis are consistent with the notion that diversity in seed size is not as great as that in seed number per plant within a species Harper, , suggesting seed number to be less responsive to selection.

This finding also suggests that the SF model may be right in assuming that the optimization of offspring size is independent of selection pressures on reproductive effort Venable, ; but see the section on environmental factors.

Thus, mechanisms to restrict seed size and not numbers might have become prominent and diversified through the course of seed plant evolution. Many functional traits affecting the size-number tradeoff are strongly influenced by environmental factors.

These traits can respond to environmental changes either through phenotypic plasticity, or through rapid evolution, and they do not have the evolutionary inertia that is typical of most life history traits discussed earlier. While short-term physiological or phenotypic adjustments can often can be classified as stress- responses, they also provide new raw material for natural selection through epigenomic changes inherited by offspring produced while the sub-optimal conditions prevail. If the same environmental conditions persist over a longer term several thousand generations , for example due to a shift in climate, then in an evolutionary sense what was stressful to the ancestors of the past becomes the new norm for those progenies that manage to survive until the present.

Therefore, analysis of stress responses is relevant also in the light of larger-scale evolutionary trends. In the following section, we discuss various environmental factors that influence plant reproductive strategies. As mentioned earlier, plant growth form a life history trait is a significant determinant of seed size, especially at the coarse scale herbs vs. The most important abiotic determinant of growth form in plants is light. To understand evolutionary trends in seed size variation and the mechanisms of size regulation, it is important to consider the influence of light on reproductive strategies at a finer scale, for example herbs of different shapes and sizes.

Herbaceous plants from forest understories can be either shade-tolerant or show a shade avoidance syndrome SAS. Shade avoidance dramatically alters plant form by stem elongation and reduced branching due to apical dominance AD.

Depending on the extent of competition for light in a given habitat, AD-induced suppression of axillary buds may translate into fewer branches, and thus reduced flowering and seed production Aarssen, ; Irwin and Aarssen, Manual removal of the apical bud in species that do not show SAS and grow in open landscapes causes a significant increase in seed weight, although seed numbers remain unaffected Naber and Aarssen, The reason may be that, since these plants lack apical dominance, suppression of apical growth does not lead to increased lateral branch growth.

Hence, the pool of carbon that would otherwise have been invested in apical growth is likely diverted to seeds, leading to greater seed weight without altering seed numbers. More recent research in crop plants has focused on AD and SAS associated reduction in grain size and number, and the underlying genetic mechanisms, since neighbor-shading is a feature of high density- monocultures Gommers et al.

A study of the relationship between seed size and the extent of shade tolerance in forest trees showed that fast-growing, deciduous, apically dominant, SAS trees bear the lightest seeds, while slow-growing, evergreen, shade-tolerant trees bear the heaviest seeds, and the trend is conserved both within and between angiosperms and gymnosperms Reich et al. The same trend was experimentally shown much earlier at a smaller scale, and it was hypothesized that large seed size provides an advantage in shaded environments by providing greater nutrient reserves to opportunistically exploit canopy gaps and by supporting seedling survival when photosynthesis is light-limited Leishman and Westoby, In addition, the larger seedlings grown from large seeds retain a sufficient advantage over fast-growing small seeds of SAS plants in dark understories of tropical forests Leishman and Westoby, The positive relationship between shade tolerance and seed size, and that between body size and seed size, can also arise due to the greater lipid content nutrient quality of large tree seeds as compared to small seeds of herbaceous plants Levin, Life history constraints including seedling and adult growth form determined by competition for light among other factors in most cases provide a sufficient basis to explain the variation in seed size vs.

Reconciling such large within-species variation with the classical notion of seed-size optimization has so far not been possible. Unfortunately, many studies do not report seed number variation, and this lack of data has hindered hypothesis testing. Therefore, it is important to consider how plant reproduction is affected by short-term responses to changes in environmental factors beyond those in light. Some studies suggest that large seeds are more viable than small seeds, and the probability of seedling emergence increases with increasing seed size Ben-Hur et al.

This trend is broadly interpreted in the context of a competition-colonization tradeoff, where large seeds have a competitive advantage due to superior ecophysiological performance, while small seeds in large numbers exploit opportunities in less competitive environments. Under the assumption that large seeds confer greater tolerance to short-term stresses, theoretical models have argued that high-fecundity plants with small seeds thrive in nutrient-surplus conditions and plants producing high-tolerance large seeds win in stressful environments Muller-Landau, ; in the context of interspecific variation.

Most of the data on seed number and quality under stress stem from agronomic studies, but such studies are not meant to answer questions pertaining to the evolution of reproductive strategies. In the following section, we discuss some broad trends in seed size-and-number variation and responses to short-term stress in non-crop plants.

