Gamete Formation & Reproductive Strategies

Gametogenesis: The Formation of Gametes

Gametogenesis is the process of gamete formation, through which haploid gametes originate from undifferentiated diploid cells via a meiotic process. This process can be either spermatogenesis or oogenesis.

Specifically, spermatogenesis is the formation of male gametes (sperm), and oogenesis is the formation of female gametes (eggs).

Spermatogenesis Stages

• Propagation: Germ cells originate by mitosis from other germ cells (spermatogonia).
• Growth: Enlarged spermatogonia transform into first-order spermatocytes (2n).
• Maturation: Meiosis occurs. Each first-order spermatocyte (2n) undergoes the first meiotic division, leading to two second-order spermatocytes (n). In the second meiotic division, these will form four spermatids (n).
• Spermiogenesis: Spermatids undergo a profound transformation, resulting in four spermatozoa.

Oogenesis Stages

• Propagation: Germ cells originate by mitosis from other germ cells (oogonia).
• Growth: The oogonia accumulate reserve substances and increase in size, transforming into first-order oocytes (2n).
• Maturation: Each first-order oocyte (2n) undergoes the first meiotic division, resulting in a large second-order oocyte (n) rich in reserve substances, and a small polar body (n). Both undergo the second meiotic division, forming an egg (n) where all nutrient reserves are concentrated, and 3 polar bodies, which degenerate and die.

Fertilization: The Fusion of Gametes

Fertilization is the fusion of male and female gametes to form the zygote and give rise to a new being.

External Fertilization

In external fertilization, male and female gametes are expelled simultaneously and in the same place into the outside environment, where some of them are fertilized. This process occurs in the majority of aquatic animals.

Internal Fertilization

Internal fertilization requires direct contact between two individuals of opposite sexes and takes place inside the female reproductive tract. Males secrete a liquid for transporting sperm, which requires a fluid medium to survive and reach the egg. In evolved animals, sperm transfer is achieved by a copulatory organ, while those that lack a copulatory organ achieve this by connecting their breeding holes. Such fertilization represents a major evolutionary step.

Special Forms of Sexual Reproduction

Parthenogenesis

Parthenogenesis is the development of a new being from a single gamete, usually a female sex cell which is not fertilized. Several types exist:

• Optional Parthenogenesis

  The egg can develop with or without fertilization. This is typical of insects such as bees. The queen bee, by which she is fertilized by males in the nuptial flight, keeps the sperm in a receptacle in her abdomen which communicates with her genital tract via a sphincter. If this sphincter does not open, sperm are not released. Therefore, if there is no fertilization of the ovules, haploid males (drones) are born. If sperm are released, fertilizing eggs, the eggs hatch and diploid females (workers or queens) are born, becoming workers or queens depending on their feeding.

• Cyclic Parthenogenesis

  Parthenogenetic generations alternate with generations which originate through fertilization. For example, in some species of rotifers, females produce eggs that can be diploid or haploid. The first type is never fertilized, and their ovules give rise only to females who may produce diploid or haploid ovules. If the latter are not fertilized, the eggs originate small males; if, in contrast, they are fertilized, the eggs are large and able to withstand cold and dryness, and give rise to diploid females that produce eggs.

• Obligate Parthenogenesis

  All generations are parthenogenetic, as occurs in some lower crustaceans.

Neoteny

Neoteny is a process characteristic of some animals that reach sexual maturity before concluding their body development. Thus, individuals can reproduce even though they still exhibit larval characteristics.

Cloning

Cloning involves inserting the nucleus of a somatic cell into an egg from which genetic material has been extracted. This egg, implanted in the uterus of a female, creates a new being identical to the somatic cell donor.

Alternating Reproduction (Metagenesis)

Alternating reproduction is a process in which generations of sexual and asexual reproduction (strobilation or budding) alternate. This is also called metagenesis.