As you are already aware gametogenesis refers to the process of formation of the two types of gametes – male and female. Gametes are haploid cells. In some algae the two gametes are so similar in appearance that it is not possible to categorise them into male and female gametes.
They are hence, are called homogametes (isogametes). However, in a majority of sexually reproducing organisms the gametes produced are of two morphologically distinct types (heterogametes). In such organisms the male gamete is called the antherozoid or sperm and the female gamete is called the egg or ovum.
Sexuality in organisms: Sexual reproduction in organisms generally involves the fusion of gametes from two different individuals. But this is not always true. From your recollection of examples studied in Class XI, can you identify cases where self-fertilisation is observed? Of course, citing such examples in plants is easy.
Plants may have both male and female reproductive structures in the same plant (bisexual) or on different plants (unisexual). In several fungi and plants, terms such as homothallic and monoecious are used to denote the bisexual condition and heterothallic and dioecious are the terms used to describe unisexual condition. In flowering plants, the unisexual male flower is staminate, i.e., bearing stamens, while the female is pistillate or bearing pistils.
In some flowering plants, both male and female flowers may be present on the same individual (monoecious) or on separate individuals (dioecious). Some examples of monoecious plants are cucurbits and coconuts and of dioecious plants are papaya and date palm.
Name the type of gametes that are formed in staminate and pistillate flowers. But what about animals? Are individuals of all species either male or female (unisexual)? Or are there species which possess both the reproductive organs (bisexual)? You probably can make a list of several unisexual animal species.
Earthworms, sponge, tapeworm and leech, typical examples of bisexual animals that possess both male and female reproductive organs, are hermaphrodites. Cockroach is an example of a unisexual species. Cell division during gamete formation : Gametes in all heterogametic species are of two types namely, male and female.
Gametes are haploid though the parent plant body from which they arise may be either haploid or diploid. A haploid parent produces gametes by mitotic division. Does this mean that meiosis never occurs in organisms that are haploid? Carefully examine the flow charts of life cycles of algae that you have studied in Class XI (Chapter 3) to get a suitable answer.
Several organisms belonging to monera, fungi, algae and bryophytes have haploid plant body, but organisms belonging to pteridophytes, gymnosperms, angiosperms and most of the animals including human beings, the parental body is diploid. It is obvious that meiosis, the reduction division, has to occur if a diploid body has to produce haploid gametes. In diploid organisms, specialised cells called meiocytes (gamete mother cell) undergo meiosis.
At the end of meiosis, only one set of chromosomes gets incorporated into each gamete. Carefully study Table 1.1 and fill in the diploid and haploid chromosome numbers of organisms. Is there any relationship in the number of chromosomes of meiocytes and gametes?