1 Reproduction
1.1 Introduction
The continued survival of a species requires the members of the species to reproduce in order to increase the number of its members and to replace members that have died. The reproductive process by which living organisms multiply differs from one species to another. However, it can broadly be classified into two main categories, sexual reproduction and asexual reproduction. The main differences between both these reproductive process are shown in table 1.1
Table 1.1 A comparison between sexual reproduction and asexual reproduction
Sexual reproduction
Asexual reproduction
Parent
Gamete
Meiosis
Progeny
Incidence
Advantage
Usually involves two unisexual parents (male and female) or hermaphrodites
Specialised reproductive cells need to be formed. That is, male and female gametes unite to form a zygote following fertilisation
A special kind of cell dividion that must take place at some stage of the life cycle of an organism that reproduces sexually. Meiosis produces haploid (n) cells that only have one-half of the normal diploid (2n) number of chromosomes. This is to ensure that the zygote formed as a result of fertilisation will have the normal diploid number of chromosomes
Progeny will not be identical to the parents. This is due to variations that occur during the formation of gametes (gametogenesis). Apart from this, the union of gametes from two different parents will produce variable genotype in the progeny
Occurs in many animals and plants
Produce progeny that is not identical to the parents
One parent
Specialised reproductive cells are not formed
There is no union of nucleus of the specialised reproductive cells. Hance, meiosis need not take place during the life cycle of organisms that reproduce asexually
Progeny is genetically identical to its parent. Except in situations where mutations may have taken place in the somatic cells
Usually happens in plants, prokaryotes and mosses
Process is faster and the necessity for two parents does not arise
The advantage of sexual reproduction compared to asecual reproduction is that sexual reproduction permits the recombination of genetic material from different parents. As a result of this, the progeny will have new features. This advantage plays an important role in maintaining the survival of a species.
in plants, there is aclear sequence in the evolutionary cycle. It starts from organisms like algae and bacteria, which reproduce asecually and ends with the higher order plants, which have a more complex life cycle. The higher order plants have complex adaptations which ensure
that the male gamete is able to meet with the female gamete to form a zygote
that the zygote will continue to survive and achieve sexual maturity
Some lower order plants such as moss, do not have specialised reproductive features. They produce millions of apores so that at least a small portion of these spores can be spread to a habitat that is suitable for them to germinate and continue life.
The higher order plants produce only a few seeds but each seed has the capacity to germinate and grow into a mature plant.
Although most animals reproduce either secually or asexually, some animals are able to reproduce both ways. However, asexual reproduction is not as common among animal species compared to plants, protists and prokaryotes. Most animals reproduce sexually.
In general, plant and animal reproduction follows a life cycle that can be seen as a sequential change in stages of life that an organism needs to undergo.
basically, reproduction involves tha transfer of genetic material (RNA or DNA) from the parents to the progeny. Sexual reproduction involves the formation of gametes (gametogenesis) and the union of the gametes (fertilisation). Asexual reproduction, on the other hand, involves the formation of spores or vegetative reproduction. Thus, it is clear that all organisms reproduce, either sexually or asexually and sometimes by both methods. Asexual reproduction is not limited to the reproduction of microorganisms. Some eukaryotic organisms can also reproduce sexually and asexually.
1.2 Sexual reproduction in plants
Plant sexual reproduction involves two basic processes.
Meiosis – A process where the diploid (2n) number of chromosomes in an cell is reduced to haploid (n) number in the male and female gametes, that are formed as a result of this process at a certain stage in the life cycle of the organism
Fertilisation – A process that involves the union of a male and a female gamete. Both the gametes are haploid (n), and form a diploid (2n) zygote at a certain stage in the life cycle of the organism
The haploid (n) generation is also known as the gametophyte generation while the diploid (2n) generation or phase is known as the sporophyte generation
The division of reproductive cells through meiosis is a pre-requisite for sexual reproduction to take place. This is bacause meiosis helps to ensure that a species is able to maintain the same number of chromosomes in its somatic or vegetative cells. Meiosis happens during gametogenesis (formation of gamete) or just after the union of gametes takes place.
