12.4: Mating Systems in Plants

Asexual Reproduction

Many plants are facultatively sexual rather than obligately sexual. Asexual reproduction is a type of reproduction where the offspring comes from one parent only, thus, inheriting the characteristics of the parent. Asexual reproduction in plants occurs in two fundamental forms vegetative reproduction and agamospermy.¹ Vegetative reproduction involves a vegetative piece of the original plant producing new individuals by budding, tillering, etc. and is distinguished from apomixis, which is a replacement of sexual reproduction, and in some cases involves seeds. Apomixis occurs in many plant species such as dandelions (Taraxacum species) and also in some non-plant organisms. For apomixis and similar processes in non-plant organisms, see parthenogenesis.

Natural vegetative reproduction is a process mostly found in perennial plants, and typically involves structural modifications of the stem or roots and in a few species leaves. Most plant species that employ vegetative reproduction do so as a means to perennialize the plants, allowing them to survive from one season to the next and often facilitating their expansion in size. A plant that persists in a location through vegetative reproduction of individuals constitutes a clonal colony. A single ramet, or apparent individual, of a clonal colony is genetically identical to all others in the same colony. The distance that a plant can move during vegetative reproduction is limited, though some plants can produce ramets from branching rhizomes or stolons that cover a wide area, often in only a few growing seasons. In a sense, this process is not one of reproduction but one of survival and expansion of biomass of the individual. When an individual organism increases in size via cell multiplication and remains intact, the process is called vegetative growth. However, in vegetative reproduction, the new plants that result are new individuals in almost every respect except genetic. A major disadvantage of vegetative reproduction, is the transmission of pathogens from parent to offspring. It is uncommon for pathogens to be transmitted from the plant to its seeds (in sexual reproduction or in apomixis), though there are occasions when it occurs.²

Seeds generated by apomixis are a means of asexual reproduction, involving the formation and dispersal of seeds that do not originate from the fertilization of the embryos. Hawkweeds (Hieracium), dandelions (Taraxacum), some species of Citrus and Kentucky blue grass (Poa pratensis) all use this form of asexual reproduction. Pseudogamy occurs in some plants that have apomictic seeds, where pollination is often needed to initiate embryo growth, though the pollen contributes no genetic material to the developing offspring.³ Other forms of apomixis occur in plants also, including the generation of a plantlet in replacement of a seed or the generation of bulbils instead of flowers, where new cloned individuals are produced. 

Sexual Reproduction

Sexual reproduction involves two fundamental processes: meiosis, which rearranges the genes and reduces the number of chromosomes, and fertilisation, which restores the chromosome to a complete diploid number. In between these two processes, different types of plants and algae vary, but many of them, including all land plants, undergo alternation of generations, with two different multicellular structures (phases), a gametophyte and a sporophyte. 

In mosses and liverworts, the gametophyte is relatively large, and the sporophyte is a much smaller structure that is never separated from the gametophyte. In ferns, gymnosperms, and flowering plants (angiosperms), the gametophytes are relatively small and the sporophyte is much larger. In gymnosperms and flowering plants the megagametophyte is contained within the ovule (that may develop into a seed) and the microgametophyte is contained within a pollen grain.

 In the evolution of early plants, abiotic means, including water and much later, wind, transported sperm for reproduction. The first plants were aquatic, and released sperm freely into the water to be carried with the currents. Ancestral land plants like liverworts and mosses have motile sperm that swam in a thin film of water or were splashed in water droplets. As taller and more complex plants evolved, modifications in the alternation of generations evolved. In the Paleozoic era progymnosperms reproduced by using spores dispersed on the wind and many gymnosperms and some angiosperms still rely on wind for gamete dispersal. The seed plants including seed ferns, conifers and cordaites have pollen grains that contain the male gametes for protection of the sperm during the process of transfer from the male to female parts. Angiosperms, or flowering plants, are the most derived and most abundant plant species and they rely on flowers producing pollen and ovules for reproduction. 