Unlike cereal crops, leguminous plants possess a simple and uniform ovary containing a few large seeds, making them ideally suited for experimental investigations on seed ecology van der Pijl, ; Ganeshaiah and Uma Shaanker, A study looking at responses of a legume subjected to various forms of abiotic stress showed that shortage of nutrients, drought, and high temperature stresses, while not significantly affecting seed number, all caused a decrease in both the mean and variance of seed size, whereas seed numbers did not differ significantly Wulff, , implying stronger maternal controls and more efficient optimization Figure 4A.

A stress-induced reduction in seed size is confirmed by other studies and appear to be general: For example, stress due to competition between maternal plants growing at high density may make parents invest limited available resources in a large number of small seeds Larios and Venable, , and a study on 30 herbaceous annuals showed that seed number was more stable than seed size under changing levels of moisture stress Germain and Gilbert, In evolutionary terms, these results are consistent with the notion that less stressful environments favor larger and fewer seeds, while higher risk of mortality in harsher habitats favors the production of more and smaller seeds Volis et al.

Figure 4. Changes in offspring size and numbers in response to changes in environmental factors sub- optimal or stressful conditions. Two alternative scenarios are represented. Scenario A combines the hypotheses presented in the models in Figures 1B,C derived from the classical Smith-Fretwell model , where resource limitation causes strengthening of maternal controls over size optimization.

Scenario B means a more complicated interaction between maternal, zygotic genetic controls over size optimization and their environment. The response of offspring number in A is expected to increase and in B it is less predictable under harsh conditions.

Unlike mean seed size, seed size variance does not always decrease in stressful environments. In contrast to the results by Wulff , discussed above , other studies show that seed size variation significantly increases under nutrient limitation Halpern, , thus indicating weaker maternal controls Figure 4B. Harper and Ogden in their seminal paper had reported a similar pattern for a legume where increasing stress levels severely impacted phenology of flowering and increased variance in plant form and seed production.

The latter results support the theory that changes in the environment beyond a species-specific tolerance threshold cause developmental instability and lead to new offspring phenotypes Simons and Johnston, Some of these observations can be discussed under the framework of bet-hedging theory Slatkin, In other words, plants may produce a large number of small seeds with a greater potential for dormancy, see below under sub-optimal conditions, such that those seeds germinate randomly over a long period of time and at least some of them germinate when favorable conditions return Cohen, ; Simons, Some observations indicate that within-species variation in seed size is positively correlated with the extent of seedling survival and stress tolerance Westoby et al.

Small seeds may have a number of advantages: The study by Larios and Venable provides evidence for greater dispersal distances, suggesting a tradeoff between dispersal and competitive ability. Small seeds can also have an advantage due to a greater potential for dormancy Venable and Brown, ; Rees, or lower probability of predation Andresen and Levey, Similarly, changes in environmental conditions including soil nutrients can alter the quantity and quality of stored nutrition in seeds Halford et al.

For example, in wild Arabidopsis , a greater gain in seed volume than seed weight under high light exposure is attributed to a doubling of seed oil content, which is less dense than carbohydrates and proteins Li et al. Other studies show that the ratio of starch to lipid in seeds could be altered under stress Ali et al. Other factors such as seed to seed coat ratio could play a significant role in determining stress tolerance and viability of seedlings Hill et al.

Therefore, plants may produce small seeds with high calorie nutrition lipid rich , or with a thicker seed coat, or with activate mechanisms of dormancy, and such seeds are likely as viable as large seeds under stressful conditions. However, there are disadvantages as well, since small seeds are generally more vulnerable to early mortality Daws et al. If the stressful conditions persist for several generations, then offspring mortality can be very high especially in the case of annuals , since all offspring originated from seeds smaller than that of their parents.

It is hard to explain a plant's strategy to sacrifice a large number of offspring instead of investing in a few large tolerant offspring. Therefore, maternal control over offspring size both theoretically and realistically extends all the way to zero size or no offspring Figure 1.

In annuals that are never exposed to multi-year stresses, maternal control may substantially weaken under stress, leading to erratic variation in seed size. The classical models argue that optimization of offspring size may be independent of parental reproductive effort. However, decrease in offspring size and increase in numbers under adverse conditions including resource limitation Figure 4B , contrast Figure 1C suggests alternative possibilities.

Weaker maternal control over seed size optimization and the resultant increase in size variation can be beneficial in annuals under stress. A handful of larger seeds with increased desiccation tolerance may support seedling growth under nutrient limitation and may ensure survival of offspring. The response of seed nutrient quantity and quality to changing environments, and what impact they have on seed survival longevity and early seedling establishment are largely unknown.

In all of these cases, seed number variation is the most difficult to predict, both due to lack of experimental data and because of complications due to variation in plant size, life span, number of branches, and overall health. Animals show a general trend toward reduction of fecundity and concomitant increase in offspring size associated with increasing organismal complexity Brockelman, Species for which no size-number tradeoff has been found typically show considerable variation in reproductive effort Fox and Czesak, Many studies have reported conservatism in egg size, with greater variation in clutch size under varying environmental conditions birds: Lack, , ; fish: Scott, ; Hester, ; Fleming and Gross, ; mammals: Jordan et al.

