Homogametes are also called isogametes, where the structure of male and female gametes (the sex cells) are morphologically similar.
Close-up of green algae
Gametes are called sex cells that play a significant role in the reproduction of an organism. Since the male and female gametes have a similar structure and size, therefore they are indistinguishable. For example, organisms like some alga and fungi have homogametic cells. Species that are homogametic have haploid life cycles; therefore, they undergo meiotic cell division (reduction). Homogametes are different from heterogametes. As the name suggests, heterogametes are the male and female gametes that can be distinguished from each other based on structure and size. Most of the organisms are heterogametes, for example, humans, most plants, etc. In heterogametic organisms, the male gates are usually mobile and move towards the female gamete. Female gametes are called eggs or ova and are stationary, whereas male gametes are called sperms.
Gametes are haploid cells. Therefore, the organisms that sexually reproduce undergo meiotic cell division. This is a reductive cell division, where diploid organisms undergo meiotic cell division to produce haploid gametes containing only half genetic information. Through syngamy (fusion of male and female gametes) and fertilisation, these haploid cells develop into diploid zygotes. The zygote then undergoes mitotic cell division, where the cells are multiplied to increase in number. The cells keep multiplying in mitotic cell division and increase the mass size of the zygote to develop into a fetus finally.
Green Cladophora a seaweed
For example, the reproduction of Cladophora occurs in the terminal section of the filament of this seaweed. Two gametophytic filaments consist of a large number of homogametes. The structure of these homogametes consists of flagella (to enable mobility towards the other gamete), a cytoplasm containing a nucleus. The two filaments release the gametes. With the help of flagella, these gametes will move towards each other. At the beak portion, the gametes will fuse to combine both the nucleus to form a zygote. The zygote will germinate to grow into another diploid sporophytic plant. The filaments from the two individual diploid plants reproduced to form a diploid Cladophora. The diploid filament aids in the development of the gametes, where the gametangia divide by meiosis to produce haploid gametes.
A diploid cladophora plant contains a structure where the spores are formed, called sporangium having diploid spores. These spores undergo meiosis cell division to produce quadriflagellate zoospore (zoospores containing four flagella) that are haploid, developing into a haploid cladophora plant. Two individual haploid plants release isogametes that fuse and fertilise to produce a diploid zygote. This zygote grows and matures into a diploid cladophora plant.
Homogametes and Heterogametes
Homogametes and heterogametes are distinguished based on the gametes’ structure, size, shape, and morphology.
- Homogametes are known as isogametes, whereas heterogametes are also known as anisogametes. As discussed earlier, the male and female gametes with indistinguishable morphology and behavior are homogametic organisms, while these are morphologically and behaviorally distinguishable in heterogametic organisms.
- Since the male and female gametes in homogametic organisms are similar, therefore they do have any differentiating terminology. In contrast, the male gametes are sperm for heterogametic organisms in animals, while the female gametes are called eggs. Meanwhile, the male gametes are pollen grains in plants, while the female gamete is the ovule.
- Additionally, sometimes female humans have homogametic sex cells because females contain a pair of XX chromosomes. In comparison, the male gametes in humans are heterogametic due to the presence of a pair of XY chromosomes that are dissimilar in morphology. Similarly, male birds contain heterogametic ZW chromosomes, while female birds contain homogametic ZZ chromosomes.