Self-Fertilization and Self-Incompatibility

Many species of plants, particularly those which produce both staminate and pistillate flowers or produce ‘perfect’ bisexual flowers, also have the ability to reproduce sexually with themselves. This is advantageous particularly if pollination services are unreliable or unpredictable, as it ensures the plant still has some fitness. Self-pollination is a form of pollination in which pollen from the same plant arrives at the stigma of a flower (in flowering plants) or at the ovule (in gymnosperms). The term selfing that is often used as a synonym, is not limited to self-pollination, but also applies to other types of self-fertilization. Plants may either be obligately self-fertilizing, or facultatively so. In facultatively selfing plants, there may be mechanisms which delay selfing, such as stamens that are initially reflexed but move to come into contact with the stigma. About 42% of flowering plants exhibit a mixed mating system in nature.⁴ In the most common kind of system, individual plants produce a single flower type and fruits may contain self-pollinated, out-crossed or a mixture of progeny types. Another mixed mating system is referred to as dimorphic cleistogamy. In this system a single plant produces both open, potentially out-crossed and closed, obligately self-pollinated cleistogamous flowers.⁵ 

Still other species are self-incompatible, and will reject their own pollen grains if they land on their own stigmatic surface. These plants are obligately outcrossing, and must successfully sexually reproduce with another member of their species. In plants with SI, when a pollen grain produced in a plant reaches a stigma of the same plant or another plant with a matching allele or genotype, the process of pollen germination, pollen-tube growth, ovule fertilization, or embryo development is inhibited, and consequently no seeds are produced. SI is one of the most important means of preventing inbreeding and promoting the generation of new genotypes in plants and it is considered one of the causes of the spread and success of angiosperms on the earth.

Outcrossing Fertilization

Plants that use insects or other animals to move pollen from one flower to the next have developed greatly modified flower parts to attract pollinators and to facilitate the movement of pollen from one flower to the insect and from the insect back to the next flower. Flowers of wind-pollinated plants tend to lack petals and or sepals; typically large amounts of pollen are produced and pollination often occurs early in the growing season before leaves can interfere with the dispersal of the pollen. Many trees and all grasses and sedges are wind-pollinated.

Plants have a number of different means to attract pollinators including color, scent, heat, nectar glands, edible pollen and flower shape. Along with modifications involving the above structures two other conditions play a very important role in the sexual reproduction of flowering plants, the first is the timing of flowering and the other is the size or number of flowers produced. Often plant species have a few large, very showy flowers while others produce many small flowers, often flowers are collected together into large inflorescences to maximize their visual effect, becoming more noticeable to passing pollinators. Flowers are attraction strategies and sexual expressions are functional strategies used to produce the next generation of plants, with pollinators and plants having co-evolved, often to some extraordinary degrees, very often rendering mutual benefit. Specialization can be advantageous because it results in more consistent pollination services. As a result, the specialized pollinator and plant can exert strong selective pressure on each other, leading to coevolution. Examples of this include the coevolution between figs and fig wasps, or yucca and yucca moths, wherein the yucca moths are both the obligate pollinators and the antagonistic herbivores of yucca. Another visually striking example is the co-evolution of long floral corollas and long beaks or proboscii in pollinators \(\PageIndex{1A+B}\).

       

Figure \(\PageIndex{1}\): (A) Sword-billed Hummingbird in Ecuador ("Sword-Billed Hummingbird" by Andrew Morffew is licensed under CC BY 2.0); (B) Rufous Hummingbird pollinating Scarlet Gilia in the Rocky Mountains ("Rufous Hummingbird" by Smallman12q is licensed under CC BY 2.0).

Plants which share pollination methods or pollinators often accumulate suites of related traits called pollination syndromes. These have evolved in response to natural selection imposed by different pollen vectors, which can be abiotic (wind and water) or biotic, such as birds, bees, flies, etc. through a process called pollinator-mediated selection.^6,7 These traits include flower shape, size, colour, odor, reward type and amount, nectar composition, and timing of flowering. For example, tubular red flowers with copious nectar often attract birds; foul smelling flowers attract carrion flies or beetles, etc. Different species which use the same pollinators may either flower synchronously, to attract pollinators more successfully, or asynchronously, to avoid heterospecific pollen transfer (or stigma gunking). The latter is an example of niche partitioning. 

The largest family of flowering plants is the orchids (Orchidaceae), estimated by some specialists to include up to 35,000 species,⁸ which often have highly specialized flowers that attract particular insects for pollination. The stamens are modified to produce pollen in clusters called pollinia, which become attached to insects that crawl into the flower. The flower shapes may force insects to pass by the pollen, which is "glued" to the insect. Some orchids are even more highly specialized, with flower shapes that mimic the shape of insects to attract them to attempt to 'mate' with the flowers, a few even have scents that mimic insect pheromones (\(\PageIndex{2}\)). 

