plants have Sex- only in ANGIOSPERMS?
Sexual Reproduction. Flowers. Pollination. Angiosperms. Herkogamy. Chasmogamy. Apomixis. Asexual reproduction.
That man could not live without plants. By providing foods. shelter, clothing and industry are the keys to the continued existence of man. Though presently there are plant species that could not exist in the wild without the intervention of man, nevertheless, plants form the basic of human needs. But, why some plants are more successful in exploring different ecological niche while others are not. All organism micro or macro must have undergone sexual reproduction at one time in their lifecycle in order to drive the engine of evolution. Otherwise, if one can not leave viable offspring in the next generation they are doomed to extinction. Sexual reproduction brings reshuffling of genes that may have a selective advantage thus, ensuring the survival of the species every generation.
Rise of Angiosperms
The exact lineage of angiosperms or flowering plants is difficult to established. Archeological evidence shows that plants started to dominate the terrestrial world 500 million years ago (mya), moreover, the rise of angiosperms was only noted in the later Mesozoic era, 150 million years ago(mya), furthermore, the ancient progenitor of angiosperms was never found. Today, angiosperms are the most successful and diverse among land plants. Phylogenetically, angiosperms belong to a large group of plants, the Phylum Anthophyta or flowering plants. The 250,000 known species is further divided into Class.Dicotyledoneae (the Dicots) and Class Monocotyledoneae ( the Monocots). The dicots are more numerous than the monocots.
Another major event during this period that coincide with the rise of angiosperms is the appearance of insects. It was believed that the success of angiosperms as land plants could be largely attributed to the co-existence with insects. As pollinators, insects is rewarded with foods (honey, nectar, pollen, etc.) and a place to live, consequently, the plants get pollinated, produced seeds thus, ensuring species continuity for generations to come. The seeds, a product of the sexual union is now the biological currency of evolution. There are three big groups of insects from 950,000 known species, namely Butterflies and moths (Lepidoptera), Flies (Diptera), and Bees and wasps (Hymenoptera) are the major pollinators of angiosperms or flowering plants.
life cycle of Angiosperms
From birth to death, in plants it begins from the Flower. In all living flowering plants, the flower is the basic unit of reproduction. The coming together of sperm and egg during fertilization, the biological union of male and female gametes signals the birth of a new individual complete with genetic information to find an ecological niche among the living.
The angiosperms or flowering plants are unique among earth's living inhabitants, being immobile, yet was able to colonize every habitable area of the world. What is its secret? these group of plants exhibits double fertilization.
During fertilization brought about by pollination, the generative nucleus inside the pollen tube divide once more to produce two haploid (n) nuclei. The pollen tube penetrates the embryo sac (deflowering?).The embryo sac is eight nucleate cells derived from the placental walls of the ovule, composed of three antipodals, two polar, two synergids(egg apparatus and one egg. Double fertilization emanates from: One sperm unites with an egg=zygote(2n), one sperm unites with two polar nuclei=endosperm(3n). Nature is very intelligent, the zygote(2n) is a diploid individual ensuring the constancy of chromosome number of the species while the endosperm(3n), a triploid whose main function is to store food, being a bigger cell, can accommodate large volume of food for the embryo (miniature plant) once detached or dispersed from the mother plant. Thus, this food provisioning ensures the survival of the species from generation to generation. Plants have sex, double fertilization, this is the secret of angiosperms or flowering plants. Most of the foods derived from plants are endosperm, the three great bowls of cereal namely, rice, wheat, and corn have a very small embryo but large eatable endosperms. These three bowls of cereal supply 60 percent of the calorie requirements of man.
Flowering plants-Monocots and Dicots
The angiosperms or flowering plants were further divided into two big groups, the Dicot class and the Monocot class. Morphological differences between these two classes are shown below. The monocots comprising about 65,000 species and the Dicots, 175,000 species. The Dicots are more primitive of the two classes with monocots apparently having derived from early dicots. Included in the dicots are the great majority of familiar angiosperms, almost all kinds of trees, shrubs, mints, peas, sunflower and other plants. Monocots include the lilies, grasses, palms, agaves, orchids, irises among others and the great three bowls of cereal: rice, corn and wheat.
