Gametogenesis

Gametogenesis

Gametogenesis is the process by which the two kinds of gametes — male and female — are formed. Gametes are always haploid cells, carrying a single set of chromosomes.

Isogametes and Heterogametes

In a few algae, the male and female gametes look identical and cannot be told apart morphologically — these are called isogametes or homogametes (illustrated for Cladophora in Figure 1.5a). In most sexually reproducing organisms, however, the two gametes are structurally distinct — heterogametes. Here the male gamete is called the antherozoid or sperm, and the female gamete is the egg or ovum, as shown for the alga Fucus and for humans in Figure 1.5b and c.

Bisexual and Unisexual Organisms

Organisms differ in whether an individual carries both male and female reproductive structures or only one:

• A bisexual organism has both male and female reproductive parts. In fungi and plants this is called the homothallic (or monoecious) condition; examples include the alga Chara, and among flowering plants, cucurbits and coconut, which bear separate male and female flowers on the same individual. Some flowers, like that of sweet potato, are themselves bisexual, containing both stamen and carpel.
• A unisexual organism has only one type of reproductive structure. The equivalent terms in fungi and plants are heterothallic and dioecious; papaya and date palm are dioecious examples, where separate male and female flowers occur on different plants. In flowering plants, a unisexual male flower (bearing only stamens) is called staminate, and a unisexual female flower (bearing only pistils) is pistillate.

Among animals, most species are unisexual — the cockroach, for instance, exists as separate male and female individuals (Figure 1.6b), each with only testes or only ovaries. A smaller number of animals are bisexual, or hermaphrodite, possessing both male and female reproductive organs in the same individual — earthworms, sponges, tapeworms and leeches are classic examples (Figure 1.6a). Figure 1.6 also contrasts the monoecious alga Chara, which bears both its male organ (antheridium) and female organ (oogonium) on the same plant, with the dioecious liverwort Marchantia, whose male and female reproductive structures occur on entirely separate thalli.

Cell Division During Gamete Formation

Although gametes are always haploid, the parent body producing them may itself be haploid or diploid, and this determines how the gametes are made. A haploid parent produces its gametes by ordinary mitosis, since the body cells are already haploid. Organisms such as many members of the Monera, Fungi, Algae and Bryophyta have a haploid plant body of this kind. In contrast, organisms whose parent body is diploid — including pteridophytes, gymnosperms, angiosperms and most animals, including humans — must undergo meiosis (reduction division) to produce haploid gametes from diploid tissue. In such diploid organisms, specialised diploid cells called meiocytes (gamete mother cells) undergo meiosis, and only a single set of chromosomes is passed into each resulting gamete. …