Figure \(\PageIndex{2}\): A bee-mimic orchid evolved to resemble a sexually receptive female bee and attract naïve male bees to pollinate it ("Bee Orchid (Ophrys apifera)" by Ian Capper is licensed under CC BY 2.0).

Examples of Pollination Syndromes

• Wind: Flowers may be small and inconspicuous, as well as green and not showy. They produce enormous numbers of relatively small pollen grains (hence wind-pollinated plants may be allergens, but seldom are animal-pollinated plants allergenic). Their stigmas may be large and feathery to catch the pollen grains.
• Water: Water-pollinated plants are aquatic and pollen is released into the water. Water currents therefore act as a pollen vector in a similar way to wind currents. Their flowers tend to be small and inconspicuous with many pollen grains and large, feathery stigmas to catch the pollen. However, this is relatively uncommon (only 2% of pollination is hydrophily) and most aquatic plants are insect-pollinated, with flowers that emerge into the air. Vallisneria is an example.
• Bee: Some bee flowers tend to be yellow or blue, often with ultraviolet nectar guides and scent. Nectar, pollen, or both are offered as rewards in varying amounts. The sugar in the nectar tends to be sucrose-dominated. A few bees collect oil from special glands on the flower.¹⁰
• Butterfly: Butterfly-pollinated flowers tend to be large and showy, pink or lavender in colour, frequently have a landing area, and are usually scented. Since butterflies do not digest pollen (with one exception), more nectar is offered than pollen. The flowers have simple nectar guides with the nectaries usually hidden in narrow tubes or spurs, reached by the long tongue of the butterflies.
• Moth: Among the more important moth pollinators are the hawk moths (Sphingidae). Their behaviour is similar to hummingbirds: they hover in front of flowers with rapid wingbeats. Most are nocturnal or crepuscular. Moth-pollinated flowers tend to be white, night-opening, large and showy with tubular corollas and a strong, sweet scent produced in the evening, night or early morning. Much nectar is produced to fuel the high metabolic rates needed to power their flight.
• Bat: There are major differences between bat pollination in the Americas as opposed to the Afro-Eurasia.  Afro-Eurasian pollinating bats are large fruit bats of the family Pteropodidae which do not have the ability to hover and must perch in the plant to lap the nectar; these bats furthermore do not have the ability to echolocate.¹¹ Bat-pollinated flowers in this part of the world tend to be large and showy, white or light coloured, open at night and have strong musty odours. They are often large balls of stamens. In the Americas pollinating bats are tiny creatures called glossophagines which have both the ability to hover as well as echolocate, and have extremely long tongues. Plants in this part of the world are often pollinated by both bats and hummingbirds, and have long tubular flowers.¹¹ In one essay, von Helversen et al. speculate that maybe some bell-shaped flowers have evolved to attract bats in the Americas, as the bell-shape might reflect the sonar pulses emitted by the bats in a recognisable pattern.¹²
• Fly: Myophilous plants tend not to emit a strong scent, are typically purple, violet, blue, and white, and have open dishes or tubes.¹³
• Sapromyophilous plants try to attract flies which normally visit dead animals or dung. Flowers mimic the odor of such objects. The plant provides them with no reward and they leave quickly unless it has traps to slow them down. Such plants are far less common than myophilous ones.¹⁴
• Beetle: Beetle-pollinated flowers are usually large, greenish or off-white in color and heavily scented. Scents may be spicy, fruity, or similar to decaying organic material. Most beetle-pollinated flowers are flattened or dish shaped, with pollen easily accessible, although they may include traps to keep the beetle longer. The plant's ovaries are usually well protected from the biting mouthparts of their pollinators.¹⁵ A number of cantharophilous plants are thermogenic, with flowers that can increase their temperature. This heat is thought to help further spread the scent, but the infrared light produced by this heat may also be visible to insects during the dark night, and act as a shining beacon to attract them.¹⁶
• Bird: Flowers pollinated by specialist nectarivores tend to be large, red or orange tubes with a lot of dilute nectar, secreted during the day. Since birds do not have a strong response to scent, they tend to be odorless. Flowers pollinated by generalist birds are often shorter and wider. Hummingbirds are often associated with pendulous flowers, whereas passerines (perching birds) need a landing platform so flowers and surrounding structures are often more robust. Also, many plants have anthers placed in the flower so that pollen rubs against the birds head/back as the bird reaches in for nectar.