In a life cycle that involves sexual reproduction, the existence of both the haploid (n) stage and diploid (2n) stage is necessary. However, the exact stage when meiosis occurs in the life cycle of an organism differs among the various species. As a result, there are three different types of life cycles depending on the length of time of the gametophyte and sporophyte generation
Haploid life cycle (figure 1.1 (a))
In a haploid life cycle, meiosis occurs just after fertilisation and the haploid stage is sustained for the most part of the organism’s life cycle. One example is the life cycle of the algae Spirogyra.
Diploid life cycle (figure 1.1 (b))
In a diploid life cycle, meiosis produces gametes. The diploid stage will remain after fertilisation for the major part of the organism’s life cycle. One example is the life cycle of the algae Fucus sp.
Haploidiplon life cycle (figure 1.1 (c))
The haploidiplon life cycle involves both the haploid and diploid stages where both stages alternate. This type of life cycle shows alternation of generations. Both the stages can be clearly distinguished
Alternation of generations is the basis for the life cycle of many plants. It is most evident in mosses (figure 1.2) and ferns. The haploid stage (gametophyte generation) is easily distinguished from the diploid stage (sporophyte generation)
However, in some plants, the alternation of generations becomes unclear when the period of a stage is shortened or does not take place. This is seen in the life cycle of certain primitive plants, such as the green algae, Oedogonium (figure 1.3). In such primitive plants, the diploid (2n) zygote does not develop to become an adult through mitosis but divide by meiosis to form zoospores after a short period following its formation. The zoozpores will later develop to become haploid (n) adults, which can be regarded as the haploid generation. As such, the sporophyte generation that is diploid (2n) is considered to be non-existent in the organism’s haploid life cycle.
On the other hand, in plants that are more advanced such as conifers (gymnosperm) and flowering plants (angiosperm) the diploid (2n) sporophyte generation is dominant and clear. However, the haploid (n) gametophyte generation happens only for a very short period and is samll in size (figure 1.4). In fact, the size of the gametophyte generation in these plants deteriorated until they were embedded in the sporophyte vegetative tissues, expecially in flowering plants.
Algae
Although most algae are aquatic, there are some that live in moist places on land, for example, on the ground and on the surface of certain plants.
Algae can either be unicellular or multicellular. The cells of algae are not differentiated into roots, stems and leaves.
All algae contain chlorophyll. However, in some species of algae, other pigments hide the green colour of the chlorophyll.
Algae have the ability to reproduce both sexually and asexually.
Asexual reproduction of algae
Unicellular algae usually reproduce asexually by binary fission
Multicellular algae that have a filamentous structure reproduce asexually by fragmentation
Most algae reproduce asexually by spore formation
Sexual reproduction of algae
The sexual reproduction of algae also involves the union of two gametes. There are three different types of gametes (figure 1.5) involved.
Isogamy: The two gametes that unite cannot be distinguished by their size, morphology or movement, for example, Chlamydomonas.
Anisogamy: The two gametes that unite are slightly different in size and movement. In most cases the male gamete is usually smalle than the female gamete, for example, Spirogyra.
Oogamy: There is a clear difference between the male and female gametes. The female gamete (egg) is unable to move and is usually bigger than the male gamete (sperm), which is mobile. The number of male gametes is also greater. An example is Fucus.
Sexual reproduction in Spirogyra
Spirogyra is a simple multicellular green algae that has a filamentous structure.
