Sexual selection in flowering plants

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Sexual selection is described as natural selection arising through preference by one sex for certain characteristics in individuals of the other sex. Sexual selection is a common concept in animal evolution but, with plants, it is oftentimes overlooked because many plants are hermaphrodites. Flowering plants show many characteristics that are often sexually selected for. For example, flower symmetry, nectar production, floral structure, and inflorescences are just a few of the many secondary sex characteristics acted upon by sexual selection. Sexual dimorphisms and reproductive organs can also be affected by sexual selection in flowering plants. [1]

Contents

Mechanisms of sexual selection

There are two main mechanisms of sexual selection in flowering plants, one being intra-sexual competition and the other being female mate choice. Intra-sexual selection is responsible for much of floral evolution and diversification. Intrasexual selection, with the help of pollinators, has also led to some of the most remarkable examples of exaggerated traits of among hermaphroditic plants. Similarly, sexual dimorphism has probably been involved in the evolution of sexual dimorphism in floral traits of dioecious plants, and likely also in the evolution of secondary sexual dimorphism through genetic correlations with floral traits. Sexual selection through female choice is perhaps a more difficult concept to apply to plants. There is evidence that females (or the female function of hermaphrodites) exercise choice, especially during fertilization. [2] These two mechanisms are the main driving forces of sexual selection in flowering plants.

Symmetry and structure

Floral Structure of Citronella Flower Lily Lilium 'Citronella' Flower.jpg
Floral Structure of Citronella Flower

Floral symmetry is often acted on by sexual selection. Floral characters are often subject to strong directional selection from pollinators, and this may disrupt developmental homeostasis in flowers that can develop into large degrees of fluctuating asymmetry. Fluctuating asymmetry in floral traits may lead to sexual selection in plants if pollinators visit symmetrical flowers in an assortative manner. Numerous studies have shown that pollinators preferentially visited more symmetrical floral patterns in flowers, and there are several believed reasons why this pollinator preference exists. First, the preference may be present due to positive reinforcement. From experience, pollinators may learn that asymmetrical flowers provide a lesser award than symmetrical ones. Second, pollinators with sensory biases could be predisposed to select symmetrical flowers. This pollinator preference can lead to symmetrical flowers that are fertilized more often. This increase in fertilization frequency increases the amount of seeds produced, and results in an increased production of offspring with symmetrical, attractive flowers. [3]

Sexual dimorphisms

There are numerous examples of sexual dimorphisms in flowering plants. Sexual dimorphic differences include bud abortion, flower size, flower number per plant, floral longevity, nutrient content of flowers, nectar production, flowering phenology and periodicity, floral fragrances, floral defense against herbivory, and various inflorescence characteristics including total flower number, daily display size, and inflorescence architecture. In animal-pollinated species, these differences affect pollinator visitation, competition for mates, and the evolution of sexual dimorphisms. [4] However, there are constraints placed on animal-pollinated species, because too much divergence could interfere with mating success if pollinators are more attracted to one sex than the other, or if the sexes attract different pollinators. Such constraints are absent from wind-pollinated plants, and the contrasting biophysical requirements for pollen dispersal and pollen capture have led to striking cases of sexual dimorphism in plant architecture and flower production in some species. [4] Below are some specific examples of sexual dimorphisms in flowering plants.

Inflorescences

Inflorescence and Corolla Differences Flower poster.jpg
Inflorescence and Corolla Differences

Inflorescences can be acted on by sexual selection in many ways, and commonly include arrangement, number, and size. [5] For example, male inflorescence in plants often produce more flowers than females . Furthermore, pollen export and ultimately paternity, often increases with flower number, even for plants with hermaphroditic flowers. Retention of older flowers with no pollinator rewards can lead to increased pollinator visitation rate and increased pollen removal. Studies suggest that there has been selection for increased pollen delivery, achieved through greater inflorescence size, and it seems probable that male-male competition is commonly part of that selection pressure. [5]

Corolla

Corollas are the petals of flowers, and also face sexual selection. Traits such as color, shape, size, and symmetry are often faced by sexually selected pressure. One example is that male flowers are often larger than female flowers, at least in some species. Although this is presumably achieved through resource allocation mechanisms, it is unlikely that resource allocation from lower cost of androecium than gynoecium leading to higher expenditure on corolla in males, is a general and complete explanation of the size differences. Some corolla enlargement may arise by means of selection on correlated characters such as pollen production. However, it is likely that male competition often contributes directly to the evolution of large male corollas, especially where pollen availability is not limiting. [5]