…and many more!

References

1.      Barrett, S.C.H. (2008). Major evolutionary transitions in flowering plant reproduction. University of Chicago Press. p. 157. ISBN 978-0-226-03816-2.

2.     ^ Fritz, R.E. & Simms, E.L. (1992). Plant resistance to herbivores and pathogens: Ecology, evolution, and genetics. Chicago: University of Chicago Press. p. 359. ISBN 978-0-226-26554-4.

3.     ^ "Archived copy". Archived from the original on 2009-10-26. Retrieved 2009-10-25.

4.      Goodwillie, C., Kalisz, S., & Eckert, C.G. (2005). The evolutionary enigma of mixed mating systems in plants: Occurrence, theoretical explanations, and empirical evidence. Annu. Rev. Ecol. Evol. Syst., 36, pp. 47–79. doi:10.1146/annurev.ecolsys.36.091704.175539.

5.     ^ Munguía-Rosas, M.A., Campos-Navarrete, M.J., & Parra-Tabla, V. (2013). The effect of pollen source vs. flower type on progeny performance and seed predation under contrasting light environments in a cleistogamous herb. PLOS ONE, 8(11): e80934. Bibcode:2013PLoSO...880934M. doi:10.1371/journal.pone.0080934. PMC 3829907. PMID 24260515.

6.      Faegri & Pijl 1980.

7.     ^ Proctor, M., Yeo, P., & Lack, A. (1996). The natural history of pollination. HarperCollins, London. ISBN 978-0-88192-352-0.

8.      Orchidaceae in Flora of North America @ efloras.org

9.     ^ Asteraceae in Flora of North America @ efloras.org

10.       Martins, A.C., Melo, G.A.R., & Renner, S.S. (2014). The corbiculate bees arose from New World oil-collecting bees: Implications for the origin of pollen baskets. Molecular Phylogenetics and Evolution, 80, pp. 88–94. doi:10.1016/j.ympev.2014.07.003. PMID 25034728.

11.      Fleming, T.H., Geiselman, C., & Kress, W.J. (2009). The evolution of bat pollination: a phylogenetic perspective. Annals of Botany, 104(6), pp. 1017–1043. doi:10.1093/aob/mcp197. PMC 2766192. PMID 19789175.

12.     ^ Von Helversen, D., Holderied, M.W., & von Helversen, O. (2003). Echoes of bat-pollinated bell-shaped flowers: conspicuous for nectar-feeding bats?. Journal of Experimental Biology, 206(6), pp. 1025–1034. doi:10.1242/jeb.00203. PMID 12582145.

13.      Kastinger, C., & Weber, A. (2001). Bee-flies (Bombylius spp., Bombyliidae, Diptera) and the pollination of flowers. Flora, 196(1), pp. 3–25. doi:10.1016/S0367-2530(17)30015-4.

14.     ^ Jones, G.D., & Jones, S.D. (2001). The uses of pollen and its implication for Entomology. Neotropical Entomology, 30(3), pp. 314–349. doi:10.1590/S1519-566X2001000300001.

15.     ^ Gullan, P.J., & Cranston, P.S. (2005). The insects: An outline of Entomology. Blackwell Publishing Ltd. p. 282. ISBN 978-1-4051-1113-3.

16.     ^ Korotkova, N., & Barthlott, W. (2009). On the thermogenesis of the Titan arum (Amorphophallus titanum). Plant Signalling and Behaviour, 4(11), pp. 1096–1098. doi:10.4161/psb.4.11.9872. PMC 2819525. PMID 19838070.

 

Contributors and Attributions 

Modified by Castilleja Olmsted (University of Pittsburgh) from the following sources:

• https://en.wikipedia.org/wiki/Plant_reproduction
• https://en.wikipedia.org/wiki/Self-incompatibility
• https://en.wikipedia.org/wiki/Self-pollination
• https://en.wikipedia.org/wiki/Pollination_syndrome
• Original text by Castilleja Olmsted