It reproduces sexually through conjugation of its filaments
The following are the various stages of reproduction in Spirogyra (figure 1.6)
i. Two filaments align side by side longitudinally
ii. Short extensions from the cell wall are formed at corresponding sites on each of the cells in a pair of cells that are side by side. The chloroplast in each cell disintegrates
iii. The short extensions then touch each other and the cell walls at the ends of the extensions disintegrate fo form conjugation tubes. The cell contents in one of the filaments cluster fo form gametes that move towards the conjugation tubes. The gamete that is able to move is known as the male gamete.
iv. The cell contents of the other filament form female gametes, which are immobile. Spirogyra is heterothallus. This means that the male gametes are produced by one filament, while the female gametes are produced by the other filament. It is very rare for one filament to produce both types of gametes.
v. The male gamete enters the female cell space through the conjugation tube. The male and female gametes unite to form a zygote.
vi. The zygote will produce a thich outer cell wall and become a zygospore, which is later released when the parental cell wall disintegrates.
vii. The zygospore remains dormant for some time before it germinates. During germination, the wall of the zygospore breaks and its contents are released as a green haploid (n) cell, which rapidly divides to form a new filament
Based on the characteristics of its gametes, the sexual reproduction of Spirogyra is of the anisogamous type.
It is clear that the mature filament of Spirogyra is of the haploid generation that produces the gametes. On the other hand, the diploid zygospore (sporophyte generation) does not develop but will undergo meiosis to form green haploid cells that divide rapidly to form filaments. Based on this, the life cycle of Spirogyra is controlled by the haploid generation (figure 1.7)
Bryophyta
The members of Bryophyta division comprise simple green terrestrial plants. These plants are normally found in areas that receive high rainfall and have a high level of humidity.
Bryophytes are divided into two classes.
Hepaticae class (livermort); example, Pellia
Muski class (moss); example, Funaria
Both classes of bryophytes show a clear alternation of generations. However, the more dominant stage in its life cycle is the haploid (n) gametophyte generation, while the diploid (2n) sporophyte only serves to be a part of the temporary spore-forming stage. The bryophytes depend on photosynthetic activity for growth. The livermort gametophytes of moss are plants small stem-like and leaf-like appendages.
The bryophytes reproduce sexually by oogamy. The archegonium is the female reproductive organ. Each female plant has several bottle-shaped archegoina. Each mature archegonium contains a single egg (oosphere). The male reproductive organ is the antheridium, and contains a number of antherozoid (spermatozoid) within the organ.
In damp condition, fertilisation between the egg and antherozoid can take place and produce a zygote that can develop to become the diploid (2n) sporophyte.
Sporophytes normally comprise sporogonium (or capsule), a stalk and a foot (apparatus for absorption) which absorbs food, water and minerals from the parent gametophyte
As the gametophyte and sporophyte generations show different morphological structures, the life cycle of bryophytes is characterised as heteromorphic alternation of generations.
Sexual reproduction of livermort, Pellia epiphylla
a) Pellia is a livermort that has a thallus and shows very little tissue specialisation. It can be found in damp areas, such as riverbanks, the sides of drains, shady areas and jungles
b) The gametophyte has one thallus, which is dark green in colour and is flat. All cells are haploid (n). the male (antheridium) and female (archegonium) sexual reproductive organs are located on the thallus of the gametophytes. The antheridium produces male gametes (antherozoid) while the archegonium produces female gametes (oosphere)
c) The fertilisation between the antherozoid and oosphere will produce a diploid (2n) zygote, which first develops to become an oospore and later develops to become a sporogonium.
d) The sporophyte stage of Pellia is the sporogonium. The sporogonium is diploid (2n) because it is a product of the zygote developing through mitotic division. In the sporogonium capsule, there are spore-forming mother cells that divide by meiosis to produce haploid (n) spores. Upon maturity, the capsule will disintegrate, releasing the spores, which are dispersed by the wind. Under moist conditions, the spores will germinate and develop to form thallus.
e) As both the alternating gametophyte and sporophyte generations differ in morphology, the life cycle of Pellia shows the heteromorphic alternation of generations
f) The structures and processes involved in the sexual reproduction of Pellia are illustrated in figure 1.8 and its life cycle is shown in figure 1.9
Sexual reproduction of Funaria sp.
