INTRODUCTION
All sexually reproducing organisms exhibit sequential events and processes which are grouped into three distinct stages, namely, the pre-fertilization, fertilization and the post-fertilization events.
- Pre-fertilization Events:- They occur prior to the process of fertilization. The two main prefertilization events are gametogenesis and gamete transfer.
(a) Gametogenesis:– It refers to the process of formation of gametes- male and female.
events in sexual reproduction
Categories of Gametes:-
(i) Isogametes:- When the fusing gametes are morphologically similar they are known as isogametes or homogametes. They are produced in some algae and fungi.
e.g., (a) Algae:- Cladophora, Chlamydomonas debaryana.
(b) Fungi:- Synchytrium, Rhizopus.
(ii) Heterogametes :- When the fusing gametes are morphologically distinct types, they are known as heterogametes. It is the feature of majority of sexually reproducing organisms.
e.g., (a) Algae:- Fucus, Volvox, Chara.
(b) All bryophytes, Pteridophytes, Gymnosperms and Angiosperms.
In such organisms, male gametes is called antherozoid or sperm and the female gamete is called egg or ovum
Cell Division During Gamete Formation:– Gametes are always haploid e.g., they possess only one set of chromosomes or genome though the parent body producing gametes may be either haploid or diploid. In haploid parent, gametes are produced by mitotic division. It is seen in several
organisms belonging to group algae, fungi as well as bryophytes. In haploid algae and some fungi, meiosis occurs in zygote or zygospore which is called zygotic meiosis.
- In plants belonging to group pteridophytes, gymnosperms and angiosperms and animals the parental body is diploid.
- Here reductional division occurs before or at the time of gamete formation, then it is which undergo meiosis are called
- If meiocyte is indulged in gamete formation, then it is called gamete mother cell.
Table:- Chromosome numbers in meiocytes (diploid, 2n) and gametes (haploid, n) of some organisms.
Name of organism | Chromosome number in meiocyte (2n) | Chromosome number in gamete (n) |
Fruit fly | 8 | 4 |
House fly | 12 | 6 |
Onion | 16 | 8 |
Maize | 20 | 10 |
Rice | 24 | 12 |
Apple | 34 | 17 |
Cat | 38 | 19 |
Rat | 42 | 21 |
Human beings | 46 | 23 |
Potato | 48 | 24 |
Dog | 78 | 39 |
Butterfly | 380 | 190 |
Ophioglossum (an adder’s tongue fern) | 1260 | 630 |
Sexuality in Organisms:-
Lower organisms:– In most of the lower sexually reproducing organisms , two fusing gametes are morphologically similar. If these gametes belong to the same parent then such organisms are called homothallic. e.g fungi (Mucor mucedo). When these gametes belong to different parents then these organisms are called heterothallic.
Higher Organisms:- In higher plants there are well-developed sex organs and there is clear distinction between male and female sex organs.The male sex organ is called stamen and female sex organ is called carpel or pistil. If male and female sex organs occur in the same flower then these plants are called bisexual, e.g China rose. If flowers possess only stamen or carpel then these plants are called unisexual. When male flower (staminate) and female flower (pistillate) are present on same plant body such plants are monoecious, e.g cucurbits, coconut and maize. However, if they are present on separate plant body then these plants are known as dioecous, e.g; date palm and papaya.
- In some of the lower plants also the monoecious and dioecious condition occur.
Sexual Reproduction in Chara and Marchantia:- The Chara is a green alga. The sex organs are highly specialized. Some workers prefer to call the male sex organ as antheridium and female as oogonium, while others did not favour this terminology. They call the male sex organ as globule and the female as nucule and this terminology is largely followed in Chara. These sex organs are exceptionally muticelled and covered by jackets
- The jacket of nucule is formed by five tube cells and the jacket of gobule is formed by eight shield cells.
- The nucule has a cap of five coronary cells.
- The sex organs are borne on the adaxial surface of the short lateral branch almost on each node.
- The nucule occupies an upper position than the globule. While most of the species of chara are monoecious, C. wallichii is dioecious.
- The globule matures prior to nucule (protandrous condition). Each antheridium produces many band shaped, spirally coiled, biflagellate antherozoids.
- The oogonium contains a single egg. The egg is laden with starch and oil globules.
- Marchantia is a dioecious bryophyte where male sex organ is antheridium and female sex organ is the
- Sex organs are borne on stalked upright receptacles or special branches called gametophores.
- Gametophores of male thallus is called antheridiophore and female thallus as archegoniophore.
(b) Gamete transfer:- After the formation of male and female gametes , compatible gametes must be physically brought together to facilitate fusion (fertilision or syngamy). In few fungi and algae, both types of gametes are motile. But in majority of organisms male gamete is motile and the female gamete is non-motile. So there is a need of a medium through which the male gametes move.
- In algae, bryophytes and pteridophytes, water is the medium for gamete transfer.
- A large number of the male gametes however, fail to reach the female gametes.
- To compensate this loss of gametes, the number of male gametes produced in several thousand times the number of female gametes produced.
In seed plants both male and female gametes are non-motile. Here pollen grains are the carrier of male gametes and ovule has egg. Pollen grain produced in anther (male part) are transferred to the stigma of female organ i.g; carpel through the process of pollination.
- Fertlisation:-
- The most vital event of sexual reproduction is the fusion of gametes. This process is called syngamy or fertilization which results in the formation of a diploid
Syngamy can occur in external medium as well as inside the body of organism. On this basis syngamy can be distinguished into two types
(a) External fertilization:- Syngamy occurs outside the body of organisms in external medium (water). It is shown by majority of aquatic organisms like most of the algae, fishesas well as amphibians.
- Organisms exhibiting external fertilization show great synchrony between the sexes and release a large number of gametes into the surrounding medium in order to enhance the chances of syngamy.
- A major disadvantage associated with it is that the offsprings are extremely vulnerable to predators threatening their survival upto adulthood.
(b) Internal fertilization:- Syngamy occurs inside the body of organisms. It is present in majority of plants like bryophytes, pteridophytes, gymnosperms and angiosperms. It occurs in few algae like Spirogyra. In all these organisms egg is formed inside the female body where syngamy occurs.
- The male gametes either through water or pollen tube are transferred to female gamete.
- In order to enhance the chances of syngamy large number of sperms are produced in these organisms and to compensate for this there is significant reduction in number of eggs produced.
Post-Fertilisation Events:- Events in sexual reproduction after the formation of zygote are called post-fertilisation events.
Zygote:-
- It is the first cell of the new generation in all sexually reproducing organisms.
- Zygote is always diploid. It is formed in the external aquatic medium in those organisms which perform external fertilization.
- Zygote is produced is produced inside the body in case where fertilization is internal.
Embryogenesis:-
- Embryogenesis is the process of development of embryo from zygote.
- Embryo is a multicellular stage in the life cycle of a plant or animal prior to formation of an independent individual. In embryogenesis, the zygote undergoes repeated cell divisions through mitosis.
- Cells undergo differentiation attaining specific shape, size and function.
- Cells differentiation occurs at specific locations resulting in production of different tissue, organs and organs system.
- In flowering plants, zygote develops into Meanwhile , wall of the ovary produces pericarp. or fruit wall.
- The ripened ovary with pericarp and seeds is called
- As the fruit begins to develop, sepals, petals, stamens, style and stigma normally sheds. After dispersal, the seeds, upon reaching suitable substratum germinate and form new plants.
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