These results suggest that in animals, selection primarily optimizes offspring size, again in accordance with optimality models, rather than fecundity. However, exceptions exist where offspring number, rather than size, is optimized.

The evolutionary advantages of invariant clutch size are not well-understood. As in the case of plants, morphological and other constraints may limit fecundity or offspring size in animals. Our analysis focuses on data rich groups reptiles and birds. Parental care complicates evolutionary interpretation of the offspring size-number tradeoff in animal reproductive strategies, because ultimately it is the overall parental investment that matters.

The contributors are well known in the fields of morphology, systematics, genetics, cell biology, and ecology, representing the full spectrum of approaches that contribute vigor to this emerging field. This new work will benefit professionals and graduate students in plant science and plant breeding, evolutionary ecology, genetics, and reproductive biology. Author : Lawrence D. Author : R. This collection of reviews by leading investigators examines plant reproduction and sexuality within a framework of evolutionary ecology, providing an up-to-date account of the field.

The contributors discuss conceptual issues, showing the importance of sex allocation, sexual selection and inclusive fitness, and the dimensions of paternity and maternity in plants.

The evolution, maintenance, and loss of self-incompatibility in plants, the nature of 'sex choice' in plants, and sex dimorphism are all explored in detail. Specific forms of biotic interactions shaping the evolution of plant reproductive strategy are discussed, and a taxonomically based review of the reproductive ecology of non-angiosperm plant groups, such as bryophytes, ferns, and algae, is presented.

Together these studies focus on the complexities of plant life cycles and the distinctive reproductive biologies of these organisms, while showing the similarities between nonflowering plants and the more thoroughly documented flowering species.

Specific forms of biotic interactions shaping. Successful reproduction is the basis not only for the stability of the species in their natural habitat but also for productivity of our crop plants. Therefore, knowledge on reproductive ecology of wild and cultivated plants is important for effective management of our dwindling biodiversity and for the sustainability and improvement of the yield in crop species. Conservation and management of our plant diversity is going to be a major challenge in the coming decades, particularly in the tropical countries which.

However, these studies are scattered across research journals and reviews from diverse areas of biology. Given its scope, the book offers a valuable resource for students, teachers and researchers of botany, zoology, ecology, agriculture and forestry, as well as conservation biologists.

Author : Mary F. The first volume to address the study of evolutionary transitions in plants, Major Evolutionary Transitions in Flowering Plant Reproduction brings together compelling work from the three areas of significant innovation in plant biology: evolution and adaptation in flowers and pollination, mating patterns and gender strategies, and asexual reproduction and polyploidy.

Spencer C. Barrett assembles here a distinguished group of authors who address evolutionary transitions using comparative and phylogenetic approaches, the tools of genomics, population genetics, and theoretical modeling, and through studies in development and field experiments in ecology.

With special focus on evolutionary transitions and shifts in reproductive characters—key elements of biological diversification and research in evolutionary biology—Major Evolutionary Transitions in Flowering Plant Reproduction is the most up-to-date treatment of a fast-moving area of evolutionary biology and ecology. The nature of weeds is the evolution of adaptive traits for seizing and exploiting locally available opportunity.

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We created them by channeling natural selection, the driver of biological change. Plants invade by dispersing, colonizing, reproducing and enduring in a locality. Weeds possess mating systems that generate variable genotypes and phenotypes that struggle for existence, the winners take all. Evolution occurs. Adaptation in weed life history is about timing: timing is everything. Adaptation in local plant communities is interference and facilitation animating strategic roles guided by functional traits.

Weed community dynamics is community assembly and ecological succession. Complex adaptive weed system formation reveals larger forces of nature: emergent behavior, physical information remembered. Knowledge of weeds is discovered, then represented in several different ways: ecological demography, life history traits. Representation is confounded by the humans that make them, their beliefs, values and models.

Case histories of three weeds explain these concepts: velvetleaf Abutilon theophrasti , triazine resistant rapeseed Brassica napus , and the foxtails Setaria species-group. Author : Edward Reekie,Fakhri A.

Much effort has been devoted to developing theories to explain the wide variation we observe in reproductive allocation among environments. Reproductive Allocation in Plants describes why plants differ in the proportion of their resources that they allocate to reproduction and looks into the various theories. This book examines the ecological and evolutionary explanations for variation in plant reproductive allocation from the perspective of the underlying physiological mechanisms controlling reproduction and growth.

An international team of leading experts have prepared chapters summarizing the current state of the field and offering their views on the factors determining reproductive allocation in plants. This will be a valuable resource for senior undergraduate students, graduate students and researchers in ecology, plant ecophysiology, and population biology. Author : J. In a young Charles Darwin took his notebook, wrote "I think"and then sketched a rudimentary, stick-like tree.