a) Funaria is a type of moss that has a higher tissue specialisation compared to the livermort. The tissue specialisation includes a simple transportation vessel for transport of food and water, and a stalk that has small leaf-like structures which can be found on the haploid gametophyte.
b) The gametophytes of Funaria are monoecious, which means both male (antheridium) and female (archegonium) sexual reproductive organs are found on the same plant. The antheridium axis has leaves arranged in the shape of a rose with wide petals and the centre is brown in colour.l this structure is called perystia and is more evident than the similar structure that surrounds the archegonium.
c) Fertilisation between the male gamete, the antherozoid, and the female gamete, the oosphere, will produce an oospore. The oospore will develop to become a diploid (2n) sporogonium, which comprises a capsule, a stalk and a foot. The spore-forming mother cells located inside the sporogonium will divide meiotically to produce haploid (n) spores.
d) Upon maturity, the capsule of the sporogonium breaks open, releasing spores that dispersed by the wind.
e) If a spores comes to rest in a suitable location, it will germinate and develop to forma branched filament called primary protonema. Cell division by the protonema will form buds that will eventually become new, true moss plants.
f) The structures and processes involved in the sexual reproduction of the true moss Funaria is illustrated in figure 1.10 and its life cycle is shown in figure 1.11
Filicinophyta: Ferns (Dryopteris sp.)
Ferns a very common in Malaysia. They are found in moist, cool and shady areas.
Ferns are diploid (2n) sporophytes, which have typical stems and leaves. These arise from an underground stem known as rhizome. The roots also extend from the rhizome. The roots penetrate the earth in all directions. The large leaves of the fern are the only parts of the plant that are visible above the ground and are also known as fronds.
Fronds are of the diploid sporophyte generation. Mature fronds are large and have brownish spots underneath the leaflets of each frond. Each dot is the reproductive organ of the fern and is called a sorus. A sorus consists of many sporangia mounted on stalks. Within each sporangium, the spore mother cells undergo meiosis, producing four spores each.
Upon maturity, the spores will be released from the sporangium for dispersal
The spores will remain dormant for sometime after they fall on a shaded, moist spot. Soon after, the spores will germinate and develop to become prothallus
As the prothallus develops from a haploid spore, the cells of the prothallus are haploid. The prothallus is the mature gametophyte generation.
During the mature gametophyte period, the prothallus will form the male (antheridia) and female (archegonia) reproductive organs, on its ventral surface. Antheridia will produce male gametes, antherozoids, while archegonia will produce female gametes, oospheres.
Fertilisation takes places within the archegonium between the antherozoid and oosphere to produce a zygote (2n) and the new sporophyte generation will be initiated. The newly formed zygote will develop by repeated cell divisions to become a young fern plant. As the young sporophyte plant gradually develops to become an independent plant, the prothallus will gradually die.
As with bryophytes, the pteridophytes also show clear alternation of generations but the dominant generation is the diploid sporophyte, which has developed its root and stem system to allow it to adapt to living under terrestrial conditions. This situation is similar to what can be found in flowering plants.
The structure of the sexual reproductive organs and the life cycle of ferns are shown in figure 1.12 and figure 1.13 respectively.
Coniferophyta, example Pinus sp.
Coniferophyta are conifer trees that produce cones. Most of these are huge plants with leaves that resemble needles. These are the predominant plants found in forests in colder regions of the earth.
Conifer trees are of the sporophyte generation and produce both male and female cones (figure 1.14). The cones are of the gametophyte generation
Male cones are smaller in size and xomprise many scaly leaves known as microsporophyll, which are arranged surrounding a central axis. Each scahly leaf will carry two sacs containing pollen grains called the microsporangium. Each microsporangium contains polen grains called the microsporangium. Each microsporangium contains polen grains that are called microspores. Microspores are haploid (n) cells produced by the diploid (2n) microspore-forming mother cells through meiotic division. Each pollen grains has an air sac, which facilitates its dispersal by the wing. The male cones will drop off from the tree once its polen grains have been dispersed.