Nectar production

European Honey Bee extracting nectar. European honey bee extracts nectar.jpg
European Honey Bee extracting nectar.
Female and Male Reproductive System Reproductive Organs in flowers.gif
Female and Male Reproductive System

Nectar production is different for every flowering plant, and has many different selective forces acting upon it. There is no specific evolutionary force that drives nectar production, but it is believed that sexual selection plays a major role. Studies have supported the idea that sexual selection is a probable explanation for at least some species with gender-biased nectar production. For example, gender-biased expression of nectar is often accompanied by a similarly biased expression of other floral characteristics. A specific example comes from the flowers of Impatiens capensis, and how they show increased longevity of the more-rewarding male phase . In other species, petals can be seen to wilt notably during the less- rewarding female phase or they change color as they pass into the less-rewarding female phase. More evidence for sexually selected nectar production relies on specific behaviors of the pollinators. If sexual selection is currently maintaining gender-biased nectar production, pollinators must be able to distinguish between male and female phase flowers. They also must visit preferentially flowers of the more-rewarding phase. For species that have discriminating pollinators, increased rewards can result in increased mating success, which would allow nectar to be a sexually selected trait. [6]

Reproductive organs

Androecium are often faced by the pressures of sexual selection. This is particularly evident when pollen is produced; there may be several sources of sexual selection on the ways that pollen is presented to pollen vectors. Because pollen is packaged in units that ensure that several to many pollen grains travel together as in pollinia, polyads, viscin threads, etc., pollen donors may be able to monopolize stigmas and the associated ovules by blocking access by other males, unless selection also favors compensating stigma enlargement. Furthermore, when pollen germination depends on some minimum number of pollen grains to overcome stigmatic inhibition, which is a mechanism that heightens male competition, there may be selection for large pollen dispersal units or certain other pollen-dispensing mechanisms. If pollinator visits are few, then there is selection to package pollen in a way that all of it can be removed by one visit . When repeated pollinator visits are typical, there may be selection for various temporal patterns of pollen presentation and various methods of pollen dispensing. Variation in stamen length, both within and among flowers influences pollen dispersal and potential male reproductive success. Increasing the opportunity for paternity by distributing pollen among pollinators may take different routes in different systems, and any of these possibilities can be viewed in a sexual selection context . [5] The gynoecium is also affected by sexual selection.  Every part from the ovaries, styles, stigma, and carpels can be faced with the pressures of sexual selection. In ovule packaging, the intensity of pollen competition depends in part on the number of pollen grains relative to the number of ovules. Although many factors may contribute to determining ovule number, one way for females to increase the level of pollen competition is to decrease ovule number while maintaining stigma size. The evolution of functional syncarpy (assuming no other attendant changes) presumably is a simultaneous increase in the number of ovules accessible from one stigma, which tended to decrease the intensity of pollen competition, and a concentration of pollen deposition on a single stigma, which tends to increase pollen competition. Further increases in pollen competition could be brought about by an increase in pollen delivery (through change of pollinator, increased attractiveness of floral display, or rewards), a decrease in ovule number or stigma size, changes in temporal patterns of stigma receptivity, or changes in the competitive environment of the carpel . [5]

Related Research Articles

<span class="mw-page-title-main">Sex</span> Trait that determines an organisms sexually reproductive function

Sex is the trait that determines whether a sexually reproducing organism produces male or female gametes. During sexual reproduction, a male and a female gamete fuse to form a zygote, which develops into an offspring that inherits traits from each parent. By convention, organisms that produce smaller, more mobile gametes are called male, while organisms that produce produce larger, non-mobile gametes are called female. An organism that produces both types of gamete is hermaphrodite.

<span class="mw-page-title-main">Petal</span> Part of most types of flower

Petals are modified leaves that surround the reproductive parts of flowers. They are often brightly colored or unusually shaped to attract pollinators. All of the petals of a flower are collectively known as the corolla. Petals are usually accompanied by another set of modified leaves called sepals, that collectively form the calyx and lie just beneath the corolla. The calyx and the corolla together make up the perianth, the non-reproductive portion of a flower. When the petals and sepals of a flower are difficult to distinguish, they are collectively called tepals. Examples of plants in which the term tepal is appropriate include genera such as Aloe and Tulipa. Conversely, genera such as Rosa and Phaseolus have well-distinguished sepals and petals. When the undifferentiated tepals resemble petals, they are referred to as "petaloid", as in petaloid monocots, orders of monocots with brightly colored tepals. Since they include Liliales, an alternative name is lilioid monocots.