Take a Quiz
Match Column A with B. Letters ONLY
Column A Column B
- Complete flower a) pollen shed before stigma receptive
- Pistillate flower b) temporal separation of male and female flowers
- Staminate flower c) flower opens before pollination
- Perfect flower d)four essential parts
- Monoecious e) female structure only
- Dioecious f) flower opens after pollination
- Cleistogamy g) staminate and pistillate flowers borne in different plants
- Chasmogamy h) male structures only
- Dichogamy i) bears male and female organs
- Protandry j) bears staminate and pistillate flowers
Excellent= 10, Very Good= 8-9, Good= 6-7, Average= 5
Flowers-the center of attraction
- In higher plants, the flower is the fundamental unit for sexual reproduction. There are four essential parts of a flower, namely,
- Petals- collectively called Corolla
- Sepals- collectively called Calyx
- Stamen-the male gametophyte(Pollen grains, Anthers, Filament)
- Pistil- the female gametophyte(Stigma, Style, Ovary)
A complete flower, therefore, is composed of four essential parts. Variation in floral structures arose if one of these essential parts is missing. For instance, a flower is called Incomplete when one of these essential is missing. If the male and female structure is present on the same flower, it is called Perfect flower, but if both male and female structures are separated, they are called Imperfect flowers. Imperfect flowers could be either, Staminate(male) or Pistillate(female) flowers respectively.
reproduction in the Angiosperms
Plant production is the real basis for agriculture. Human beings has been taking advantage of all plant reproductive processes- both sexual and asexual, In the most general terms, reproduction is the total sequence of events required for the replication of cells and the organisms.
Richardson and Stubbs, 1977
Do you Know?
that Ismail, the Blood Thirsty (Sultan of Morocco, 1672-1727)
is reported to have fathered 888 offsprings during his reign?
sexual Reproduction in Angiosperms
There are two kinds of reproduction employed by flowering plants, that is, asexual and sexual reproduction respectively. The former is based on somatic propagation while the later is based on the modes of pollinations. We will be focusing our discussion on sexual reproduction. Pollination is an important strategy in sexual reproduction, there are two types of pollination, namely self and cross-pollination. The less technical definition says that. the transfer of pollen grains from the anthers to the stigma of the same flower is called self-pollination while the transfer of the pollen grains from the anthers to the stigma of another flower is called cross-pollination. Self-pollination or selfing is the most drastic form of inbreeding, that is the mating of closely related individuals that lead to homozygosity. Experts believed that selfing is the end of an evolutionary path. Cross-pollination allows reshuffling of genes that may possessed evolutionary adaptive environmental fitness compared with inbreeders.
The meaning of sex here is not the erotic one but rather a function to effect the union of male and female gametes in the process of sexual reproduction. Sex remains an enigma within a mystery. It is baffling for the reason other than the fact that, in atypical and isogamous species with no male investment in young, a female could have as many grandchildren were she an asexual (apomict). From various definition of sex, two broad categories tend to predominate, that is, a) that sex enables the spread /creation of advantageous traits and b) that sex enables the efficient removal of deleterious genes. Simply sex is defined as putting together genes from two different individuals into one and producing recombinant genotypes. these new gene combinations could be an important asset for the survival of the species.
one sexual type
two sexual type
Plant sex, Flower sex and others
Because of the diversity of floral structures brought about by adapted evolutionary changes through time, some plants are classified as Monoecious(one house) when both male and female structures are both found on the same plant or same flower (hermaphrodite), the hermaphrodites are more numerous, accounting 72% of all sexual types. Moreover, if male and female structures are on separate plants are called Dioecious (two houses), male and female plants exist, examples of this condition are: papaya(Carica papaya), while monoecious plants include rice, corn, and wheat. Plants which bears female flowers only are called Pistillate, while plants that have male flowers only are Staminate, therefore, it is not uncommon to find different sexual types co-exist with one another.
According to Barrett and Harder(1996), sex in flowering plants is complicated by three distinct features of their biology:
- First, being sessile, plants requires vectors to transfer male gametes(pollen) between individuals. Animals are most commonly employed as pollinators because of their flexibility that can be manipulated by plant traits.
- Second, most plants are hermaphrodite (male and female structures present), so, are capable of selfing sometimes at the expense of outcrossing.
- Third, owing to their modular construction of plants, male and female gametes are packed in bewildering array of their structural and temporal combinations at the flower, inflorescence, plant or population level and despite the hermaphroditic condition.