The felame cones are larger than the male cones and these remain on the tree for three consecutive years. Each female cone consists of scally leaves known as megasporophyll. Two ovules can be found on the top surface of each megasporophyll. Each ovule comprises a clump of cells arranged around a single megaspore mother cell. In the first year, the mother cell will divide by meiosis to produce four megaspores. However, only one of the four will remain while the other three will deteriorate. Pollination will take place under these conditions.
The remaining megaspore will develop to become an embryo sac, that is, a megagametophyte, which contains two archegonia.
In the second year, microspores (pollen grains) will be blown by the wind in the direction of the female cone. The microspore might enter an ovule through a fine opening known as microphyll and be trapped inside the obule. The trapped pollen grain will form a thin pollen tube that grows into the tissues of the female cone untul it reaches the vicinity of the archegonium. This may take a full year in most pines.
The pollen tubes will rupture inside each archegonium allowing the nucleus of the haploid male gamete (pollen) to fuse with the nucleus of the haploid egg cell (megaspore) to form a diploid zygote
As the male gamete (microspore) needs to move around to search and fuse with a female gamete megaspore, this type of fertilisation is known as zoidogamous, which is also common in pteridophytes. This shows that an evolutionary link exists between coniferophyta and filicinophyta.
After fertilisation, two of more embryos might be formed in each embryo sac (megagametophyte) but only one will remain while the rest will deteriorate.
In the third year, seed development is completed and the seed is dispersed before the dry brown cone falls from the tree.
The reproductive structures of the coniferophyte, Pinus, are shown in figure 1.14 while its life cycle is summarised in figure 1.15
Angiospermophyta (Flowering Plants)
In Angiospermophyta, sexual reproduction takes place in the flower, which is the gametophyte stage of the plant.
Flower are the reproductive organs of flower plants. A flower has hermaphrodite structure, consisting of both male and female reproductive organs. In flowers, the male reproductive organ consists of the stamen and is known as the androecium while the female reproductive organ consists of a number of carpels and is referred to as the gynoecium. However there are some flowers that are unisexual, where each tree can only produce either female flowers or male flowers.
Structure of flower: flowers have evolved from compressed shoots with four whorls of modified leaves separated by very short internodes. Flowers are highly specialised for direct or indirect sexual reproduction. The four floral organs comprise sepals, petals, stamen and carpels (in sequence from the outside to the inside of a flower). These organs meet at a central point which is the receptacle causing it to swell slightly. The four whorls form the four important parts of the flower (figure 1.16)
Sepals: sepals are green, leaf-like structures. They make up the outermost whorl of the flower. Sepals form the calyx.
Petals: petals form the second whorl, inside the sepal whorl. They are usually brightly coloured to attract insects and birds to help with pollination. The petals group together to form the corolla.
Androecium: the androecium comprises a number of stamens, and is the male part of the flower. Each stamen has an anther, which contains pollen sacs (figure1.17) that are located at the ends of a long and flexible filament. Each pollen sac contains a group of diploid pollen forming parent cells. Each pollen has the ability to divide meiotically fo form a tetrad, shich comprises four haploid (n) pollen. The nucleus of each haploid pollen will divide mitotically to form two nuclei, the generative nucleus and the tube nucleus.
Gynoecium (pistil): The pistil is located in the innermost whorl and is the female part of the flower. The pistil consists of a carpel, which can be apocarpous (separate) or syncarpous (fused). Each carpel comprises an oval-shaped ovary with a lumen, a style and a stigma (figure 1.18). The stigma releases an sticky fluid that can trap any pollen g rain that falls onto its flat surface. Each ovary contains one or more ovules that comprise parenchyma cells called nucellus. The nucellus is covered all over by one or two layers of integument except at the part of the micropyle, where it is exposed to facilitate the entry of a pollen tube. The female gamete or egg cell found inside each mature ovule is called ovum.