<span class="mw-page-title-main">Sexual dimorphism</span> Condition where males and females exhibit different characteristics

Sexual dimorphism is the condition where sexes of the same species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most dioecious species, which consist of most animals and some plants. Differences may include secondary sex characteristics, size, weight, color, markings, or behavioral or cognitive traits. Male-male reproductive competition has evolved a diverse array of sexually dimorphic traits. Aggressive utility traits such as "battle" teeth and blunt heads reinforced as battering rams are used as weapons in aggressive interactions between rivals. Passive displays such as ornamental feathering or song-calling have also evolved mainly through sexual selection. These differences may be subtle or exaggerated and may be subjected to sexual selection and natural selection. The opposite of dimorphism is monomorphism, when both biological sexes are phenotypically indistinguishable from each other.

<span class="mw-page-title-main">Pollination</span> Biological process occurring in plants

Pollination is the transfer of pollen from an anther of a plant to the stigma of a plant, later enabling fertilisation and the production of seeds, most often by an animal or by wind. Pollinating agents can be animals such as insects, for example beetles or butterflies; birds, and bats; water; wind; and even plants themselves. Pollinating animals travel from plant to plant carrying pollen on their bodies in a vital interaction that allows the transfer of genetic material critical to the reproductive system of most flowering plants. When self-pollination occurs within a closed flower. Pollination often occurs within a species. When pollination occurs between species, it can produce hybrid offspring in nature and in plant breeding work.

<span class="mw-page-title-main">Pseudocopulation</span> Biological process

Pseudocopulation describes behaviors similar to copulation that serve a reproductive function for one or both participants but do not involve actual sexual union between the individuals. It is most generally applied to a pollinator attempting to copulate with a flower. Some flowers mimic a potential female mate visually, but the key stimuli are often chemical and tactile. This form of mimicry in plants is called Pouyannian mimicry.

<span class="mw-page-title-main">Plant reproductive morphology</span> Parts of plant enabling sexual reproduction

Plant reproductive morphology is the study of the physical form and structure of those parts of plants directly or indirectly concerned with sexual reproduction.

<span class="mw-page-title-main">Sequential hermaphroditism</span> Sex change as part of the normal life cycle of a species

Sequential hermaphroditism is one of the two types of hermaphroditism, the other type being simultaneous hermaphroditism. It occurs when the organism's sex changes at some point in its life. In particular, a sequential hermaphrodite produces eggs and sperm at different stages in life. Sequential hermaphroditism occurs in many fish, gastropods, and plants. Species that can undergo these changes do so as a normal event within their reproductive cycle, usually cued by either social structure or the achievement of a certain age or size. In some species of fish, sequential hermaphroditism is much more common than simultaneous hermaphroditism.

<span class="mw-page-title-main">Commelinaceae</span> Family of flowering plants

Commelinaceae is a family of flowering plants. In less formal contexts, the group is referred to as the dayflower family or spiderwort family. It is one of five families in the order Commelinales and by far the largest of these with about 731 known species in 41 genera. Well known genera include Commelina (dayflowers) and Tradescantia (spiderworts). The family is diverse in both the Old World tropics and the New World tropics, with some genera present in both. The variation in morphology, especially that of the flower and inflorescence, is considered to be exceptionally high amongst the angiosperms.

<span class="mw-page-title-main">Flower</span> Reproductive structure in flowering plants

A flower, also known as a bloom or blossom, is the reproductive structure found in flowering plants. Flowers consist of a combination of vegetative organs – sepals that enclose and protect the developing flower, petals that attract pollinators, and reproductive organs that produce gametophytes, which in flowering plants produce gametes. The male gametophytes, which produce sperm, are enclosed within pollen grains produced in the anthers. The female gametophytes are contained within the ovules produced in the carpels.

<span class="mw-page-title-main">Ornithophily</span> Pollination by birds

Ornithophily or bird pollination is the pollination of flowering plants by birds. This sometimes coevolutionary association is derived from insect pollination (entomophily) and is particularly well developed in some parts of the world, especially in the tropics, Southern Africa, and on some island chains. The association involves several distinctive plant adaptations forming a "pollination syndrome". The plants typically have colourful, often red, flowers with long tubular structures holding ample nectar and orientations of the stamen and stigma that ensure contact with the pollinator. Birds involved in ornithophily tend to be specialist nectarivores with brushy tongues and long bills, that are either capable of hovering flight or light enough to perch on the flower structures.

