Sexual reproduction, as heard, is not a simple process. It is a slow and highly complex process. For the ease of studying, a three-staged classification has been assigned to sexual reproduction. 'Pre-fertilization events' is the first stage of sexual reproduction and is highly important for the remaining stages to occur. This stage of sexual reproduction refers to all the events that takes place before fertilization, that is, fusion of male and female gametes. Pre-fertilization events of sexual reproduction involves two major events, namely, gametogenesis and gamete transfer.
The process of formation of gametes is called gametogenesis. In sexual reproduction, there are two types of gametes, namely, male gametes and female gametes. However, there are few exceptions. There are cases in which it is impossible to differentiate the gametes into male or female. This is because the gametes are so similar in appearance that it is very difficult, almost impossible, to distinguish them as male and female gametes. This happens in the case of certain algae, for example, Cladophora. In such cases, the gametes cannot be classified into male and female and are hence, called isogametes or homogametes. However, such exceptions are very few in number and in most of the cases, the two parents produce two different types of gametes that are morphologically very different. Such gametes are called heterogametes. In case of heterogametes, the female gamete is called egg or ovum and the male gamete is called antherozoid or sperm.
Does sexual reproduction always mean gametes from two different individuals?
Do you think that gametes always come from two different individuals in sexual reproduction all the time? No, the gametes do not come from two different individuals all the time. There are cases in which the gametes come from the same individual. This happens more often in the case of plants. In plants, both male and female reproductive structures might be present in the same flower or both male and female flowers might be located on the same plant. In both the cases, the condition is called bisexual or monoecious. Monoecy promotes self fertilization. In such cases, the gametes for sexual reproduction are coming from the same individual. For example, coconuts and cucurbits exhibits monoecy.
However, if the male and female flowers are located on different plants, them such a condition is called unisexual or dioecious. Dioecy prevents self fertilization and promotes cross fertilization. For example, date palm and papaya exhibits dioecy. However, in the case of animals, the bisexual condition is often referred to as hermaphrodites. Earthworms and leeches are examples for hermaphrodites. Humans and majority of the remaining animals are examples for unisexual species.
Mitosis or meiosis
All gametes are haploid in nature. However, this does not mean that the parental body that produces these gametes are also haploid in nature. The parental body that produces these gametes might be haploid or diploid in nature.
For haploid organisms, gametogenesis is all about mitosis. These organisms produce gametes by mitosis. Haploid parental body is more common in the case of organisms with relatively simple organisations. In general, majority of the monerans, fungae, algae and bryophytes posses haploid parental body and hence, produces gametes by mitosis.
In diploid organisms, the process gets even more complex. In the case of diploid organisms, reduction division or meiosis is a must, as the gametes are always haploid in nature. Thus, diploid organisms produce their gametes by meiosis. The remaining categories of the plant kingdom, that is, pteridophytes, gymnosperms and angiosperms along with majority of the animals posses diploid parental body. Thus, they require meiosis for the production of gametes. A unique characteristic property of diploid organisms is the presence of meiocytes. Meiocytes are nothing but specialized cells found in diploid organisms, that undergo meiosis to form gametes.
This is the second and final event of the first stage of sexual reproduction, that is pre-fertilization events. Gamete transfer is the process of bringing together, the two different types of gametes. This is done to facilitate fusion of the two types of gametes (fertilization). In some cases, both the male and female gametes are motile and travels towards each other for fusion. However, in majority of the cases, the female gametes are non-motile and male gametes, that are motile, travels towards female gametes for fusion. However, in the case of flowering plants, both male and female gametes are non-motile. In this case, the non-motile male gametes are transferred to the non-motile female gametes with the help of pollen grains and pollen tube. Each and every transfer of the gametes is done to facilitate the ultimate event of sexual reproduction, that is, fertilization.