Because of vector-mediated transfer and elaborate sexual system, plant mating can be highly "promiscuous" with individuals mating with many sexual partners including themselves.
types of Mating sYSTEMS
The primary mating system in plants are, outcrossing(cross-pollination), autogamy(self-pollination) and apomixis (asexual reproduction without fertilization). Likewise, in a mixed mating system. plants use two or even more three mating systems are not uncommon. A mating system is a way in which a group is structured in relation to sexual behavior. The precise meaning depends on the context. In plants, the term refers to the degree and circumstances that promote cross- pollination (Wikipedia, the free Encyclopedia).Flowering plants are predominantly hermaphroditic with most species producing flowers that contain both a female (pistil) and a male(stamen) sexual organs. These sexual structures can be separated spatially in a flower (Herkogamy) or can function at different times (Dichogamy), reducing the likelihood of self-pollination. In Apomixis, sexual organs or related structures take part but no union of gametes(reproductive cells), thus no fertilization occurs. The resulting seed is vegetatively produced. although there is no union of gametes, pollination is necessary to initiate the development of the endosperm. Forms of apomixis include the following:
- Parthenocarpy-is the phenomenon wherein fruits develop even without pollination, thus no seeds are formed..
- Apogamy-is the development of the embryo from haploid nuclei other than the egg. Generally, apogamy results from the fusion of two cells of the embryo sac.
- Apospory-is the formation of the embryo sac directly from a somatic cell without reduction and formation of spores.
mechanisms or devices of Pollination Control
Devices for the control of the mating system, whether to encourage inbreeding or outbreeding, are present in different ways in higher plant species and these are:
- Cleistogamy-stigma pollinated before the flower opens, favors self-pollination. Example-lettuce, annual fescue, several rice varieties.
- Chasmogamy-stigma pollinated after the flower opens, favors cross-pollination. Example-tomato
- Dichogamy-temporal maturity of male and female structure favors cross-pollination. Example-corn
- a) Protandry-pollen dehisced before stigma becomes receptive. Example-carrots. soybeans
- b)Protogyny-stigma receptive before pollen is dehisced. Example-avocado, walnut
- Monoecious-staminate(male)and pistillate(female) flowers are borne on the same plant favors self-pollination. Example-rice,
- Dioecious-staminate and pistillate flowers are borne on separate plants favors cross-pollination. Example-papaya, durian, rambutan
Mechanism of Self-incompatibility in flowering plants
Self-incompatibility mechanisms are designed to prevent self-pollination. Experts believed that inbreeding is the end of the evolutionary path. There are two general types of self-incompatibility in higher plants, namely;
- a) Gametophytic- the reaction is determined by pollen genotype. Stylar Inhibition.
- b) Sporophytic- the reaction is determined by sporophyte genotype.Stigmatic Inhibition
- Similar genotypes preclude self-pollination(see figure).
Ode to an Apomict: THE DANDELION
Daughters I have many
Yet husband and father none
A perfect matriarchy
I set in the sun
And send my plumed children
Floting one by one
Sexual reproduction is important for the continued existence of the species. It serves as the vehicle for evolution to operate in the development and survival of the species, whether plants or animals. The investment in sex may be measured quantitatively in terms of new gene combinations brought about by the union of gametes and the frequency of this new genotypes in the sexually interbreeding population. Individuals that cannot transmit their genetic endowment to the next generation may become extinct and the demise of the population. The question of "promiscuity" in plants is of no consequence, the flowers as sexual organs are will structure designed for either, self and cross pollination that may accept gametes from different sexual partners, including themselves.
Chasmogamy-pollination event happened in an opened flower
Dicots-are plants under the Class Dicotyledoneae of sub-Phylum Angiospermae or flowering plants, many known plants belong to this group, trees shrubs, food crops, etc.
Dioecy - a sexual polymorphism in which population contain male and female plants
Evolution- a change in gene frequency over time
Flower- a modified shoot designed for sexual reproduction
Herkogamy- spatial separation of sexual structures in a flower
Hermaphrodite- individual that can be male or female, occurrence may be sequential or simultaneous
Isogamy-is a form of sexual reproduction that involves gametes of similar morphology( similar in shape and size) differing only in the expression of alleles in the population.
Monocots-are plants that belong to Class Monocotyledoneae sub- Phylum Angiospermae or flowering plants include the tallest grass of the world-Bamboo and the three great bowls of cereals, namely, rice, corn, and wheat.
Sex -refers to the union(syngamy) of gametes, individuals need not have distinct sexes attributed to differences in size or structure of gametes
Barnett, Spencer. 2002. The evolution of plant sexual diversity(a Review).Genetics 3: 274-284.
Cresti, M and A. Trezzi (eds.) Sexual reproduction in plants.pp.48-88.Springer-Verlag.
Faure, J.2001. Double fertilization in flowering plants: discovery, study methods, and mechanism. CNR Acad. Sci.Vol.3.
Hartfield, M and P, Keightley.2012. The current hypothesis for the evolution of sex and recombination. Integrative Zoology 7:192-209.
Ward, J. et al.2014.Variation in sex allocation and floral morphology in an expanding distylous plant hybrids.International J. of Plant Sciences.175(5): 518-525.
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