<span class="mw-page-title-main">Pollination syndrome</span> Flower traits that attract pollinators

Pollination syndromes are suites of flower traits that have evolved in response to natural selection imposed by different pollen vectors, which can be abiotic or biotic, such as birds, bees, flies, and so forth through a process called pollinator-mediated selection. These traits include flower shape, size, colour, odour, reward type and amount, nectar composition, timing of flowering, etc. For example, tubular red flowers with copious nectar often attract birds; foul smelling flowers attract carrion flies or beetles, etc.

Plant reproduction is the production of new offspring in plants, which can be accomplished by sexual or asexual reproduction. Sexual reproduction produces offspring by the fusion of gametes, resulting in offspring genetically different from either parent. Asexual reproduction produces new individuals without the fusion of gametes, resulting in clonal plants that are genetically identical to the parent plant and each other, unless mutations occur.

This page provides a glossary of plant morphology. Botanists and other biologists who study plant morphology use a number of different terms to classify and identify plant organs and parts that can be observed using no more than a handheld magnifying lens. This page provides help in understanding the numerous other pages describing plants by their various taxa. The accompanying page—Plant morphology—provides an overview of the science of the external form of plants. There is also an alphabetical list: Glossary of botanical terms. In contrast, this page deals with botanical terms in a systematic manner, with some illustrations, and organized by plant anatomy and function in plant physiology.

<span class="mw-page-title-main">Gynodioecy</span> Coexistence of female and hermaphrodite within a population

Gynodioecy is a rare breeding system that is found in certain flowering plant species in which female and hermaphroditic plants coexist within a population. Gynodioecy is the evolutionary intermediate between hermaphroditism and dioecy.

Floral biology is an area of ecological research that studies the evolutionary factors that have moulded the structures, behaviour and physiological aspects involved in the flowering of plants. The field is broad and interdisciplinary and involves research requiring expertise from multiple disciplines that can include botany, ethology, biochemistry, and entomology. A slightly narrower area of research within floral biology is sometimes called pollination biology or anthecology.

<span class="mw-page-title-main">Monocotyledon reproduction</span> Flowering plant reproduction system

The monocots are one of the two major groups of flowering plants, the other being the dicots. In order to reproduce they utilize various strategies such as employing forms of asexual reproduction, restricting which individuals they are sexually compatible with, or influencing how they are pollinated. Nearly all reproductive strategies that evolved in the dicots have independently evolved in monocots as well. Despite these similarities and their close relatedness, monocots and dicots have distinct traits in their reproductive biologies.

<span class="mw-page-title-main">Pollinator-mediated selection</span> Process in which pollenators effects a plants evolution

Pollinator-mediated selection is an evolutionary process occurring in flowering plants, in which the foraging behavior of pollinators differentially selects for certain floral traits. Flowering plant are a diverse group of plants that produce seeds. Their seeds differ from those of gymnosperms in that they are enclosed within a fruit. These plants display a wide range of diversity when it comes to the phenotypic characteristics of their flowers, which attracts a variety of pollinators that participate in biotic interactions with the plant. Since many plants rely on pollen vectors, their interactions with them influence floral traits and also favor efficiency since many vectors are searching for floral rewards like pollen and nectar. Examples of pollinator-mediated selected traits could be those involving the size, shape, color and odor of flowers, corolla tube length and width, size of inflorescence, floral rewards and amount, nectar guides, and phenology. Since these types of traits are likely to be involved in attracting pollinators, they may very well be the result of selection by the pollinators themselves.

<span class="mw-page-title-main">Monoecy</span> Sexual system in seed plants

Monoecy is a sexual system in seed plants where separate male and female cones or flowers are present on the same plant. It is a monomorphic sexual system comparable with gynomonoecy, andromonoecy and trimonoecy, and contrasted with dioecy where individual plants produce cones or flowers of only one sex and with bisexual or hermaphroditic plants in which male and female gametes are produced in the same flower.

<span class="mw-page-title-main">Pollination of orchids</span>

The pollination of orchids is a complex chapter in the biology of this family of plants that are distinguished by the complexity of their flowers and by intricate ecological interactions with their pollinator agents. It has captured the attention of numerous scientists over time, including Charles Darwin, father of the theory of evolution by natural selection. Darwin published in 1862 the first observations of the fundamental role of insects in orchid pollination, in his book The Fertilization of Orchids. Darwin stated that the varied stratagems orchids use to attract their pollinators transcend the imagination of any human being.

<span class="mw-page-title-main">Floral morphology</span>

In botany, floral morphology is the study of the diversity of forms and structures presented by the flower, which, by definition, is a branch of limited growth that bears the modified leaves responsible for reproduction and protection of the gametes, called floral pieces.

References

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