Development on pollen and the male gamete
Pollen development occurs in each of the pollen sacs located on the anther. Initially, each sac contains a group of diploid parent cells known as pollen-forming parent cells. Each pollen-forming parent cell will later divide by meiosis to form a tetrad, which comprises four haploid cells that will develop to become pollen.
The haploid nucleus in each pollen will divide by mitosis to form two haploid nuclei, one of which will form the generative nucleus while the second one will form the tube nucleus (figure 1.19)
In addition to the development described above, the pollen wall will thicken untul the outer surface of the mature pollen appears to be completely curved. These curves allow the pollen to easily stick on to the surface of the stigma.
During pollen germination on the stigma, the tube nucleus will position itself at the end of the growing pollen tube, while the generative nucleus will divide mitotically to become two male nuclei (figure 1.22)
The development of the ovule and embryo sac inside the ovary
Each ovule starts as an extension of the ovary wall at the base of the carpel.
During its development, the young ovule will enlarge and swell to form a receptacle that is known as funicle, which is bound to the ovary wall through a specialised tissue called placenta.
In the beginning, each consists of a mass of uniform cells called nucellus. In addition to the continued development of the nucellus, the wall of the ovule will divide into two protective intetuments which surround the softer nucellus tube below it (figure 1.20). It will leave a tiny hole called micropyle at the end with the calaza at the other end.
The embryo sac, which protects the female gamete or the haploid egg cell, is located in the centre of the ovule.
The embryo sac originates from a single cell known as the diploid embryo sac-forming parent cell. This parent cell will later divide meiotically to form a row of four haploid cells. Three of the cells will deteriorate and only one will develop to bewcome the young embryo sac (figure 1.20)
While the young embryo sac is rapidly developing, its nucleus will undergo three successive mitotic dividions to produce eight daughter nuclei. Four of these nuclei will migrate to each end of the sac.
Later, one out of the four daughter nuclei from each end will move to the centre of the embryo sac. These two nuclei are known as polar nuclei. They play a role in the formation of endosperm storage tissues.
In addition to the formation of the polar nucleus, the remaining six nuclei will remain at both ends of the embryo sac.
This means that a mature embryo sac has three nuclei at each end and two nuclei in the centre. Moreover all the nuclei are haploid.
One of the three nuclei from one end will become the functional mature egg cell while the remaining two non-functional nuclei, that is, the synergids, will eventually deteriorate.
In the meantime, the three nuclei at the second end, known as antipodal cells, will not be involved in any further changes inside the embryo sac.
Transfer of pollen- pollination
There are two types of pollination, self-polination and cross-pollination. Self pollination refers to the process where the pollen in the anther is transferred to the stigma of the same flower while cross-pollination occurs when the pollen in the anther is transferred to the stigma of a different flower.
Before pollination takes place, the anther wall will become dry causing the cell to shrink. This will in turn cause some tension in the anther wall. The mounting tension will split the anther open along its side (figure 1.21)
The part where the splitting starts to occur is the stomium and is the weak point on the anther. The tension in the anther wall causes it to split open releasing pollen which is then transferred to stigma ghrough various specific agents.
Angiosperms have several adaptations to prevent self-polination in the flowering plant.
Male and female flowers are found on separate plants.
The flower possesses several structures that prevent self-polination
In hermaphrodite flowers, the stamen and stigma mature at different times. When the stigma matures first, it is called protogyny. When the stamen matures first, it is known as protandry.
To avoid self-polination, angiosperms exhibit isolation mechanisms. Due to this, if the pollen from the same plant falls onto the stigma, the pollen tube will not form alternatively, the pollen tube in such a case takes much longer to reach the embryo sac compared to the pollen tubes of pollen from another plant.
8. Pollen germination and the growth of a pollen tube- fertilisation
